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Poulton, P. R., Johnston, A. E., Macdonald, A. J., White, R. P. and
Powlson, D. S. 2018. Major limitations to achieving 4 per 1000 increases
in soil organic carbon stock in temperate regions: evidence from long-
term experiments at Rothamsted Research, UK. Global Change Biology.
24 (6), pp. 2563-2584.
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P R I M A R Y R E S E A R C H A R T I C L E
Major limitations to achieving
“
4 per 1000
”
increases in soil
organic carbon stock in temperate regions: Evidence from
long-term experiments at Rothamsted Research, United
Kingdom
Paul Poulton
1
|
Johnny Johnston
1
|
Andy Macdonald
1
|
Rodger White
2
|
David Powlson
1
1
Department of Sustainable Agriculture
Sciences, Rothamsted Research, Harpenden,
UK
2
Department of Computational and
Analytical Sciences, Rothamsted Research,
Harpenden, UK
Correspondence
David S. Powlson, Department of
Sustainable Agriculture Sciences,
Rothamsted Research, Harpenden, UK.
Email: david.powlson@rothamsted.ac.uk
Funding information
Biotechnology and Biological Sciences
Research Council (BBSRC), Grant/Award
Number: BBS/E/C/000J0300
Abstract
We evaluated the
“
4 per 1000
”
initiative for increasing soil organic carbon (SOC) by
analysing rates of SOC increase in treatments in 16 long-term experiments in south-
east United Kingdom. The initiative sets a goal for SOC stock to increase by 4
&
per
year in the 0
–
40 cm soil depth, continued over 20 years. Our experiments, on three
soil types, provided 114 treatment comparisons over 7
–
157 years. Treatments
included organic additions (incorporated by inversion ploughing), N fertilizers, intro-
ducing pasture leys into continuous arable systems, and converting arable land to
woodland. In 65% of cases, SOC increases occurred at
>
7
&
per year in the 0
–
23 cm
depth, approximately equivalent to 4
&
per year in the 0
–
40 cm depth. In the two
longest running experiments (
>
150 years), annual farmyard manure (FYM) applica-
tions at 35 t fresh material per hectare (equivalent to approx. 3.2 t organic C
/
ha/year)
gave SOC increases of 18
&
and 43
&
per year in the 23 cm depth during the first
20 years. Increases exceeding 7
&
per year continued for 40
–
60 years. In other
experiments, with FYM applied at lower rates or not every year, there were increases
of 3
&
–
8
&
per year over several decades. Other treatments gave increases between
zero and 19
&
per year over various periods. We conclude that there are severe limi-
tations to achieving the
“
4 per 1000
”
goal in practical agriculture over large areas.
The reasons include (1) farmers not having the necessary resources (e.g. insufficient
manure); (2) some, though not all, practices favouring SOC already widely adopted;
(3) practices uneconomic for farmers
—
potentially overcome by changes in regulations
or subsidies; (4) practices undesirable for global food security. We suggest it is more
realistic to promote practices for increasing SOC based on improving soil quality and
functioning as small increases can have disproportionately large beneficial impacts,
though not necessarily translating into increased crop yield.
K E Y W O R D S
4 per 1000, carbon sequestration, climate change mitigation, long-term experiments,
management practices, organic amendments, Rothamsted, soil organic carbon
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
©
2018 The Authors.
Global Change Biology
Published by John Wiley & Sons Ltd
Received: 31 October 2017
|
Accepted: 23 December 2017
DOI: 10.1111/gcb.14066
Glob Change Biol.
2018;1
–
22.
wileyonlinelibrary.com/journal/gcb
|
1
Page 3
1
|
I N T R O D U C T I O N
The
“
4 per 1000 initiative: Soils for Food Security and Climate
”
, also
known as
“
4 per mille
”
or
“
4
&
”
, for increasing soil organic carbon
(SOC) stocks was launched by the French Ministry of Agriculture in
2015 in preparation for the Paris climate conference of the United
Nations Framework Convention on Climate Change (UNFCCC). It is
described in several web sites including:
1.
https://www.4p1000.org/4-1000-initiative-few-words
2.
http://newsroom.unfccc.int/lpaa/agriculture/join-the-41000-initia
tive-soils-for-food-security-and-climate
It is also described by Chabbi et al. (2017) and Minasny et al.
(2017). The aim of the initiative is to promote land management
practices leading to an increase in the stock (i.e. quantity as opposed
to concentration) of SOC at the rate of 4
&
(0.4%) of the initial value
per year for 20 years. It was originally suggested that, if this rate of
C sequestration was achieved for all soils globally to a depth of
40 cm, the C removed from the atmosphere would equal annual
CO
2
–
C emissions from fossil fuels of 8.9 Gt and thus
“
halt the
annual increase in CO
2
in the atmosphere
”
(http://4p1000.org/under
stand). Later enunciations of the initiative have recognized that
increases in SOC are only likely in soils that are being actively man-
aged for agriculture (and possibly under managed forestry) and, even
in these, such a rate of increase may not be achievable everywhere
(Chabbi et al., 2017; Chambers, Lal, & Paustian, 2016; Lal, 2016;
Minasny et al., 2017). It is suggested that C sequestration in agricul-
tural soils alone, as opposed to all soils, might be limited to
c.
1 Gt/
year (Chabbi et al., 2017; Smith, 2016). There is also some uncer-
tainty about the soil depth referred to in the initiative; in some pub-
lications, the 0
–
100 cm depth is mentioned in addition to 0
–
40 and
0
–
30 cm (Chabbi et al., 2017). Minasny et al. (2017) published a
compilation of data from 20 regions of the world showing opportu-
nities and limitations for achieving the 4
&
rate of SOC increase.
The initiative has been generally welcomed as laudable because
any contribution to climate change mitigation is helpful and equally,
or perhaps more importantly, any increase in SOC is virtually certain
to improve the quality and functioning of many soils. This is espe-
cially relevant in the light of the reported widespread incidence of
land degradation globally (UNCCD, 2017). However, there have been
significant criticisms of the initiative by several authors (van Groenin-
gen, van Kessel, Hungate, & Oenema, 2017; Baveye, Berthelin, Tes-
sier, & Lemaire, 2018; de Vries, 2018; VandenBygaart, 2018; White,
Davidson, Lam, & Chen, 2018), suggesting that there are many situa-
tions where the 4
&
rate of increase in SOC is not feasible for land
managers in practical situations and that some of the experimental
examples given by Minasny et al. (2017) are not representative of
what is practically achievable in wide-scale agriculture. There also
appear to be differences of opinion as to whether the 4
&
goal is a
specific target, an aspiration goal or even
“
more of a concept (or
even a slogan)
”
as suggested by Minasny et al. (2018).
The aims of this paper were twofold. First, to report the rates
of SOC increase in several long-term agricultural field experiments
run by Rothamsted Research at three sites in southeast England
between 1843 and 2013. In temperate climates, SOC changes
slowly in response to changes in management so periods of years
or decades are required to reliably detect and quantify rates of
change (Macdonald et al., 2015; Storkey et al., 2016). We use data
from 16 separate experiments on three different soil types, giving
over 110 treatment comparisons. It is recognized that the sources
of data are geographically constrained but the cropping systems are
diverse, and treatments cover a wide range of management prac-
tices relevant to temperate regions, though do not include reduced
tillage. The results represent the largest concentration of long-term
SOC data globally, and the soil types and climates are broadly rep-
resentative of temperate regions globally. Second, we attempt to
use the long-term data to provide an evidence-based assessment of
the likely range of situations where the 4
&
target might be
achieved in practical agriculture, and for how long increases at this
rate might continue.
2
|
M A T E R I A L S A N D M E T H O D S
2.1
|
Experimental sites and soils
Soil samples from 16 experiments with a cool temperate climate in
southeast United Kingdom were analysed for their organic carbon
content. Much of the data have not been published previously and
for several experiments, data have been updated. The soil types are
silty clay loam at Rothamsted, sandy loam at Woburn and sandy clay
loam at Saxmundham (see Table 1 for soil classifications). Long-term
average annual rainfall is
c.
700, 650 and 610 mm at Rothamsted,
Woburn and Saxmundham respectively. Until the late 1980s, the
mean annual temperature was
c.
9.0
–
9.4
°
C at the three sites, but, at
Rothamsted and Woburn, where meteorological recording has con-
tinued, it has increased by about 1
°
C in the last 25
–
30 years (Scott,
Macdonald, & Goulding, 2014; Johnston, Poulton, Coleman, Macdon-
ald, & White, 2017). In all-arable experiments, soils were cultivated
by inversion ploughing to a depth of 20
–
22 cm.
Table 1 lists the experiments with references to publications giv-
ing details. In subsequent Tables, data are grouped according to
treatment, taking data from different experiments where required
rather than discussing each experiment separately. Further informa-
tion appropriate to this paper is in Supporting Information.
2.2
|
Soil sampling and analysis
Most soils were sampled to 23 or 25 cm, i.e. a little deeper than cur-
rent plough depth, with a semicylindrical gouge auger (2
–
3 cm diam-
eter), taking many cores per plot. However, early samples on
Broadbalk and Hoosfield were taken from several positions within
each large plot with an open-ended iron box sampler (typically
30.5
9
30.5
9
22.9 cm deep). Each
“
box
”
sample provided a large
2
|
POULTON
ET AL
.
Page 4
mass of soil which made it possible to determine the weight of fine
soil (
<
6 mm, the standard procedure at Rothamsted at the time for
samples that were to be archived) per unit volume. Similar box sam-
ples have been taken from many long-term experiments at Rotham-
sted, Woburn and Saxmundham and the measured soil weight is
used for these and other experiments on the same soil type. Soils
were air-dried and subsamples ground to
<
355
l
m for nitrogen (N)
and carbon (C) analysis. Organic carbon was determined by Tinsley
(1950) or by modified Walkley-Black (corrected to make the data
equivalent to Tinsley; Kalembasa & Jenkinson, 1973). Later soils, and
some earlier archived soils, were analysed for total C by combustion
(LECO Corp., St Joseph, Michigan, USA) and a correction made for
inorganic C, determined by manometry (Skalar Analytical BV, Breda,
Netherlands). Standard soils are included with each batch of soils
sent for analysis. For recent analyses, the mean and standard error
of the standard soil is 1.46% OC
0.005.
