Prout et al. (2022). Changes in organic carbon to clay ratios in different soils and land uses in England and Wales over time

–

4

. Over millennial time scales, cultivation has caused losses of soil carbon of

approx. 116 Gt C

,

6

, and soils are currently losing carbon in many parts of the world. For example, an analysis

of data in the National Soil Inventory (NSI) of England and Wales found widespread losses of carbon from soils

across both countries during the 1980s and 1990s

, mostly due to past changes in land management but possibly

also linked to warming during that period

,

9

. On the other hand, soils in some regions show gains in carbon under

both managed and natural vegetation

10

–

14

. Gauging realistic targets for SOC sequestration at local to national

scales, and monitoring progress against targets, requires practicable indices which allow for governing factors

and are based on readily measurable soil properties

Soil clay concentration is a key determinant of SOC concentration under given land use and environmental

conditions. This is because SOC is protected from microbial attack by adsorption on clay-mineral surfaces

and by occlusion within soil aggregates, and both of these are functions of clay concentration

15

–

17

. An analysis

of 252 Polish arable soils, 305 French arable soils and 51 French pasture soils showed that a SOC/clay ratio of

1/10 marked the capacity for SOC protection by clay

18

. Soil physical conditions (bulk density, water retention,

clay dispersibility) could be better explained by calculated properties of ‘complexed’ and ‘non-complexed’ SOC

and clay. The 1/10 ratio corresponded to an approximate maximum for correlations between these and the soil

physical properties. This idea was developed further and found, from an analysis of 161 Swiss arable and pasture

soils, that SOC/clay ratios of 1/8, 1/10 and 1/13 marked boundaries between

Very Good

,

Good

,

Moderate

and

Degraded

levels of soil structure, respectively

19

. We have confirmed the value of an index of SOC status based on

OPEN

1

Department of Sustainable Agriculture Sciences, Rothamsted Research, West Common, Harpenden AL5

2JQ, Hertfordshire, UK.

2

School of Water, Energy & Environment, Cranfield University, Cranfield MK43 0AL,

Bedfordshire, UK.

3

Innovative Solutions for Decision Agriculture (iSDA), Rothamsted Campus, West Common,

Harpenden, AL5 2JQ, Hertfordshire, UK.

4

Computational and Analytical Sciences Department, Rothamsted

Research, West Common, Harpenden AL5 2JQ, Hertfordshire, UK.

*

email: g.kirk@cranfield.ac.uk

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these threshold values for 3809 soils in the NSI of England and Wales under arable crops, ley grass, permanent

grass and woodland

. Though highly weathered tropical soils with clay fractions dominated by sesquioxides

may behave differently

21

, the index appears to work very successfully for a wide range of temperate soils across

Northern Europe. By extension it should work in other temperate parts of the world.

In this paper we consider the use of the index for gauging changes in SOC over time under different land uses,

and its use for assessing SOC status across different soil types under given land uses. We assessed changes in the

SOC/clay index between the two samplings of the NSI of England and Wales over 12–25 years, during which

both large losses and gains of SOC were found, largely due to historic changes in land management

. This is one

of the most comprehensive datasets globally for this purpose, covering a wide range of soils and land uses, and

with the same sampling and analytical methods used at both samplings. We also analyse changes in the index in

more frequent sampling of two long-term experiments in contrasting soils under ley-arable cropping with dif-

ferent organic matter managements. Due to past cropping history, one of these experiments started at low SOC

on a soil low in clay and the other started with a higher SOC and clay. We show how management systems that

increase or decrease SOC can be normalised by using the SOC/clay index, and how long management changes

take to bring about changes.

Methods

National soil inventory.

The NSI was first sampled between 1978 and 1983. Topsoil (0–15 cm depth) sam-

ples were collected as 25 samples on a 20 × 20 m grid at each site. These sites were located at intersections of an

orthogonal 5 km grid over the entire area of England and Wales, excluding urban areas and water bodies (

. Sufficient subsets of the sites (approximately 40% of the original sites) were resampled at inter-

vals from 12 to 25 years after the original sampling to be able to detect changes in SOC concentration ≥ 2 g kg

−1

with 95% confidence, taking account of the accuracy of the laboratory methods (± 1 g kg

−1

), the size of the origi-

nal sampling and the distribution of SOC in the original sampling

. This was done in three phases: in 1994–1995

for arable and ley grass sites (853 of the original 2578 sites), in 1995–1996 for managed permanent grassland sites

(771 of the original 1579), and in 2003 for non-agricultural sites (including deciduous and coniferous woodland;

555 of the original 1505). Soil from both samplings was analysed using the standard methods of the Soil Survey

of England Wales

23

: organic carbon by the Walkley–Black method, clay by the pipette method and pH in water

at 1:2.5 soil:solution ratio. To check for differences in analytical precision between the samplings, stored samples

from 10% of the original sites were reanalysed for SOC, and good agreement (± 1 g kg

−1

) with the original values

was found across the full range of values

. How accurately the sites could be relocated was assessed by revisiting

10 sites following the original site descriptions and recording positions with a global positioning system; accu-

racy was better than 20 m on enclosed land and better than 50 m on open land

.

For the analyses presented here, we only considered arable, ley grass, permanent grass and woodland sites

which had the same land use at the two sampling dates. We excluded sites (a) classified as peat or organic as

defined by SOC concentration > 120–180 g kg

−1

for clay concentration 0–600 g kg

−1

24

, (b) without measurements

of clay concentration or pH, and (c) with SOC/clay ratio > 0.361 to agree with Prout et al.

. To allow for differ-

ences in sampling interval between sites, we adjusted SOC values in the second sampling for a notional sampling

interval of 15 years, assuming the rate of change over the actual interval was constant. We calculated annual rates

of change in SOC and SOC/clay ratio from the change in SOC between the samplings divided by the accurate

time interval. Since clay concentrations were only measured on soils from the original sampling, we assumed

no change between samplings. We excluded sites for which changes in SOC/clay per year were greater than 0.02.

This gave 1418 sites, whose distributions across England and Wales are shown in Fig.

We divided sites according to SOC/clay thresholds of 1/8, 1/10 and 1/13 as the boundaries between

Very

Good

,

Good

,

Moderate

and

Degraded

levels of soil structure, respectively, following Johannes et al.

19

. In our earlier

paper

we showed that these thresholds were supported by the SOC/clay ratios of soils from the first NSI sam-

pling that differed in structural condition, which we determined using a method derived from the Agricultural

Land Classification of England and Wales

26

.

Long-term experiments.

Detailed land management practices were not recorded in the NSI, but the land

use was the same at the two dates 12–25 years apart. To give more information on how more detailed changes

in land management affect the SOC/clay index at more frequent time intervals, we used data from two long-

term experiments under arable, ley grass and permanent grass. The two experiments have run over similar time

periods, overlapping the period of the NSI samplings. One is on a sandy loam soil (Woburn) and the other on a

silty clay loam (Harpenden).

Woburn ley‑arable experiment.

The Woburn experiment was established at Woburn Experimental Station,

Woburn, Bedfordshire in 1938–1942. Details and treatments are in Table

. We considered six treatments: arable

(with or without fallows), lucerne (or sainfoin) converted to 3-year grass-clover ley, grazed ley converted to

3-year grass ley with inorganic nitrogen (N) additions, and alternating cycles of arable, lucerne, and grazed ley

converted to 8-year leys (either grass-clover or with inorganic N). Farmyard manure (FYM) additions (fresh

weight of 38 Mg ha

−1

year

−1

) were applied to the first test-crop on one of the two paired-rotation plots in each

experimental block until the mid-1960s; the SOC measurements of with- and without FYM plots were averaged

for each treatment. Soils were sampled every fifth year. The SOC values used here are averages of five experi-

mental blocks for each treatment. Only the first of the two 8-year ley treatment cycles was used for this analysis.

Highfield ley‑arable experiment.

The Highfield experiment was established at Rothamsted Research (formerly

Rothamsted Experimental Station), Harpenden, Hertfordshire in 1948. Details and treatments are in Table

. We

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Figure 1. Distribution of National Soil Inventory sites selected for analyses (

n

= 1418). The map was produced

using QGIS 3.0.1-Girona

25

Table 1. Summary of treatments in the long-term experiments.

a

Clay concentrations for Woburn from Catt

et al

27

, and for Highfield from Jensen et al

28

.

b,c

Change in treatment occurred at approximately the same time

between samplings in all treatments of each experiment respectively.