T A B L E 1
Experimental site details
Site, experiment
and number
Duration
Clay
(%)
Sampling
depth (cm)
Crop
Treatments include
Reference
Rothamsted
a
, Hertfordshire, United Kingdom
Broadbalk (1)
1843
—
ongoing
20
–
38
23
W. wheat
Rates of N, FYM, straw
Johnston and Garner (1969),
Rothamsted Research (2006)
Hoosfield Barley (2)
1852
—
ongoing
18
–
27
”
S. barley
Rates of N, FYM, FYM residues
Jenkinson and Johnston (1977)
Fosters
Ley-arable (3)
1949
—
ongoing
”
”
Arable crops
and leys
All-arable rotations,
3-year leys
+
3-year
arable rotations, or
permanent grass since 1949
Johnston (1973), Johnston
et al. (2009)
Highfield
Ley-arable (4)
1949
—
ongoing
”
”
Arable crops
and leys
All-arable rotations,
3-year leys
+
3-year
arable rotations, or
permanent grass since 1838
Johnston (1973), Johnston
et al. (2009)
Amounts of Straw (5)
1986
–
2015
”
”
W. wheat
Wheat straw incorporated
since autumn 1986
Powlson, Glendining, Coleman,
and Whitmore (2011)
Broadbalk
Wilderness (6)
1881
—
ongoing
20
–
25
”
Regenerating
woodland
None
Poulton et al. (2003)
Geescroft
Wilderness (7)
1886
—
ongoing
”
”
Regenerating
woodland
None
Poulton et al. (2003)
Park Grass (8)
1856
—
ongoing
18
–
27
”
Permanent
pasture
FYM
Warren and Johnston (1964),
Fornara et al. (2011)
Exhaustion Land (9)
1852
—
ongoing
18
–
27
”
Arable crops
b
FYM, FYM residues
Johnston and Poulton (1977),
Johnston et al. (2017)
Woburn
c
, Bedfordshire, United Kingdom
Amounts of Straw (10)
1986
–
2015
c. 14
”
W. wheat
Wheat straw incorporated
since autumn 1986
Powlson, Glendining, et al.
(2011)
Organic Manuring (11)
1965
—
ongoing
8
–
13
”
Arable crops
and leys
Various organic amendments
Mattingly et al. (1974)
Green Manuring (12)
1936
–
67
c.13
20
Arable crops
Various organic amendments
Chater and Gasser (1970)
Market Garden (13)
1942
—
ongoing
10
–
12
23
Various crops
Various organic amendments
from 1942 to 1967
Johnston (1975)
Ley-arable (14)
1938
—
ongoing
11
–
16
25
Arable crops
and leys
All-arable rotations,
3- and 8-year leys
+
2-year
arable rotations, FYM
every 5 years until mid-1960s
Johnston et al. (2017)
Saxmundham
d
, Suffolk, United Kingdom
Rotation II (15)
1899
–
1986
c. 25
20
Arable crops
FYM every 4 years
Mattingly, Johnston, and
Chater (1969)
Rotation I (16)
1899
–
2009
”
”
Arable crops
FYM
Williams and Cooke (1971)
a
The soil at Rothamsted is a flinty silty clay loam over Clay-with-flints and is classified as a Chromic Luvisol (IUSS Working Group WRB, 2015).
b
Potatoes were grown continuously from 1876 to 1901; FYM was applied annually during this period.
c
The soil at Woburn is a sandy loam and is classified as a Cambic Arenosol (IUSS Working Group WRB, 2015).
d
The soil at Saxmundham is a sandy clay loam derived from boulder clay and is classified as a Eutric Gleysol (IUSS Working Group WRB, 2015).
POULTON
ET AL
.
|
3
Page 5
2.3
|
Correcting amounts of SOC for changes in soil
bulk density
When measuring changes in the quantity (stock) of organic C in the
soil over time, it is important, where possible, to sample the same
weight of mineral soil each time (Jenkinson, Poulton, & Bryant, 2008),
on the assumption that most organic matter is held on the mineral par-
ticles. Where bulk density has not changed and, therefore, the weight
of soil to a defined depth has not changed, the soil is sampled to the
same depth. But, where bulk density has changed allowance must be
made for this as discussed in detail in Johnston et al., 2017. In some of
the experiments discussed here bulk density has declined due to addi-
tion of bulky organic material or a period in pasture, and later soil sam-
ples should have been taken to a slightly greater depth so that SOC is
determined in the same weight of mineral soil. If this is not done, and
the same weight of soil is used for the start and end of the measured
period, this will overestimate the amount of C being sequestered. If
the soil weight has been measured at the start and end of a period, but
no allowance is made for the mass of extra soil which should have
been sampled, and the amount of C it contained, then the amount of C
being sequestered will be underestimated. Where bulk density has
declined and where the weight of soil has been determined we have
made a simple correction for that change. If, for example, the soil
weight to a depth of 23 cm at the beginning and end of a period in
ley-arable cropping was 3,770 t/ha and 3,470 t/ha respectively, then
the difference, 300 t/ha, represents the amount of
“
extra
”
soil that
should have been sampled at the end (see Johnston et al., 2017). The
amount of C in this extra soil can be calculated (it will have the same
concentration of C as at the start) and added to that in the sampled
layer. Where the amounts of SOC have been adjusted for bulk density
changes, they are given in the appropriate Tables.
2.4
|
Relating our data for increases in SOC with
the 4
&
target
The rates of increase in SOC calculated from our data are for soil sam-
ples taken to a depth of 20, 23 or 25 cm. Where possible, we have
taken account of any extra soil that should have been sampled where
bulk density declined over time as a result of the treatment (see
above). However, the
“
4 per 1000
”
initiative refers to increases in SOC
to a depth of 40 cm. In Supporting Information, we describe how we
can adjust our data to that for 0
–
40 cm. Based on these calculations,
we suggest that, for most of the topsoil data presented here, an
increase of 7
&
equates to 4
&
when expressed on a 0
–
40 cm basis.
Where the topsoil contains a higher concentration of OC (e.g. in grass-
land or woodland sites or where large amounts of manure have been
applied) an increase of
c.
5
&
will equate to 4
&
for the 0
–
40 cm layer.
2.5
|
Statistical analysis
All the experiments reported here are, or were, long-term but for
some we present changes over relatively short (e.g. 3
–
7 years),
sometimes consecutive, periods (see Tables 5, 6, 9, S1
–
S5). Many of
the experiments were established before the introduction of modern
experimental design so that treatments were not necessarily repli-
cated or randomized. Thus, the degree to which conventional statis-
tics can be used to assess whether the observed changes are
statistically significant is limited. Where errors have been calculated,
and published, these are given in the appropriate tables. In other
cases, where possible, the standard error of the mean of the differ-
ences between replicate plots for the stock of SOC at the start and
end of each period is given together with the standard error of the
annual rate of increase.
It is clear from these long-term experiments that measuring SOC
differences between treatments with sufficient precision to assign
statistical significance is challenging due to soil spatial variability and
the large background of SOC against which changes occur. Reliably
detecting the relatively small changes likely to result from application
of the
“
4 per 1000
”
initiative will be even more difficult for the rea-
sons discussed by Smith (2004).
2.6
|
Fitting curves to the accumulation of soil
organic carbon on farmyard manure-treated plots on
the Hoosfield Barley and Broadbalk Wheat
experiments
It is well established that the accumulation of SOC does not increase
indefinitely but moves towards an equilibrium value (Johnston, Poul-
ton, & Coleman, 2009; Smith, 2014; Gollany et al., 2011) so the
accumulation of SOC can be represented by a curve. It was possible
to fit such curves to the accumulation of SOC in farmyard manure
(FYM)-treated plots in the Hoosfield Barley and Broadbalk Wheat
experiments because the soils have been sampled sufficiently fre-
quently. An advantage of this procedure was that it enabled data
from the two or three FYM treatments in the experiments that were
started at different times to be amalgamated, thus giving some
degree of replication. Changes in SOC between each sampling point
are given in Table S1, making allowance for the SOC in any soil to a
greater depth that should have been sampled due to decreasing soil
bulk density (see above). For both experiments the exact starting
value has been estimated using the approach described by Jenkinson
and Johnston (1977) because no samples were taken at the start of
these two experiments. The FYM-treated plots are not replicated
but soil from the several large box samples (see above) taken period-
ically from each large plot have been analysed separately for SOC.
For each experiment, a simple limiting exponential model was fitted
(GenStat
â
, 2016) using the following equation:
org C
¼
a
þ
b
r
time
where org C is SOC, in t/ha,
a
is an asymptote representing the
maximum capacity of the soil for SOC under the specific manage-
ment, the slope coefficient,
b
, represents the available capacity of
the soil to take up SOC and
r
is the exponential coefficient, repre-
senting the rate at which the soil accumulates SOC.
On Hoosfield, one plot has received FYM every year since 1852,
another since 2001. Using the start of each treatment as year zero
4
|
POULTON
ET AL
.
Page 6
the accumulation of SOC stocks over time was modelled for the
combined dataset.
Broadbalk is more complicated; three plots receive (or have
received) FYM; one since autumn 1843, one since autumn 1884 and
one between autumn 1967 and autumn 2000 only. However, FYM
was not applied to the two plots that started in 1843 and 1884 when
these plots were fallowed, usually 1 year in five between 1926 and
1967, to control weeds (Johnston & Garner, 1969). Thus, for these
two plots there is a break in the annual application of FYM between
soil sampling in 1914 and that in 1967. This has been overcome by
considering five separate series of data; 1843
–
1914 and 1967
–
2000
for the FYM plot since 1843; 1885
–
1914 and 1967
–
2000 for the plot
since 1884; and 1967
–
2000 for the plot receiving FYM since 1967
(Table S1). However, there is no reason to suppose that after annual
applications of FYM resumed the sequestration of SOC would not
continue as before. We therefore considered that the separate series
of measurements should belong to a common model. To construct a
common time frame for the separate series, the
b
parameter was
allowed to vary and then the calendar time values for each series were
adjusted accordingly, back to the first set (1843
–
1914), while the
a
and
r
parameters were estimated from all the data, assuming that
these would be the same for each series. The simple limiting exponen-
tial model (GenStat
â
, 2016) was then fitted to the time-shifted data.
Each fitted curve accounts for a large proportion of the variance,
94% and 89% for Hoosfield and Broadbalk respectively, suggesting
that any inferences drawn from the data are reliable. From each
curve the amount of SOC in the soil after e.g. 20, 40, 60 years was
estimated, and the average rate of accumulation in each 20-year per-
iod calculated assuming a linear increase in SOC during that period,
though recognizing that this is an approximation. These values were
then used in comparisons with data from other experiments. Associ-
ated standard errors were calculated using GenStat
â
(2016), but
they should be regarded as approximate.
3
|
R E S U L T S
3.1
|
Comparing our data with the 4
&
initiative
The
“
4 per 1000
”
initiative relates to SOC in the 0
–
40 cm depth of soil
but data from the long-term experiments described in this paper are
mostly derived from sampling to a depth of 0
–
23 cm, slightly deeper
than the usual plough depth for arable soils at these sites. In Support-
ing Information, we describe how 0
–
23 cm data can be approximately
related to 0
–
40 cm. In general, for most topsoil data presented, an
increase of 7
&
in the 0
–
23 cm layer is equivalent to 4
&
in 0
–
40 cm
depth. Where the topsoil has a higher SOC concentration (
>
c.
1.3%)
an increase of
c.
5
&
will equate to 4
&
for 0
–
40 cm soil.