Woburn

Highfield

Location

52° 1

′

12

″

N, 0° 37

′

12

″

W

51° 80

′

N, 00° 36

′

W

Establishment

1938–1942

1948 (1959 for bare fallow)

Previous history

> 62 years of arable

> 100 years of permanent grass

Soil

a

Sandy loam, 114–164 g clay kg

−1

Flinty silty clay loam, 233–335 g clay kg

−1

Structure

Three years of treatment crops followed by 2 years of test crops

Three years of treatment crops followed by 3 years of test crops

Treatments

Arable (no fallows)

Arable crop rotations with 1-year hay

in rotation until 1975

Arable

Arable crop rotations

Arable (with fallows)

Arable crop rotations with 1-year

root crop until 1975 and 1-year fallow

until 1995

Bare fallow

Routinely ploughed and kept free of

vegetation

Lucerne/LC3

b

Lucerne or sainfoin until 1975 after

which 3-year grass-clover ley (LC3)

Grazed ley/LC3

c

Changed from grazed ley to grass-

clover ley (no inorganic nitrogen)

from 1962

Grazed ley/LN3

b

Grazed ley until 1975 after which

3-year grass with inorganic nitrogen

(LN3)

Cut grass/LN3

c

Changed from cut-grass ley to grass-

ley with inorganic N additions from

1962

Alternating/LC8

b

Alternating/LN8

b

Alternating cycles (arable, lucerne,

or grazed ley) until 1975 after which

a 10-year structure of either 8-year

grass-clover ley (LC8) or 8-year grass

with inorganic N (LN8)

Old grass

Old grass was undisturbed pasture and

reseeded grass was broken up and re-

sown to long-term grass; 3-year cycles

(2 years sheep grazing, 1 years hay

with aftermath grazing); grazing was

discontinued in 1962 (old grass) and

1963 (reseeded grass)

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considered the following treatments: arable, ley grass, reseeded grass, old grass, and bare fallow. From 1961, FYM

additions (fresh weight of 30 Mg ha

−1

year

−1

) to sub-plots of potato crops were made to whole plots instead, then

discontinued in 1970. The experiment was designed with four blocks and for each treatment the SOC/clay ratio

was averaged across blocks. Incomplete sampling between years meant that the number of SOC measurements

averaged was not always four. The bare fallow treatment comprised 4 plots, for which the SOC measurements

per sampling year are averaged here. Where clay concentration was not measured directly in a plot, we used the

clay concentration of the closest plot.

Changes in carbon stocks.

Estimates of carbon stocks (Mg C ha

−1

to 25 cm) were calculated for each SOC/clay

threshold using the mean clay concentration of each long-term dataset (138 g kg

−1

for Woburn, and 263 g kg

−1

for Highfield), and the difference between each SOC/clay threshold (including a value of SOC/clay = 1/16, which

emerged as a common ratio for long-term arable management at the two sites; see “

Results

”). For Woburn, a

topsoil weight of 3770 t ha

−1

was used for continuous arable soils

29

. The bulk density of Highfield soil was esti-

mated to be 1.12 g cm

−3

(topsoil weight = 2880 Mg ha

−1

) at the start of the experiment using starting SOC, tex-

ture measurements of Jensen et al.

28

, and a pedotransfer function for non-cultivated soils

30

(the corresponding

function for cultivated soils gave good agreement with the bulk density back-calculated from the soil weight for

the arable soil of Woburn). We calculated equivalent soil masses so that changes in bulk density with SOC were

accounted for. The standard deviations of SOC/clay values for each of the long-term experiments are presented

in the Supplementary Tables S1, S2, and S3.

Statistical analysis.

R version 4.0.2

31

was used to process the data and produce figures (package: ggplot2)

32

.

Regression analysis was used to test how much of the variance in rate of change of SOC/clay could be explained

by land use, average annual precipitation, major soil group, and mean pH between samplings. These varia-

bles were derived in the same way as Prout et al.

, except that average annual precipitation was averaged over

1910–2003 (extended to include the second sampling period). Empirical cumulative distribution functions, with

pointwise bootstrapped 95% confidence intervals, were used to assess the difference in rate of change of SOC or

SOC/clay by index class and land use. A chi-squared goodness of fit test was used to determine the representa-

tiveness of the smaller subset (n = 1418) compared to that of the larger subset in a previous study (n = 3809)

; the

results are in Supplementary Table S4. Genstat

33

was used to compare counts of index classes between the two

time points of the subset (n = 1418) using a chi-squared test.

Results

National soil inventory.

Distribution of SOC/clay.

In both NSI samplings, SOC/clay ratios increased in

the order arable < ley grass < permanent grass ≈ woodland (Fig.

). However, the values tended to be smaller in

the second sampling for arable, ley grass and permanent grass, and larger for woodland. The medians at each

sampling date remained in the same index class for each land use respectively:

Moderate

(SOC/clay ≥ 1/13–1/10)

for arable,

Good

(SOC/clay ≥ 1/10–1/8) for ley grass, and

Very Good

(SOC/clay > 1/8) for both permanent grass

and woodland. However, the 25th percentile for each land use decreased to the next index class down for each

land use (except for the second sampling of woodland which was also in the

Very Good

class). The specific

changes in SOC/clay and index class are explored and analysed in the following sections.

Rates of change in SOC and SOC/clay.

The rates of change in SOC (Fig.

a) and SOC/clay (Fig.

b) were

explored for each land use grouped by index class at the first sampling. The rates of change for both SOC and

SOC/clay have the same cumulative frequencies of positive and negative rates for each index class (NB clay

concentration did not change) and therefore the following applies to both. The

Very Good

class (SOC/clay > 1/8)

had larger proportions of negative rates than the other index classes for all land uses except woodland where

proportions were smaller. The other index classes had more similar curves to each other. In general, however, the

proportion of negative rates followed the order of

Very Good

>

Good

>

Moderate

>

Degraded

. The proportions of

the

Very Good

class with negative rates were similar for arable, ley grass and permanent grass (75, 73, and 73%

respectively), however smaller proportions of negative rates were observed for all other index classes of ley and

permanent grass compared with those of arable. Woodland soils mostly had positive rates of change, but nega-

tive rates were more frequent for

Very Good

soils.

The rates of change on the x-axis differ between Fig.

a,b. The general trends between index classes for each

land use were similar between the two, with the

Very Good

index class tending to have a greater range of nega-

tive rates than the other index classes. The difference between the two rates of change was most evident for the

separation of the

Very Good

and

Good

curves, particularly those of ley grass. For the rate of change in SOC

(Fig.

a), the two means and respective confidence intervals overlapped, however for SOC/clay (Fig.

b), the

separation of the curves indicates a difference between classes. Considering the change in rate variable: the very

high positive rate of change in SOC for the

Degraded

class of the ley grass data (8.0 g kg

−1

year

−1

; Fig.

a) was

brought into line with the rest of the data when clay concentration was accounted for (Fig.

b). The next highest

rate of increase in SOC for

Degraded

ley grass soils was 2.7 g kg

−1

year

−1

, in line with the highest rates for

Very

Good

and

Moderate

ley grass soils.

Regression analysis was used to assess several factors other than land use, that might affect changes in SOC/

clay. However, this showed that less than 1% of the variation in rate of change of SOC/clay could be explained

by average annual precipitation, mean pH, major soil group or carbonate score. Because such a small amount of

the variation was explained by these factors, this result was not considered further.

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Changes in index class between samplings.

The proportions of each land use falling in each index class are in

Table

. Arable had fewer soils classed as

Very Good

than

Degraded

, whereas permanent grass and woodland

had a majority of

Very Good

soils, and ley grass was intermediate. The proportions in the subset of the NSI used

in this analysis (

n

= 1418) were similar to those in the larger subset used in our earlier study

(

n

= 3809) (Sup-

plementary Table S4).

The distribution of index classes by land use changed between the NSI samplings (

Χ

2

(15) = 32.24, p < 0.001).

From the first to the second sampling, the proportion of soils in the

Very Good

class decreased under arable and

ley grass by close to 5%, and permanent grass by almost 10%. The increased proportion of the

Moderate

class

under ley and permanent grass also contributed to the statistic. Arable soils in the

Degraded

class increased from

43 to 47%. In contrast, woodlands showed an increase in the proportion of the

Very Good

class and a decrease

in the other index classes. The proportion of the

Very Good

class that changed index class followed the trend:

arable > ley grass > permanent grass > woodland; the inverse was evident for the

Degraded

class (Table

).