3.2
|
Effects of farmyard manure additions on SOC
stocks
Applying animal manure to soil is probably the oldest and most
thoroughly researched means of adding nutrients and, concurrently,
increasing the concentration and stock of SOC. Ten experiments on
three soil types provide data on the rate of SOC increase in
Tables S1
–
S3, with one that includes FYM applications in conjunc-
tion with different crop rotations. In the oldest of the Rothamsted
experiments in Table 1, Broadbalk (started autumn 1843, growing
winter wheat); and Hoosfield (started 1852, spring barley), FYM
has, with some exceptions (see Materials and Methods), been
applied every year at 35 t fresh material per hectare,
ca
. 8 t dry
matter per hectare. This amount was tested for experimental pur-
poses to supply large amounts of nutrients, not because it would
be recommended as a practical treatment: the significance of this is
discussed later.
Figurs 1 and 2 show the accumulation of SOC resulting from
FYM applications in the Hoosfield and Broadbalk experiments
respectively, the curves being constructed by amalgamating data
from treatments in which FYM applications started at three different
times (Broadbalk) and at two different times (Hoosfield) as described
in
“
Materials and Methods
”
. The data used are shown in detail in
Table S1. From the fitted curves, the rates of SOC increase,
expressed as
&
per year compared to the initial contents, have been
calculated for different periods (Table 2).
Both experiments started with low stocks of SOC: 30.1 t/ha
(1.15% C) in Hoosfield and 28.8 t/ha (1.00% C) in Broadbalk
(Table S1). This is because the fields had been in arable cropping for
several hundred years before the start of the experiments, probably
in 5-course rotations where FYM was applied 1 year in five. On
Broadbalk it is known that no manure was applied in the 5 years
prior to the start of the experiment in autumn 1843 (Lawes & Gil-
bert, 1895). In both experiments SOC initially increased rapidly with
F I G U R E 1
Accumulation of soil organic carbon (SOC), t/ha,
resulting from an annual application of farmyard manure (FYM),
35 t/ha in the Hoosfield Barley Experiment, Rothamsted. The
amount of SOC has been corrected for the change in soil bulk
density at the end of each sampling period and an exponential curve
fitted. FYM since 1852, (
9
); FYM since 2001, (
○
). The start of each
treatment is considered as year zero
POULTON
ET AL
.
|
5
Page 7
average rates of 18
&
and 43
&
per year in the first 20 years of
applying 35 t/ha FYM annually in Hoosfield and Broadbalk, respec-
tively. The rates remained above 7
&
per year (equivalent to 4
&
per
year in 0
–
40 cm soil) for 40 years in Hoosfield and 60 years in
Broadbalk (Table 2). After this, the rates declined as SOC stocks
tended towards new equilibrium values and after about 80
–
100 years of applying this amount of FYM annually, the rate of
change in SOC was not significantly different from zero.
The final SOC stock attained after applying FYM for many years
was larger in Hoosfield (80 t C/ha) than in Broadbalk (75 t C/ha);
see Figures 1 and 2. We can offer no complete explanation for this
which may be due to small differences in soil properties or manage-
ment between the experiments. In addition, on Broadbalk no crop
was grown and no FYM applied 1 year in five for a 42-year period
when bare fallowing was used to control weeds (see Supporting
Information and Table S1), so total organic C inputs over the course
of the experiment were less than on Hoosfield.
In the Exhaustion Land experiment at Rothamsted, adjacent to
Hoosfield and on the same soil type, FYM was applied from 1876 to
1901 at the same annual rates as on Broadbalk and Hoosfield. Soil
samples taken in 1903 showed that the average rate of increase in
SOC was 27
&
(Tables 3 and S2), a value comparable to the rates in
Hoosfield and Broadbalk over similar periods.
Results from other experiments, at Saxmundham and Woburn
(Table 3), better reflect FYM use in commercial practice, either
because smaller amounts were applied annually and/or FYM was not
applied every year. Although the treatments in these experiments
were replicated, disadvantages are that the data are less detailed
than in Broadbalk and Hoosfield as soils were sampled on fewer
occasions and, in some cases, the treatments were applied for
shorter periods. In the two experiments at Saxmundham, where the
soil contains 25% clay, SOC increased significantly. The rates of
F I G U R E 2
Accumulation of soil organic carbon (SOC), t/ha,
resulting from an annual application of farmyard manure (FYM),
35 t/ha in the Broadbalk Wheat Experiment, Rothamsted. The
amount of SOC has been corrected for the change in soil bulk
density at the end of each sampling period. Each of the three series
of data started in different years: Series 1. FYM from 1843 to 1914
(
9
), and again from 1967 to 2000, (
○
). Series 2. FYM from 1885 to
1914 (
+
), and again from 1967 to 2000, (
Δ
). Series 3. FYM from
1967 to 2000 (
□
). To create a common timeframe, the calendar
years for Series 2 and 3 data were shifted back to coincide with
Series 1 before fitting the exponential model
Experiment
and number
Period
(years)
Predicted amount of org C
c
Rate of increase
(t ha
1
year
1
)
Annual increase
(
&
/ha)
At start (t/ha)
At end (t/ha)
Hoosfield
Spring
barley (2)
1
–
20
37.6
1.4
51.4
2.0
0.69
8.4
21
–
40
51.4
2.0
61.0
2.6
0.48
9.3
41
–
60
61.0
2.6
67.7
2.5
0.34
5.5
61
–
80
67.7
2.5
72.3
2.1
0.23
3.4
81
–
100
72.3
2.1
75.5
1.5
0.16
2.2
101
–
120
75.5
1.5
77.7
1.2
0.11
1.5
121
–
140
77.7
1.2
79.3
1.2
0.08
1.0
141
–
160
79.3
1.2
80.4
1.4
0.06
0.7
Broadbalk
Winter
wheat (1)
1
–
20
23.4
3.5
43.4
1.7
1.00
42.7
21
–
40
43.4
1.7
56.2
1.2
0.64
14.7
41
–
60
56.2
1.2
64.3
1.0
0.40
7.2
61
–
80
64.3
1.0
69.4
1.4
0.26
4.0
81
–
100
69.4
1.4
72.7
2.0
0.16
2.4
101
–
120
72.7
2.0
74.8
2.6
0.10
1.4
a
For Hoosfield:
a
=
82.8 (
SE
3.13);
b
=
45.2 (
SE
2.89);
r
=
0.982 (
SE
0.005).Variance accounted
for
=
94%;
SE
of observations
=
4.6.
b
For Broadbalk:
a
=
78.5 (
SE
4.50);
b
=
55.1 (
SE
4.90);
r
=
0.978 (
SE
0.004).Variance accounted
for
=
89%;
SE
of observations
=
4.0.
c
Data to which the exponential models were fitted included any additional C present in the soil to an
“
equivalent
”
depth at the end of each period (see Supporting Information and Table S1).
T A B L E 2
Predicted amounts of organic
C in topsoil, together with standard
errors, from exponential models, org
C
=
a
+
b
9
r
time
, fitted to soils given
35 t FYM ha
1
year
1
; Hoosfield
a
and
Broadbalk
b
6
|
POULTON
ET AL
.
Page 8
TABL
E
3
Changes
in
amount
of
organ
ic
C
from
add
ing
fa
rmyard
manu
re
Experiment
and
number
Crop
Treatment
a
Sampling
period
b
Amount
of
organic
C
c
in
measured
depth
Rate
of
increase
(t
C
h
a
1
year
1
)
Annual
increase
(
&
/yr)
Notes
At
start
(t/ha)
At
end
(t/ha)
Difference
d
(t/ha)
Saxmundham
RII
(16)
Various
arable
crops
FYM
every
4th
year
1899
–
1968
(43.9)
53.2
9.3
1.25
0.13
0.018
3.0
25.1
t/ha
every
4th
year
Woburn
Green
Manuring
(12)
Various
crops
FYM
every
2nd
year
1936
–
54
29.2
36.4
7.2
0.40
13.6
25.1
t/ha
every
2nd
year
Saxmundham
RI
(15)
Various
arable
crops
FYM
every
year
1899
–
1969
(36.5)
57.8
21.3
1.75
0.30
0.025
8.4
15.1
t
h
a
1
year
1
(30.1
t/ha
since
1965)
Exhaustion
Land
(9)
Arable
crops
FYM
1876
–
1903
(24.9)
43.8
18.9
0.68
27.1
35
t
h
a
1
year
1
1876
–
1901
Woburn
Organic
Manuring
(11)
Various
arable
crops
NPKMg
1965
–
1971
e
25.8
24.6
1.2
0.37
0.21
0.062
8.1
No
organic
amendment
”
NPKMg
1980
–
1986
24.4
22.7
1.7
0.86
0.28
0.143
12.0
”
”
NPKMg
2002
–
2012
26.8
23.2
3.5
0.73
0.35
0.073
13.2
”
”
FYM
1965
–
1971
e
25.2
35.3
10.1
1.36
1.68
0.227
66.4
50
t
h
a
1
year
1
+
N
applied
1965
–
1970
”
FYM
1980
–
1986
29.4
36.8
7.4
1.35
1.23
0.225
42.2
50
t
h
a
1
year
1
+
N
applied
1981
–
1985
”
FYM
10
t
h
a
1
year
1
since
2003
2002
–
2012
28.4
28.1
0.3
1.15
0.03
0.115
0.2
10
t
h
a
1
year
1
+
N
”
FYM
25
t
h
a
1
year
1
since
2003
2002
–
2012
35.
4
40.0
4.6
0.96
0.46
0.096
13.1
25
t
h
a
1
year
1
+
N
Park
Grass
(8)
Permanent
pasture
FYM
every
4th
year
since
1905
1923
–
1959
79.9
94.8
14.9
5.54
0.41
0.154
5.2
35
t
h
a
1
year
1
every
1959
–
2002
94.8
104.2
9.4
0.59
0.22
0.014
2.4
4th
year
”
FYM
every
4th
year
since
1905
1923
–
1959
77.9
98.0
20.1
6.77
0.56
0.188
7.4
35
t
h
a
1
year
1
every
4th
year
+
NPK
in
other
years
1959
–
2002
98.0
97.4
0.7
1.61
0.02
0.037
0.1
”
FYM
every
4th
year
since
1905
1923
–
1959
87.6
85.1
2.5
0.07
0.8
35
t
h
a
1
year
1
every
4th
year
+
fishmeal
(0.75
t/ha
DM)
every
4th
year
1959
–
2002
85.1
86.9
1.8
2.87
0.04
0.067
0.6
a
Unless
stated,
organic
amendments
were
applied
in
autumn.
b
Soils
were
sampled
in
autumn,
before
any
organic
amendments
were
applied;
except
Park
Grass
which
was
sampled
in
early
spring.
For
Saxmundham
RI
and
RI
I,
starting
values
(in
parentheses)
are
based
on
values
from
plots
with
NPK
but
no
FYM
input
(RI)
or
no
fertilizer
or
FYM
(RII),
measured
in
1969
and
1968
respectively.
For
the
Exhaustion
Land
the
star
ting
value
(in
parentheses)
is
the
mean
of
data
from
control
plots
with
no
fertilizer
or
FYM
input
or
from
plots
receiving
NPK,
measured
in
1903.
c
See
Table
S2
for
soil
weights
and
%
O
C
determined
by
Walkley-Black
(corrected),
Tinsley
or
combustion.
d
SE
of
the
mean
of
the
differences
between
replicates
where
applicable.
e
Sampled
in
spring
1964;
there
is
assumed
to
have
been
no
change
between
spring
1964
and
autumn
1965.
POULTON
ET AL
.
|
7
Page 9
increase were 3
&
per year averaged over 69 years where FYM was
applied at 25.1 t/ha every 4 years and 8
&
per year averaged over
70 years where it was applied at 15.1 t/ha every year (Table 3).
Thus, with an annual FYM application of 15.1 t/ha, the
“
4
&
”
goal
(measured as 7
&
for the 0
–
23 cm soil depth) was exceeded over an
extended period on this soil type, and it is likely that the rate of
increase was much greater in the earlier years of each experiment.
In a much sandier soil (
c.
13% clay) in the Woburn Green Manur-
ing
Experiment,
starting
at
a
lower
SOC
content,
applying
25.1 t FYM/ha every 2 years increased SOC at an average rate of
14
&
per year, during a much shorter period of 18 years (Table 3).
These results contrast with those in the Woburn Organic Manuring
Experiment, also on a sandy soil and starting with only about 0.7%
SOC. Treatments receiving only inorganic fertilizers lost SOC during
different phases of the experiment. Large manure applications of
50 t/ha annually led to very rapid and significant increases of SOC
(66
&
or 41
&
per year; Tables 3 and S2) when measured over
shorter periods. In a later phase of the experiment, applying
25 t FYM ha
1
year
1
plus N fertilizer increased SOC significantly,
at 13
&
per year over 10 years, but when only 10 t ha
1
year
1
was
applied there was no increase. (Mattingly, Chater, & Poulton, 1974).
In the Park Grass experiment, started in 1856 on a site that had
been in permanent pasture since
c.
1700, applying FYM to the sur-
face began in 1905 on three plots. FYM, at 35 t/ha, was applied
every 4 years, either with or without additional inputs (see Support-
ing Information). Unfortunately, soil samples on these particular plots
were not taken until 1923. Over the next 36 years there was a mod-
est increase in SOC on two of the treatments and in the following
43 years there was little or no further change (Tables 3 and S2). In
the Woburn Ley-arable Experiment, started in 1938, different 5-year
crop rotations are compared, and initially in each rotation either no
FYM or FYM applied once every 5 years at 38 t/ha was tested. In
the rotations comprising 3 year of either lucerne or grazed grass fol-
lowed by 2-year arable, FYM increased SOC by 6
&
or 12
&
per year
over 30 years (Table 4). Where only arable crops, mainly cereals,
were grown, the rate of increase in SOC was smaller (3
&
per year)
and where more root crops were grown in the 5-year rotation there
was no increase in SOC from applying FYM. Johnston et al. (2017)
considered that these differences were due to the additional number
of soil cultivations needed to grow root crops. Changes in SOC in
the different rotations are described by Johnston et al. (2017) and
summarized here in a later section.
3.3
|
Effects of various organic amendments on
SOC stocks
Table 5 shows rates of SOC increase following annual applications
of various organic amendments (the quantities added expressed as
fresh weight) in two experiments on a sandy soil. The Woburn Mar-
ket Garden Experiment was started in 1942 and data are available
for a 25-year period (Table S3). The soil contained only 0.87% C,
equivalent to 31.1 t C/ha initially. Table 5 also shows the effect of
adding compost for 10 years in the Woburn Organic Manuring
Experiment where the initial SOC content was higher: 1.13%
(Table S3), equivalent to 39 t C/ha.
In the Woburn Market Garden Experiment, four different organic
materials (FYM and three different composts) were tested initially
and all gave large annual rates of SOC increase in the first 9 years
of 51
&
–
114
&
per year for annual applications of 35 t/ha annually
of fresh material and even larger where a double rate was applied
(Table 5). In the subsequent 9
–
18 years, SOC increases in excess of
7
&
per year continued (8
&
–
21
&
per year for the lower application
rate). In 1960 two major changes were made to the experiment: only
FYM and vegetable compost continued as organic amendments and
the rate of N fertilizer applied annually to all treatments increased.
During the third measurement period (18
–
25 years after the start of
the experiment), SOC in the inorganic fertilizers (NPK) treatment
increased at the rate of 25
&
per year, whereas in the FYM and veg-
etable compost treatments the rates of increase were smaller (11
&
and 3
&
per year). These smaller increases were presumably because
the starting value of SOC in the organic amendment treatments in
1960 were considerably greater than in NPK (49 and 59 compared
T A B L E 4
Changes in amount of organic C from adding farmyard manure every 5 years to continuous arable or ley-arable rotations; Woburn
Ley-arable experiment (14)
a
Crop
rotation
b
Treatment
No. of plots
sampled
Sampling period
No. of
years
Amount of organic C
Rate of
increase
(t C ha
1
year
1
)
Annual
increase
(
&
/yr)
Notes
At start
(t/ha)
At end
c
(t/ha)
Ar
FYM every
5th year
5
1938
–
1965/74
30.5
37.0
37.0
0.00
0.0
213 t/ha applied from
1938 to the mid-1960s
Ah
”
5
1938
–
1965/74
30.5
37.0
40.2
0.11
2.8
”
Lu3
”
5
1938
–
1965/74
30.5
37.0
43.4
0.21
5.7
”
L3
”
5
1938
–
1965/74
30.5
37.0
50.1
0.43
11.7
”
a
Adapted from Tables 2, 4 and S2, Johnston et al., 2017. Although there was not a significant effect of FYM for individual rotations, when all rotations
were considered together, there was a significant effect.
b
Ar: 5-year arable crops including root crops. Ah: 5-year arable including 1-year hay. Lu3: 3-year lucerne ley followed by 2-year arable. L3: 3-year grazed
grass/clover ley followed by 2-year arable.
c
For L3, the amount of organic C at the end includes additional C present in the soil to an
“
equivalent
”
depth; i.e. so that the same mass of mineral soil
was being considered both at the start and end of the period.
8
|
POULTON
ET AL
.
Page 10
TABL
E
5
Cha
nges
in
am
ount
of
organ
ic
C
fro
m
adding
vari
ous
organ
ic
am
endmen
ts
Experiment
and
number
Crop
Treatment
a
Sampling
period
b
Amount
of
organic
C
c
in
measured
depth
Rate
of
increase
(t
C
h
a
1
year
1
)
Annual
increase
(
&
/yr)
Notes
At
start
(t/ha)
At
end
(t/ha)
Difference
d
(t/ha)
Woburn
Organic
Manuring
(11)
Various
arable
crops
Compost
2002
–
2012
38.8
52.9
14.1
2.42
1.41
0.242
36.3
40
t
h
a
1
year
1
compost
e
Woburn
Market
Garden
(13)
(Series
A)
Market
Garden
crops
NPK
1942
–
1951
31.1
33.6
2.5
0.28
8.9
No
organic
amendment
1951
–
1960
33.6
32.2
1.4
0.16
4.7
”
1960
–
1967
32.2
37.4
5.2
0.87
0.82
0.124
25.4
No
organic
amendment
but
amount
of
N
applied
doubled
”
FYM
1942
–
1951
31.1
45.3
14.2
1.59
51.1
311.4
t/ha
applied
in
total
1951
–
1960
45.3
48.6
3.3
0.36
7.9
401.8
t/ha
applied
in
total
1960
–
1967
48.6
52.1
3.5
1.51
0.51
0.216
10.5
175.8
t/ha
applied
in
total
”
Vegetable
compost
1942
–
1951
31.1
50.0
18.9
2.10
67.7
311.4
t/ha
applied
in
total
1951
–
1960
50.0
59.1
9.1
1.03
20.6
401.8
t/ha
applied
in
total
”
1960
–
1967
59.1
60.8
1.7
2.49
0.20
0.355
3.4
175.8
t/ha
applied
in
total
”
Sewage
sludge
1942
–
1951
31.1
62.8
31.7
3.53
113.7
311.4
t/ha
applied
in
total
1951
–
1960
62.8
68.8
6.0
0.67
10.7
401.8
t/ha
applied
in
total
”
Sludge
compost
1942
–
1951
31.1
53.6
22.5
2.50
80.5
311.4
t/ha
applied
in
total
1951
–
1960
53.6
63.2
9.6
1.07
20.0
401.8
t/ha
applied
in
total
”
FYM
(double
rate)
1942
–
1951
31.1
60.7
29.6
3.29
106.0
622.7
t/ha
applied
in
total
1951
–
1960
60.7
72.8
12.1
1.35
22.2
803.5
t/ha
applied
in
total
”
1960
–
1967
72.8
77.5
4.7
4.3
0.66
0.614
9.1
351.5
t/ha
applied
in
total
”
Vegatable
compost
(double
rate)
1942
–
1951
31.1
60.3
29.2
3.25
104.7
622.7
t/ha
applied
in
total
1951
–
1960
60.3
71.4
11.1
1.23
20.4
803.5
t/ha
applied
in
total
”
1960
–
1967
71.4
80.2
8.8
2.70
1.28
0.385
17.9
351.5
t/ha
applied
in
total
”
Sewage
sludge
(double
rate)
1942
–
1951
31.1
81.4
50.3
5.59
180.1
622.7
t/ha
applied
in
total
1951
–
1960
81.4
104.4
23.0
2.54
31.2
803.5
t/ha
applied
in
total
”
Sludge
compost
(double
rate)
1942
–
1951
31.1
67.8
36.7
4.09
131.5
622.7
t/ha
applied
in
total
1951
–
1960
67.8
84.8
17.0
1.90
28.1
803.5
t/ha
applied
in
total
a
Unless
stated
organic
amendments
were
applied
in
autumn.
b
Unless
stated,
soils
were
sampled
in
autumn,
before
any
organic
amendments
were
applied.
c
See
Table
S3
for
soil
weights
and
%
O
C
determined
by
Walkley-Black
(corrected),
Tinsley
or
combustion.
d
SE
of
the
differences
between
replicates
where
applicable.
e
Commercially
available
compost
supplying
c.
24
t
D
M
h
a
1
year
1
and
300
kg
N
h
a
1
year
1
.
POULTON
ET AL
.
|
9
Page 11
to 32 t C/ha; Table 5), and the SOC content was nearer its equilib-
rium value in the soils with organic amendments. The effect of N
fertilizer on SOC stock is discussed later.
In the Woburn Organic Manuring Experiment, the rate of SOC
increase in the first 10 years of compost application (at 40 t/ha
annually) was slightly lower than in the Market Garden Experiment
(36
&
per year), possibly because of the higher initial SOC content
Table 5).