Despite having similar numbers in the

Very Good

class, 18% fewer of the ley grass changed class compared

to arable. The extent of decreases from

Very Good

, increases from

Degraded

, and changes in either direction for

Good

or

Moderate

classes could also be seen for each land use (Fig.

4

). More arable soils became

Degraded

than

grassland soils, irrespective of initial class. A higher number of

Very Good

class soils changed to a lower class

under permanent grass, but the changes tended to be to

Good

or

Moderate

classes, and permanent grass had a

higher proportion remain

Very Good

than arable or ley grass. A larger number from the

Good

class moved down

index classes than moved up under arable or ley, though under permanent grass the split was even. A larger

proportion of the

Moderate

class became

Degraded

under arable than grassland. The number of

Degraded

sites

increasing to another index class showed a similar trend for each of these three land uses, with few

Degraded

soils achieving

Good

status, and very few becoming

Very Good

. Though arable soils had the largest number of

sites improving from

Degraded

to

Moderate

it also had the largest number of

Degraded

sites and the largest pro-

portion remaining

Degraded

. Fewer woodland soils changed class and most of the changes were to a better class

than when first sampled. As the woodland sites tended to be

Very Good

already, had a smaller sample size, and

changes in index class tended to be upward, the smaller numbers changing class was to be expected.

Figure 2. Boxplots of SOC/clay for each land use and sampling of the NSI. Notches extend to 1.58 ×

interquartile range/square-root(n) either side of the median and indicate 95% confidence around the median.

Whiskers extend to the furthest point within 1.5 × interquartile range below the 25th-, or above the 75th

percentiles. Observations outside of the whisker ranges are represented as points. The y-axis was truncated to

0.41 meaning that 11 points (across the data) are not visible.

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Figure 3. Empirical cumulative frequency distributions of change per year of (a) SOC, and (b) SOC/clay ratio

in the two samplings of the National Soil Inventory, 1978–2003. Line colours of (a) and (b) indicate the SOC/

clay index class from the first NSI sampling. Shaded areas represent bootstrapped 95% confidence intervals.

Index classes correspond to SOC/clay ratios:

Very Good

≥ 1/8,

Good

= 1/10–< 1/8,

Moderate

= 1/13–< 1/10, and

Degraded

< 1/13. Sample counts for the index classes of each land use are displayed in the corresponding colours.

The vertical line indicates rate of change equal to zero.

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Long-term experiments.

Figure

a,b show the changes in SOC concentration over time in the two long

term experiments on soils with contrasting clay contents. Woburn had been in long-term arable management

and Highfield in long-term grass before the experiments began. Considering the flattening out of the curves for

the ley and arable treatments from approximately 1980 onwards, SOC concentrations in the Woburn experiment

were approximately half of those for similar treatments in the Highfield experiment. After normalising for clay

concentration, similarities between the two sites are apparent. The SOC/clay results for each site are discussed

below.

Woburn.

Despite a history of arable management on the Woburn experiment site, SOC/clay ratio decreased

over 70 years in the arable treatments, mostly remaining < 1/13, i.e.

Degraded

. The treatment without fallows

started each treatment cycle with 1-year hay crop, which was evidently sufficient to maintain a higher SOC/

clay ratio than the other arable treatment. After the hay rotation was stopped in 1975, SOC/clay decreased

to approximately the level of the other arable treatment which decreased further from this point due to the

introduction of fallow rotations. Apart from the 3-year grazed-ley (Grazed ley/LN3), which increased SOC/

clay from

Degraded

to high in the

Moderate

class, the other treatments were similar to the arable treatment with

1-year hay for approximately the first 35 years (Fig.

c). The two 8-year ley treatments were under an alternat-

ing rotation which included a grazed grass ley, but SOC/clay remained low, possibly due to the other treatments

between grazed rotations. After the mid-1970s, however, the 3-year ley with clover (previously lucerne) and

the 8-year leys showed similar, but more substantial increases. The peaks and troughs of the 8-year leys after

1970 correspond with samplings in the 8th and 3rd year under ley respectively. The replacement of grazing with

inorganic N fertilizer resulted in a decline in SOC/clay for the grazed-ley treatment but it remained higher than

the other ley treatments for approximately 15 year until they caught up. There was little difference between the

shorter and longer leys from the 1990s onwards, though the shorter leys had lower SOC/clay in the 2000s, and

little difference between the N-fertilized and clover leys of the longer rotations, both on a trajectory towards

SOC/clay = 1/10 after the second phase of 8-year under ley grass. It is notable that in each case the 8-year leys

decreased to a similar SOC/clay as the 3-year ley.