3.4
|
Effects of nitrogen fertilizer additions on SOC
stocks
Increases in SOC following an increase in the annual mineral fertil-
izer N application are seen in the Broadbalk Wheat Experiment
(Tables 6 and S4) although, due to lack of replication, we cannot
ascertain whether these changes are statistically significant. In
1968, a plot that had received 48 kg N/ha since 1852 started to
receive 192 kg N/ha, and during the next 20 years, SOC stock
under continuous winter wheat increased at an average rate of 5
&
per year, and during the following 13 years it continued to increase
at a similar average rate (Table 6). The N rate change from 1968
coincided with a change of crop cultivar to a short straw variety
that gave considerably greater grain yields at all N rates, so the
effects of the change in variety and increase in N rate cannot be
unequivocally separated. However, total above-ground dry matter
production was almost equal between the new and old varieties at
a given N rate (Austin, Ford, Morgan, & Yeoman, 1993) so it seems
likely that organic C inputs to soil in roots and stubble were little
changed due to the change in crop variety per se. It is therefore
likely that the observed increase in SOC is associated with the
increase in N fertilizer application. This could be caused by
increased organic C input to soil resulting from increased crop
growth or an increase in %N in crop residues caused by the
increased N application facilitating increased C retention in soil, or
a combination of these mechanisms.
In 1985, two other treatments had their long-term annual N
application rates increased: one from 144 to 240 kg N/ha and one
from 96 to 288 kg N/ha. The average rates of SOC increase mea-
sured over the next 13 years were 9
&
and 14
&
per year respec-
tively.
As discussed in the previous section, increasing annual fertilizer
N in the Woburn Market Garden Experiment, on a sandy soil, led to
an average rate of increase in SOC stock of 25
&
per year over
9 years in the NPK treatment (Table 5). Although when expressed as
&
per year this rate appears greater than in the Broadbalk examples,
the absolute increase in the Woburn soil of 5.2 t C/ha is in the same
range as in Broadbalk (2.2
–
5.6 t C/ha over the first 13
–
20 years;
Table 6).
It is important to note that, in the situations described above,
crop growth was not constrained by lack of other nutrients, inappro-
priate soil pH, or any other soil factors. Applying N fertilizer alone
would not be expected to increase SOC if such conditions were not
met.
3.5
|
Effects of straw incorporation, green manures
or cover crops
Straw incorporation, at an average annual rate of 3 t/ha, was started
in one section of the Broadbalk Experiment in 1987 on plots that
initially had only 1.13% C. During the first 12 years, SOC increased
significantly, at a rate of 8
&
per year, averaged over plots receiving
96
–
192 kg N/ha (Table 6).
In the Woburn Green Manuring Experiment, one treatment
received straw at 3.77 t ha
1
year
1
every 2 years (Table 6). This
increased SOC by an average of 6
&
per year over the next
18 years. In the Woburn Organic Manuring Experiment, straw was
incorporated at different times and effects were compared with
treatments receiving only inorganic fertilizers (shown in Table 3). In
plots receiving only inorganic fertilizers, there were small decreases
in SOC. Straw application led to variable increases ranging from zero
to 24
&
per year measured over 6 or 10 years but, due to the vari-
ability, only the increase in SOC with largest rate of applied straw
was statistically significant (Table 6).
In 1987, experiments comprising three rates of straw incorpora-
tion were started at both Rothamsted (18%
–
27% clay) and Woburn
(14% clay). Where the amount of straw applied was equal to that
produced, the average rates of SOC increase measured over
22 years were small and not significantly different from zero
(Table 7). Where the rates of straw applied were two or four times
the amount produced the increases were larger (5
&
–
10
&
per year)
but statistically only the four-times rate increased SOC significantly
compared to no straw.
Three experiments, all on sandy soil with an initial SOC concen-
tration of
<
1%, provide limited sets of data on the impact of green
manures or cover crops on SOC (Tables 6 and S2). In one case,
green manures led to a small increase (7
&
per year over 18 years)
and in another a combination of green manures and grass leys within
a mainly arable rotation increased the rate to 29
&
per year, mea-
sured over 6 years and probably just statistically significant. In
another treatment, inclusion of overwinter cover crops in 4 years
out of 10 failed to increase SOC (Table 6).
3.6
|
Changing from continuous arable cropping to
arable/ley rotations
The Woburn Ley-arable experiment (Johnston et al., 2017), started
in 1938, compares the effects of 3-year arable cropping and 3-year
or 8-year leys on the yields of two subsequent arable crops. Soils on
which 3-year lucerne leys were grown until 1970 showed only a
very modest increase in SOC in the 0
–
23 cm soil layer over
30 years. This has been attributed to the wide-spaced rows of
lucerne with its main tap root rather than a dense mass of shallower
roots, typical of grasslands (Johnston et al., 2017). When the lucerne
ley was replaced by a grass/clover mixture, SOC increased by at
least 4
&
per year over the next 35 years (Table 8). Where the 3-
year ley was grazed by sheep as opposed to being cut and the her-
bage removed, the rate of SOC increase was greater: 9
&
per year
10
|
POULTON
ET AL
.
Page 12
TAB
L
E
6
Changes
in
amoun
t
o
f
organ
ic
C
from
inc
reasing
the
am
ount
o
f
fertilizer
N
app
lied,
add
ing
straw
or
growi
ng
green
ma
nures
or
cover
crops
Experiment
and
number
Crop
Treatment
a
Sampling
period
b
Amount
of
organic
C
c
in
measured
depth
Rate
of
increase
(t
C
h
a
1
year
1
)
Annual
increase
(
&
/yr)
Notes
At
start
(t/ha)
At
end
(t/ha)
Difference
d
(t/ha)
Broadbalk
(1)
W.
wheat
Sects
1
&
9
N
applied
increased
from
48
to
192
kg/ha
in
1968
1967
–
1987
e
28.7
31.2
2.5
0.13
4.5
Also,
a
change
from
long
to
short
straw
cultivars
in
1967
1987
–
2000
31.2
33.4
2.2
0.17
5.3
”
N
applied
increased
from
144
to
240
kg/ha
in
1985
1987
–
2000
29.5
33.1
3.6
0.28
9.4
”
N
applied
increased
from
96
to
288
kg/ha
in
1985
1987
–
2000
30.1
35.7
5.6
0.43
14.4
Broadbalk
(1)
W.
wheat
Sect
0
Wheat
straw
incorporated
since
autumn
1986
f
1988
–
2000
32.5
35.7
3.2
1.30
0.26
0.108
8.1
c.
36
t/ha
straw
applied
in
total
Woburn
Green
Manuring
(12)
Various
crops
Barley
straw
incorporated
in
alternate
years
1936
–
1954
29.2
32.3
3.1
0.17
5.8
3.77
t/ha
every
2
years
Woburn
Organic
Manuring
(11)
Various
arable
crops
Wheat
straw
incorporated
autumn
1965
–
1970
1965
–
1971
g
28.5
32.5
4.0
0.80
0.69
0.133
24.1
45
t/ha
straw
applied
in
total
”
Wheat
straw
incorporated
autumn
1980
–
1985
1980
–
1986
28.2
29.6
1.4
1.28
0.23
0.213
8.1
45
t/ha
straw
applied
in
total
”
Wheat
straw
incorporated
since
autumn
2002
2002
–
2012
31.4
31.5
0.1
0.45
0.03
0.045
1.1
75
t/ha
straw
applied
in
total
Woburn
Green
Manuring
(12)
Various
crops
Green
manures
1936
–
1954
29.2
32.6
3.4
0.19
6.5
Spring
sown
lupins
or
rape
or
undersown
clover
or
ryegrass
Woburn
Organic
Manuring
(11)
Various
arable
crops
Green
manures
1965
–
1971
g
26.2
30.4
4.2
1.62
0.75
0.269
28.5
Ryegrass
in
1964
and
1965;
undersown
trefoil
in
1966
and
red
clover
in
1968
and
1971
”
Cover
crops
2002
–
2012
30.3
24.8
5.5
1.06
0.55
0.106
18.2
Overwinter
cover
crops
in
2003/2004,
2006/2007,
2008/2009
and
2011/2012
a
Unless
stated,
organic
amendments
were
applied
in
autumn;
fertilizer
N
was
applied
in
spring.
b
Unless
stated,
soils
were
sampled
in
autumn,
before
any
organic
amendments
were
applied.
c
See
Table
S4
for
soil
weights
and
%
O
C
determined
by
Walkley-Black
(corrected),
Tinsley
or
combustion.
d
SE
of
the
differences
between
replicates
where
applicable.
e
Sampled
in
autumn
1966;
there
is
assumed
to
have
been
no
change
between
autumn
1966
and
autumn
1967.
f
Mean
of
plots
receiving
96,
144
and
192
kg
N/ha
+
P,
K
and
Mg.
g
Sampled
in
spring
1964;
there
is
assumed
to
have
been
no
change
between
spring
1964
and
autumn
1965.
See
Table
3
for
effects
on
control
treatment
receivi
ng
NPKMg.
POULTON
ET AL
.
|
11
Page 13
during 31 years (Table 8). But when the grazed grass/clover ley was
changed to a grass ley given a small N fertilizer addition there was
no further increase in SOC during the subsequent 35 years. Two
cycles of 8-year grass or grass/clover leys, followed by 2-year arable
cropping, were started in the 1970s. SOC increased at rates of 7
&
–
9
&
per year over the next 35 years (Table 8).
Long leys (6 or 8 years) were also grown in the Woburn Organic
Manuring experiment (Mattingly et al., 1974) where SOC increased
significantly; increases ranged from 14
&
to 34
&
per year, measured
at the end of the 6- or 8-year period in the ley (Table 9). As noted
for organic additions, SOC increases tended to be greater when the
initial starting value was lower.
Ley-arable experiments were started on two sites on silty clay
loam soil at Rothamsted in 1949 (Johnston, 1973). One site, Fosters,
had been in long-term arable cropping and had an initial SOC con-
tent of about 1.5%. In this experiment, some plots were put into
permanent grass, some into a rotation of 3-year leys followed by 3-
years of arable crops and some plots continued to grow arable crops
each year. Rotations that included a 3-year grass
+
N ley or a 3-year
lucerne ley followed by 3 years of arable crops caused little increase
in SOC compared to continuous arable (Table 9). Rotations that ini-
tially included grazed leys, but were replaced by cut grass/clover leys
in the early 1960s, gave small increases in SOC (2
&
–
7
&
per year;
Table 9). Soil subject to land use change from arable to permanent
grass in 1949 gave an annual SOC increase of 14
&
per year over
the following 37 years which then declined to 3
&
per year over the
next 20 years (Table 9). The second Ley-arable experiment at
Rothamsted on Highfield was established on a site which had been
in long-term grass since 1838 (Lawes & Gilbert, 1885). The topsoil
on this site contained about 2.75% SOC in 1949. All plots ploughed
out of permanent grass lost SOC during the next 39 years whether
in permanent arable or ley-arable cropping (Table 9). Rotations which
included 3-year leys declined at a similar rate to soils in continuous
arable rotations (Johnston et al., 2009).
3.7
|
Land use change from arable cropping to
woodland
Figure 3 (summarizing data from Poulton, Pye, Hargreaves, & Jenkin-
son, 2003) shows SOC changes at two sites at Rothamsted following
a major land use change from long-term arable cropping to wood-
land through natural reversion. At one site, Broadbalk Wilderness,
the soil initially contained calcium carbonate and remained above pH
7 for the entire period of over 110 years. In total this site accumu-
lated 56% more SOC than the other site, Geescroft Wilderness, in
which soil pH declined from 7.1 at the start of reversion in 1883 to
4.4 in 1999. During the first 20 years after reversion to natural veg-
etation, Geescroft and Broadbalk accumulated SOC at 15
&
and
19
&
per year respectively, declining to 10
&
and 5
&
per year in the
final 30-year measurement period. Besides the increase in SOC,
much carbon was also accumulated in the above-ground vegetation
(Poulton et al., 2003). On these two woodland sites SOC had not
reached a new equilibrium, even after
>
100 years (Figure 3). By con-
trast, when there was a change from old arable land to permanent
T A B L E 7
Changes in organic C from adding various amounts of straw. Rothamsted and Woburn, Amounts of Straw experiments
a
Site, experiment
and number
Treatment
b
Treatment
period
Organic C
c
Rate of
increase
e
(t C ha
1
year
1
)
Annual
increase
(
&
/yr)
Notes
At end
(% C)
At end
d
(t/ha)
Rothamsted
Amounts
of straw (5)
No straw
1987
–
2008
1.86
53.2
—
—
No straw added
Wheat straw incorporated since
autumn 1986
1987
–
2008
1.96
56.1
0.13
2.4
100 t/ha straw applied
Wheat straw
9
2 incorporated
since autumn 1986
1987
–
2008
2.07
59.2
0.27
5.1
201 t/ha straw applied
Wheat straw
9
4 incorporated
since autumn 1986
1987
–
2008
2.14
61.2
0.36
6.8
402 t/ha straw applied
SED
0.078 (differences in % C significant at
p
=
.05)
Woburn
Amounts
of straw (10)
No straw
1987
–
2008
1.00
34.4
—
—
No straw added
Wheat straw incorporated
since autumn 1986
1987
–
2008
1.10
37.8
0.16
4.5
87 t/ha straw applied
Wheat straw
9
2 incorporated
since autumn 1986
1987
–
2008
1.21
41.6
0.33
9.5
175 t/ha straw applied
Wheat straw
9
4 incorporated
since autumn 1986
1987
–
2008
1.22
42.0
0.34
10.0
340 t/ha straw applied
SED
0.091 (differences in % C not significant)
a
Adapted from Table 2, Powlson, Glendining, et al. (2011).
b
Straw was applied in autumn after soils were sampled.
c
% OC determined by combustion.
d
Using soil weights of 2,860 and 3,440 t/ha for Rothamsted and Woburn respectively.
e
Compared to treatment without added straw.
12
|
POULTON
ET AL
.
Page 14
grassland, SOC had reached an equilibrium after about 100 years
(Johnston et al., 2009).
4
|
D I S C U S S I O N
4.1
|
General considerations
Any increases in SOC will almost certainly improve soil functioning
and quality. There is considerable evidence that even small increases
can have disproportionately large and beneficial effects on soil bio-
logical activities and physical properties such as water infiltration,
aggregate stability and ease of tillage (Haynes, Swift, & Stephen,
1991; Snyder & Vazquez, 2005; Blair, Faulkner, Till, & Poulton,
2006; Thierfelder & Wall, 2012; Thierfelder, Cheesman, & Rusi-
namhodzi, 2013; Verhulst et al., 2010; Watts, Clark, Poulton, Powl-
son, & Whitmore, 2006). For example, even where zero tillage
increases SOC
concentration
in near-surface soil (e.g. 0
–
10 cm), but
with minimal impact on SOC
stock
to a greater depth, various soil
physical properties can be considerably improved (Powlson, Stirling,
Thierfelder, White, & Jat, 2016; Thierfelder & Wall, 2012). As
pointed out by Janzen (2015), it is the process of decomposition of
organic matter entering soil that delivers improvements in soil struc-
ture and functioning and the release of nutrients to crops: it is not
necessary that a large increase in SOC stock is attained.
Where large amounts of FYM have been applied over many
years much of the FYM-C has been lost even though the increase in
SOC has been large. For example, in the silty clay loam soil on
Broadbalk only 11% of the 480 t organic carbon added in FYM since
1843 has been retained in the soil to a depth of 46cm (Table 10),
yet the soil properties are drastically altered compared to that with
no manure addition. In the treatment where FYM had only been
applied since 1968, SOC was still far from reaching a new equilib-
rium level and a much larger proportion (35%) of the added C could
be found. In the sandy loam at Woburn, modelling with RothC-26.3,
showed that 5%, at most, of the C added in plant residues over a
70-year period is still present in the soil (Johnston et al., 2017) but,
again, this caused measurable changes in soil properties. However,
improvements in soil physical properties caused by increased SOC
do not necessarily translate into consistently increased crop yields
(Hijbeek et al., 2017).
In contrast to the impacts of increasing SOC on soil properties and
quality, the requirements for SOC increases for mitigating climate
change through soil carbon sequestration are more stringent. First, it is
necessary that SOC
stock
(i.e. the quantity of organic C in soil) is
increased, not just the
concentration
of C in the surface layer. Second,
it is essential that the additional C sequestered in soil would otherwise
have been in atmospheric CO
2
and is not simply being transferred
from one terrestrial location to another. Other well-known caveats
must also be observed including the fact that the rate of SOC increase
slows as the new equilibrium value is approached (Johnston et al.,
2009; Powlson et al., 2012; Smith, 2014) and that increases are
reversed if the modified management practice is not continued
T A B L E 8
Changes in amount of organic C in continuous arable or ley-arable rotations. Woburn Ley-arable experiment (14)
a
Crop
rotation
b
Treatment
Sampling period
No. of
years
Amount of organic C
c
Rate of
increase
(t C ha
1
year
1
)
Annual
increase
(
&
/yr)
Notes
At start
(t/ha)
At end
d
(t/ha)
Ar
Arable with root crops
1938
–
1965/74
30.5
36.95
35.25
0.06
1.5
Arable after
long-term arable
AF
Arable with 2-year fallows
1965/74
–
2000/09
35
35.25
30.91
0.12
3.5
Ah
Arable with 1-year hay
1938
–
1965/74
30.5
36.95
38.64
0.06
1.5
”
AB
Arable
1965/74
–
2000/09
35
38.64
35.63
0.09
2.2
Lu3
3-year lucerne ley
+
2-year arable
1938
–
1965/74
30.5
36.95
40.34
0.11
3.0
Ley-arable rotations
after long-term
arable
LC3
3-year grass/clover ley
+
2-year arable
1965/74
–
2000/09
35
40.34
46.24
0.17
4.2
L3
3-year grazed ley
+
2-year arable
1938
–
1965/74
30.5
36.95
47.18
0.34
9.1
”
LN3
3-year grass ley with N
+
2-year arable
1965/74
–
2000/09
35
44.24
44.76
0.01
0.3
LC8
8-year grass/clover ley
+
2-year arable
1965/74
–
2000/09
35
41.28
51.69
0.30
7.2
”
LN8
8-year grass ley with N
+
2-year arable
1965/74
–
2000/09
35
39.77
52.61
0.37
9.2
”
Rotation X year group
F
-ratio
(10 302)
=
35.40,
p
<
0.001
SED for all amounts of organic C
=
1.84
a
Adapted from Tables 2 and 4, Johnston et al. (2017). Data are the mean of plots without FYM and with FYM once every 5th year from 1938 to the
mid-1960s.
b
Ar: 5-year arable crops including root crops; becomes AF: 5-year arable including 2-year bare fallow. Ah: 5-year arable including 1-year hay; becomes
AB: 5-year arable. Lu3: 3-year lucerne ley followed by 2-year arable; becomes LC3: 3-year grass/clover ley followed by 2-year arable. L3: 3-year grazed
grass/clover ley followed by 2-year arable; becomes LN3: 3-year grass
+
N ley followed by 2-year arable. LC8: 8-year grass/clover ley followed by 2-year
arable. LN8: 8-year grass
+
N ley followed by 2-year arable.
c
% OC determined by Tinsley or combustion.
d
Includes any additional C present in the soil to an
“
equivalent
”
depth; i.e. so that the same mass of mineral soil was being considered at both the start
and the end of each period.
POULTON
ET AL
.
|
13
Page 15
TAB
L
E
9
Changes
in
organ
ic
C
a
s
a
re
sult
of
a
cha
nge
from
long
-term
arab
le
or
grasslan
d
to
ley
-arable
rota
tions,
perman
ent
pasture
o
r
conti
nuous
arab
le
Experiment
and
number
Crop
Treatment
Sampling
period
a
Amount
of
organic
C
b
in
measured
depth
Org
C
in
extra
soil
c
(t/ha)
Total
at
end
with
extra
organic
C
(t/ha)
Difference
in
organic
C
a
t
start
and
end
(t/ha)
Rate
of
increase
(t
C
h
a
1
year
1
)
Annual
increase
(
&
/ha)
Notes
At
start
(t/ha)
At
end
(t/ha)
Woburn
Organic
Manuring
(11)
Ley
8-yr
grass
ley
with
N
1965
–
1971
d
28.0
30.0
2.4
32.4
4.4
0.96
0.75
0.160
27.0
Ley
after
long-term
arable
6-yr
grass/clover
ley
(previously
grass
ley
with
N)
1980
–
1986
29.8
29.8
2.6
32.6
2.6
0.47
0.43
0.078
14.5
Ley
after
9-year
arable
8-yr
grass/clover
ley
1965
–
1971
d
26.3
29.4
2.3
31.7
5.4
0.53
0.9
0.089
34.4
Ley
after
long-term
arable
6-yr
grass/clover
ley
(previously
grass/clover
ley)
1980
–
1986
28.7
29.1
2.5
31.6
2.8
0.60
0.47
0.100
16.5
Ley
after
9-year
arable
Grass/clover
Grass/clover
since
2003
2002
–
2012
31.3
35.0
2.7
37.7
6.4
2.12
0.64
0.212
20.6
Grass/clover
after
ley-arable
crops
Fosters
Ley-arable
(3)
Arable
cropping
NPKMg
1950
–
1987
41.0
37.8
0.0
37.8
3.2
0.09
2.1
Arable
after
1987
–
2008
37.8
37.9
0.0
37.9
0.1
0.01
0.2
long-term
arable
Ley-arable
cropping
3-yr
lucerne
ley
+
3-yr
arable
rotation
since
1949
1950
–
1987
41.0
42.4
0.0
42.4
1.4
0.04
0.9
Ley-arable
cropping
after
long-term
arable
3-yr
grass
ley
with
N
+
3-yr
arable
rotation
since
1949
1950
–
1987
41.0
43.5
0.0
43.5
2.5
0.07
1.6
”
3-yr
grazed
grass
ley
+
3-yr
arable
from
1949
to
1961/1962
then
3-yr
grass/clover
ley
+
3-yr
arable
since
1962/63
1950
–
1987
41.0
43.7
0.0
43.7
2.7
0.07
1.8
”
1987
–
2008
43.7
50.2
0.0
50.2
6.5
0.31
7.1
”
Grass
Grass
with
N
since
1949
1950
–
1987
41.0
60.2
2.1
62.3
21.3
0.57
14.0
Permanent
grass
with
N
after
long-term
arable
1987
–
2008
60.2
63.5
0.0
63.5
3.3
0.16
2.6
(Continues)
14
|
POULTON
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Page 16
TAB
L
E
9
(Contin
ued)
Experiment
and
number
Crop
Treatment
Sampling
period
a
Amount
of
organic
C
b
in
measured
depth
Org
C
in
extra
soil
c
(t/ha)
Total
at
end
with
extra
organic
C
(t/ha)
Difference
in
organic
C
a
t
start
and
end
(t/ha)
Rate
of
increase
(t
C
h
a
1
year
1
)
Annual
increase
(
&
/ha)
Notes
At
start
(t/ha)
At
end
(t/ha)
Highfield
Ley-arable
(4)
Arable
cropping
NPKMg
1948
–
1987
61.3
39.6
5.4
34.2
27.1
0.69
11.3
Arable
after
grass
since
1838
1987
–
2008
39.6
37.9
1.7
36.1
3.5
0.16
4.2
Ley-arable
cropping
3-year
lucerne
ley
+
3-year
arable
rotation
since
1949
1948
–
1987
61.3
45.0
8.4
36.6
24.7
0.63
10.3
Ley-arable
cropping
after
grass
since
1838
3-year
grass
ley
with
N
+
3-year
arable
rotation
since
1949
1948
–
1987
61.3
45.9
8.4
37.5
23.8
0.61
9.9
”
3-year
grazed
grass
ley
+
3-year
arable
from
1949
to
1961/1962
then
3-year
grass/clover
ley
+
3-year
arable
since
1962/1963
1948
–
1987
61.3
45.7
8.4
37.3
24.0
0.61
10.0
”
1987
–
2008
45.7
49.4
0.0
49.4
3.7
0.18
3.9
”
Grass
Improved
grass
with
N
since
1949
1948
–
1987
61.3
66.5
0.0
66.5
5.2
0.13
2.2
Permanent
grass
with
N
after
grass
since
1838
1987
–
2008
66.5
73.2
0.0
73.2
6.7
0.32
4.8
a
Unless
stated,
soils
were
sampled
in
autumn.
b
See
Table
S5
for
soil
weights
and
%
O
C
determined
by
Walkley-Black
(corrected),
Tinsley
or
combustion.
c
Where
soil
weight
in
the
sampled
depth
has
decreased,
soils
should
have
been
sampled
deeper
so
that
the
same
mass
of
mineral
soil
was
being
considered.
W
here
the
soil
weight
has
increased
the
soil
should
have
been
sampled
less
deeply;
see
text
for
further
explanation.
d
Sampled
in
spring
1964;
there
is
assumed
to
have
been
no
change
between
spring
1964
and
autumn
1965.See
Table
3
for
effects
on
control
treatment
receivi
ng
NPKMg.
POULTON
ET AL
.
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15
Page 17
indefinitely (Powlson, Glendining, et al. 2011; Powlson, Whitmore, &
Goulding, 2011; Mackey et al., 2013).
In considering the climate change mitigation potential of any
change in land management practice, it is the overall impact on all
greenhouse gas fluxes that must be assessed, not only changes in
SOC (Smith et al., 2008). Some practices leading to C sequestration
may increase emissions of trace greenhouse gases, especially N
2
O.
For example, a recent global meta-analysis of experiments with
manure showed that, on average, manure addition increased N
2
O
emission by 33% compared to inorganic N fertilizer and this could
largely offset the benefit of increased SOC stock (Zhou et al., 2017).
However, the trend was smaller with FYM than with manures con-
taining a larger proportion of readily mineralizable N such as poultry
manure.
Of the 114 treatment comparisons within the long-term experi-
ments reported in this paper, almost two-thirds showed SOC stock
increases
>
7
&
per year (or 5
&
in soils with larger initial SOC con-
tent); these increases, mostly measured in the 0
–
23 cm soil layer,
being roughly equivalent to 4
&
goal specified for the 0
–
40 cm layer
in the
“
4 per 1000
”
initiative. The increases were predominantly
from organic inputs (manure, compost or straw) or from inclusion of
pasture leys instead of continuous arable cropping. However, in eval-
uating the practicality of the
“
4 per 1000
”
initiative, considerable
caution is required in transferring results from experiments such as
these, and those reported by Minasny et al. (2017), to real world sit-
uations. Experimental results need to be evaluated in the light of the
following practical and logistical considerations:
1.
Is the practice suitable for a wide range of soil types and envi-
ronmental conditions and possible for farmers to adopt in practi-
cal situations?
2.
Is the practice profitable for a farmer? If the answer is
“
not under
current conditions
”
but the practice is highly beneficial, either for
climate change mitigation or long-term soil quality improvement
or food security, there could be an argument for changes in pol-
icy or financial arrangements to promote uptake of the practice.
3.
What are the implications for global food security if a practice is
widely adopted?
4.
Is the practice already widely used in the regions concerned, thus
giving limited opportunities for further adoption?
Below, we address the land management practices considered in
this paper from the viewpoint of these considerations.
0
10
20
30
40
50
60
70
80
90
1880
1920
1960
2000
a
/ht
,
C
ci
na
gr
O
Year
F I G U R E 3
Accumulation of soil organic carbon (SOC), t/ha, in
arable soils that were allowed to revert to woodland from the
1880s. The amount of SOC at the end of each sampling period has
been corrected for the change in soil bulk density. Broadbalk
Wilderness, (
■
); Geescroft Wilderness, (
♦
). Rothamsted
T A B L E 1 0
The retention of FYM-derived C in the soil
Experiment and
number
Treatment
Years applied
Soil
sampled
FYM-C
applied (t/ha)
Additional C in soil
compared to controls
a
FYM-derived C recovered
compared to controls
a
In topsoil
0
–
23 cm (t/ha)
In subsoil
23
–
46 cm (t/ha)
In topsoil
0
–
23 cm (%)
In subsoil
23
–
46 cm (%)
Broadbalk (1)
FYM 1843
Since 1843
b
2000
477
39.8
13.2
8
3
FYM 1885
Since 1885
b
2000
341
33.8
12.5
10
4
FYM 1968
1968
–
2000
2000
106
28.1
8.7
27
8
Hoosfield (2)
FYM 1852
Since 1852
1998
470
55.8
17.7
12
4
FYM residues
1852
–
71
1998
64
10.5
0.6
16
1
Exhaustion
Land (9)
FYM
1876
–
1901
1903
83
18.9
—
23
—
FYM residues
1876
–
1901
1974
83
6.7
—
8
—
a
For Broadbalk and Hoosfield, control treatments received N, P, K, Mg fertilizers; for the Exhaustion Land the control is the mean of plots receiving
either no fertilizers or N, P, K, Mg fertilizers.
b
Between 1926 and 1967, sections of the experiment were fallowed each year to control weeds. The two sections in continuous wheat being consid-
ered here (sampled in 2000) were fallowed nine times in this period (eight times on section 9; 10 times on Section 1); FYM was not applied in these
years.
16
|
POULTON
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4.2
|
Removal of land from agriculture
Removing land from arable cropping, and allowing natural regenera-
tion to deciduous woodland, led to large accumulations of organic C
in soil in addition to that in trees; SOC stock doubled or trebled in a
little over a century (Figure 3 and Poulton et al., 2003). Initially the
rates of increase in these woodland reversion sites were very large
and still exceeded 4
&
per year (in the 0
–
23 cm soil layer) during the
final 30 years of measurement. Similarly, conversion from arable to
permanent grass caused an increase of 55% in SOC in 58 years
(Table 9). Pi
~
neiro, Jobb
agy, Baker, Murray, and Jackson (2009) also
reported substantial increases in SOC at 142 sites in the United
States where agricultural land was set aside. There are obviously
severe limitations to the area of land that can be removed from agri-
culture globally if food security goals are to be met. But, in limited
situations where soils are either of low productivity or are fragile
and prone to erosion, conversion from agriculture to forest or grass-
land may be a logical strategy (Albanito et al., 2016; Smith et al.,
2013). A good example is the
“
grain for green
”
programme in China
that reduced soil erosion and led to considerable increases in SOC
(Chadwick et al., 2015; Song, Peng, Zhou, Jiang, & Wang, 2014).
4.3
|
Addition of manures and other organic
materials
The annual application of FYM at 35 t fresh material per hectare on
Broadbalk since 1843 and Hoosfield since 1852 led to a high rate of
SOC increase for several decades (Figures 1 and 2; Table 2) but such
increases are unlikely to be achieved in practical farming situations.
Few farmers would have such large quantities of animal manure
available each year for all fields on their farm and, even if they did,
they may be prevented from applying such amounts continuously
because of the risk of nitrate and phosphate pollution (Goulding,
Poulton, Webster, & Howe, 2000; Hesketh & Brookes, 2000) and/or
government legislation (e.g. www.gov.uk/guidance/nutrient-manage
ment-nitrate-vulnerable-zones). However, on the positive side, even
the more practically relevant application regimes (lower rates and/or
applied every second or fourth year) in the Saxmundham and
Woburn experiments led to rates of increase in SOC of 8
&
–
14
&
per year even when averaged over 20
–
70 years (Table 3). Thus,
manure applications are a very effective way of increasing SOC,
especially if used on soils with low initial SOC content where the
potential for increase is greatest; the reduced rate of increase as
SOC stock increased with time was very clear in all these experi-
ments. This has important implications for farming practice: from the
viewpoint of SOC management (and recycling of nutrients) it would
be desirable to have a mixed landscape comprising farms with grass
and arable fields in close proximity, as was the case some decades
ago in northern Europe. Such mixed farming facilitates the applica-
tion of manures to arable fields which tend to have a lower SOC
content, and so benefit the most from organic additions, but is con-
trary to the current tendency in many countries for specialization,
with animal and arable enterprises being spatially separated.
Specialization is favoured by a range of practical (soil type and cli-
mate) and economic factors. If this trend is to be reversed to achieve
environmental benefits, including increased SOC stocks, it is likely
that significant changes in policies and financial incentives will be
required. For example, if policies facilitated alterations in farm struc-
tures such that the estimated 300 Mt of solid manure produced in
the European Union (Foged et al., 2011) were distributed more
evenly this would be beneficial for soil quality.
While manure applications are very effective at increasing SOC
and improving a wide range of soil functions, it should be recognized
that these increases will generally not be delivering climate change
mitigation but are rather a transfer of C from one location to
another (Schlesinger, 2000). Globally, virtually all manure is currently
being applied to soil at some location, though often in a suboptimal
way. So, while there is certainly scope to make more rational and
efficient use of manures both for soil C enhancement and for nutri-
ent supply, almost all manure is already being used to some extent.
Thus, it is incorrect to assume that all SOC increases observed in
experiments on manure application can be transferred to practical
situations and fully treated as climate change mitigation: at least part
of the benefit will already be accruing (Powlson et al., 2012; Powl-
son, Glendining, et al., 2011; Powlson, Whitmore, et al., 2011).
Where manure is being used inefficiently, or is applied to soil with
an already high SOC stock, there is an opportunity to manage it dif-
ferently by applying instead to low-SOC soils with potential for
some degree of climate change mitigation plus numerous other ben-
efits for soil quality, nutrient supply and decreased water pollution
(Chadwick et al., 2015). Although, of course, there are major practi-
cal barriers to transporting manure, even over moderate distances.
The different organic amendments tested in experiments on a
sandy loam soil (sewage sludge, now commonly called biosolids, and
various composts) all led to high rates of SOC accumulation
(Table 5). As with most of the FYM treatments, the application rates
were large so the absolute rates of SOC increase cannot be directly
transferred to practical farming but there is a strong indication that,
for a given application rate, they deliver larger increases in SOC than
FYM and the effect is longer lasting. This is presumably because
these materials have already undergone greater decomposition than
FYM during composting or sewage treatment so the organic C
applied to soil will be somewhat more recalcitrant (Johnston, 1975).
An important factor regarding these materials is that they represent
organic resources that are not currently widely utilized. In many
countries, a significant proportion of food waste and similar organic
materials is currently disposed of in landfill where decomposition
returns CO
2
or CH
4
to the atmosphere (Bijaya, Barrington, & Marti-
nez, 2006). Their greater use as a soil amendment can contribute to
genuine climate change mitigation in addition to soil improvement
(Powlson et al., 2012). However, it is recognized that some of these
materials may have alternative uses that mitigate climate change in
other ways, such as feedstock for anaerobic digestion to generate
biogas as a substitute for fossil fuel. For example, about 8% of man-
ure in the EU is currently processed for such purposes (Foged et al.,
2011). To some extent these two uses can be combined if the
POULTON
ET AL
.
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17
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digestate residue is applied to soil as a source of C and crop nutri-
ents. Similarly, some organic wastes such as poultry litter are cur-
rently incinerated to generate electricity or heat, the residue being
used as source of crop nutrients.
4.4
|
Retention of crop residues
Straw additions had variable, though generally positive, impacts on
SOC stocks (Tables 6 and 7). Irrespective of soil type (silty clay loam at
Rothamsted or sandy loam at Woburn), any increases tended to be
greater where there was less SOC at the start of the experiment. Else-
where, in temperate climatic regions, straw addition has also given
positive but generally small increases in total SOC stock (Powlson
et al., 2012; Powlson, Glendining, et al., 2011; Powlson, Whitmore,
et al., 2011; van Groenigen et al., 2011). For example, in the 25 straw
incorporation experiments of 6- to 56-year duration reviewed by
Powlson, Glendining, et al. (2011) and Powlson, Whitmore, et al.
(2011), the increase in SOC was only statistically significant in six
cases. However, as mentioned earlier, there is evidence that even
small increases in SOC can have disproportionately large and benefi-
cial effects on soil physical and biological properties (Houot, Molina,
Chaussod, & Clapp, 1989; Malhi & Lemke, 2007; Ketcheson & Beau-
champ, 1978; Thierfelder & Wall, 2012). In many regions, much cereal
straw is already returned to soil (e.g. an estimate of 50% in the United
Kingdom in 2008; Copeland & Turley, 2008) with much of the remain-
der being used for animal bedding and eventually returned to soil as
FYM, so the scope for additional straw return to soil for climate
change mitigation is limited. In smallholder agriculture in tropical
regions the use of crop residues for animal feed or bedding is regarded
as a significant barrier to direct return of crop residues to soil as part
of wider adoption of conservation agriculture (Powlson et al., 2016;
Thierfelder et al., 2013; Giller et al., 2011).
4.5
|
Conversion from continuous arable to ley-
arable cropping
In the experiments considered here this change of management
often led to increases in SOC exceeding 7
&
per year (equivalent to
4
&
per year in the 0
–
40 cm depth), sometimes for several decades
(Tables 8 and 9). However, leys of just 3 years generally had only
small effects. Increases in SOC from growing leys compared to con-
tinuous arable crops is a genuine transfer of additional C from atmo-
sphere to soil due to additional inputs of photosynthate entering the
soil during the pasture phases of the rotation, mainly through the
root mass. The issues when considering the wider adoption of this
system mainly concern profitability at the farm scale. As discussed
above in the context of manure use, many farmers in developed
countries apparently find that the practical and economic benefits of
specialization outweigh the less immediate benefits of improved soil
quality that can be gained from a mixed animal/arable farming sys-
tem. An expansion of mixed systems would require changes in policy
and financial incentives and presupposes that there is a consumer
demand for the animal products derived from the grass phase. Such
a change would only be logical from consideration of climate change
if it was accompanied by a decrease in the number of animals fed
on grain in more intensive animal productions systems and probably
an overall decrease in consumption of meat and dairy products
(Baj
zelj et al., 2014). Such changes face major social and behavioural
barriers and are unlikely to be achieved rapidly.
4.6
|
Addition of N fertilizer
In three examples from the Broadbalk Experiment, increasing the
annual application of N fertilizer (with adequate supplies of other
nutrients) caused increases in SOC
>
4
&
per year in the 0
–
23 cm soil
layer that continued for periods of between 13 and 33 years (Table 6).
This is consistent with earlier studies on Broadbalk (Glendining et al.,
1996) and reviews of numerous experiments worldwide showing
increased SOC in soils receiving N fertilizer compared to unfertilized
or unbalanced fertilizer applications (Ladha, Reddy, Padre, & van Kes-
sel, 2011; Han, Zhang, Wang, Sun, & Huang, 2016). It is presumed that
this is due to increased inputs of organic C into soil from roots, exu-
dates and above-ground crop residues resulting from increased crop
growth. It may also be that increased %N in these residues leads to
greater conservation of organic C in soil organic matter. For regions of
the world where fertilizer applications are currently low or nonexis-
tent, this finding demonstrates a practical opportunity for increasing
SOC with simultaneous benefits for crop production, provided other
aspects of soil fertility, water availability and crop protection are in
place. However, in such regions (e.g. Africa), there are major infrastruc-
ture and economic barriers to overcome to achieve rational fertilizer
use. Where it does occur, it will be driven by food security goals, but
improved soil quality will be a welcome co-benefit. Increases in SOC
derived in this way are unlikely to deliver climate change mitigation
because of the greenhouse gas emissions associated with N fertilizer,
especially direct and indirect N
2
O emissions, though yield-scaled
greenhouse gas emissions (i.e. emission per unit of agricultural pro-
duct) may decrease (Linquist, van Groenigen, Adviento-Borbe, Pit-
telkow, & van Kessel, 2012). In addition, there are large CO
2
emissions
associated with the manufacture of N fertilizer (Schlesinger, 2000), for
example 3
–
8 t CO
2
-equivalent per t N under European manufacturing
conditions (Brentrup & Palli
ere, 2008). In regions where agriculture is
well developed (e.g. Europe, North America), N fertilizer is already
widely used, often at near-optimum rates, so there is little opportunity
to achieve increased SOC stocks through this mechanism. In regions
where N is frequently overused (e.g. China) the overriding aim is to
decrease applications to decrease water pollution and greenhouse gas
emissions (Zhang et al., 2013): it is clearly not acceptable to justify
excessively large N applications on the grounds of increasing SOC.
4.7
|
Practical limitations to achieving SOC
increases of 4 per 1000 compared to initial SOC
stock in agricultural soils
Any policies or initiatives, such as
“
4 per 1000
”
, that lead to
increased SOC and remove CO
2
from the atmosphere are to be
18
|
POULTON
ET AL
.
Page 20
welcomed. Even small increases in SOC are likely to improve soil
quality and functioning for sustainable crop production, ecosystem
services or both. But only in some cases will there be a concurrent
benefit for climate change mitigation. Results from the long-term
experiments reviewed here demonstrate several land management
practices that can increase SOC stocks, often at rates well above the
4
&
per year rate (in the 0
–
40 cm depth). However, as discussed
above, there are many situations where these practices cannot be
widely adopted, either because they are impractical for farmers or
because of wider global issues. The limitations influencing wide-
spread adoption vary between different regions of the world but
mostly fall into one or more of the following categories:
1.
Farmers do not have the necessary resources available, e.g. insuf-
ficient manure due to lack of animals or insufficient crop residues
because they are required for other purposes, e.g. smallholder
farmers in Africa (Thierfelder et al., 2013; Giller et al., 2015).
2.
The practice is already widely used, giving limited scope for
achieving increased SOC accumulation through greater adoption.
An example is the return of crop residues to soil in many situa-
tions in Europe and North America.
3.
Widespread adoption would have a negative impact on global
food security, e.g. converting agricultural land to forest or grass-
land.
4.
The necessary change of management would be uneconomic to
the farmer under current conditions or impractical for some other
reason. To implement such changes would probably require
changes in government policies, regulations or subsidies to pro-
mote the practice.
For these reasons, we conclude that promoting the
“
4 per 1000
”
initiative as a major contribution to climate change mitigation is
unrealistic. Of course, any contribution to mitigating climate change
is beneficial but the reasoning we have set out, and that of Baveye
et al. (2018), strongly indicate that the magnitude is far smaller than
claimed, or implied, by the proposers of the initiative. We suggest
that a more logical rationale for promoting practices that increase
SOC is the urgent need to preserve and improve the functioning of
soils, both for sustainable food security and wider ecosystem ser-
vices. As discussed earlier, there is significant evidence that even
small increases in SOC can have disproportionately large impacts on
a range of soil physical properties so even very partial success in
meeting the
“
4 per 1000
”
target can be highly beneficial in this
respect. We recognize that this is a more nuanced argument with
less obvious political appeal but it is more firmly based on sound evi-
dence. It is also in line with attempting to meet the UN Sustainable
Development Goals (http://www.un.org/sustainabledevelopment/sus
tainable-development-goals). We also emphasize that SOC increases
are not necessarily the key issue when considering the success of
climate change mitigation through land management practices: the
impacts of management changes on emissions of trace greenhouse
gases, especially N
2
O, must be given equal weight (Smith et al.,
2008; Zhou et al., 2017; Tian et al., 2016).
A C K N O W L E D G E M E N T S
Rothamsted Research receives strategic funding from the UK
Biotechnology and Biological Sciences Research Council (BBSRC).
The Rothamsted Long-term Experiments National Capability is sup-
ported by the BBSRC Grant BBS/E/C/000J0300. The authors also
thank the Lawes Agricultural Trust, the managers of the Electronic
Rothamsted Archive, Tony Scott, Steve Freeman and Chris Hall for
their assistance and numerous past and present farm and analytical
staff for conducting the experiments and carrying out the analysis.
O R C I D
David Powlson
http://orcid.org/0000-0001-8776-2339
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Poulton P, Johnston J, Macdonald A,
White R, Powlson D. Major limitations to achieving
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