Table 2. Percentages of sites with a given index class under each land use in each sampling.

Land use

n

Percentage of sites with indicated SOC/clay index class
First sampling

Second sampling

Very Good Good Moderate Degraded Very Good Good Moderate Degraded

Arable

504 25.6

13.9

17.9

42.7

20.7

12.5

20.0

46.8

Ley grass

284 45.1

21.1

15.1

18.7

41.2

17.6

21.1

20.1

Permanent grass

532 70.5

15.6

8.3

5.6

60.9

18.8

15.2

5.1

Woodland

98 62.2

16.3

14.3

7.1

77.6

12.2

5.1

Table 3. Changes in index class between the two NSI samplings by land use and first sampling index class.

Note the

Very Good

class could only move to a lower class and the

Degraded

class to a higher one, but the

Good

and

Moderate

classes could move either up or down.

Land use

Index class at first sampling
Very Good Good Moderate Degraded

Arable
Number at first sampling

129

70

90

215

Number that changed class

61

50

57

55

% that changed class

47

71

63

26

Ley grass
Number at first sampling

128

60

43

53

Number that changed class

37

40

25

24

% that changed class

29

67

58

45

Permanent grass
Number at first sampling

375

83

44

30

Number that changed class

86

51

25

18

% that changed class

23

61

57

60

Woodland
Number at first sampling

61

16

14

7

Number that changed class

2

13

11

4

% that changed class

3

81

79

57

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Highfield.

Starting from long-term grass management and therefore a higher index class (SOC/clay > 1/10),

SOC/clay ratios decreased under arable, ley grass, and bare fallow treatments, whereas they increased under

grassland treatments (retained old grass or reseeded grass) (Fig.

d). During the first 20 years, the index class

in arable plots decreased from

Good

to the

Moderate

-

Degraded

threshold, then decreased further over the next

15 years before steadying at SOC/clay ≈ 1/16. This was similar to the Woburn arable treatments (mean SOC/clay

across both Woburn treatments from 1982 to 2007 was 1/16), despite the Highfield soils having 110–160 g kg

−1

higher clay concentrations. SOC/clay for the ley treatments decreased over the same periods as arable but

remained in the

Moderate

class until around 1980. Between 1987 and 2008 there were no measurements in these

treatments, but they still had SOC/clay close to the

Moderate

-

Degraded

threshold when next sampled, and the

ley with clover showed further increases. The continuous bare fallow treatment started 10 years after the other

treatments. However, within 4 years, SOC/clay decreased from

Good

to the level that arable and ley treatments

took nearly 20 years to reach, and the soil was lower in the

Degraded

class than the arable soils within 10 years.

The SOC/clay ratio continued to decline over the following 45 years. The bare fallow illustrates how far SOC can

fall in this soil in the absence of crop inputs. The SOC concentration associated with the last samplings presented

for bare fallow was approximately 10 g kg

−1

. This was comparable to, though slightly higher than, the arable treat-

ments of Woburn which showed signs of equilibrating at 8–9 g kg

−1

. Probably due to the initial ploughing and

reseeding, the SOC/clay ratio of reseeded grass decreased across the

Good

-

Moderate

threshold in the first 3 years

followed by a general increase, approaching the

Very Good

class over 60 years. The SOC/clay of the old grass

treatment showed an increasing trend over the course of the experiment, from the

Good

index class up to

Very

Good

over 60 years. Both treatments were grazed until 1962–1963, which might explain some of the increase up

to this point and subsequent plateau.

Carbon stocks.

Using the clay concentrations of the long-term experiments as examples, the carbon stocks

at each SOC/clay threshold and the difference that would result from a change in SOC/clay class (threshold

to threshold) were calculated (Table

4

). SOC/clay = 1/16 was included as a possible starting point for an arable

scenario. Changes in SOC/clay equivalent to SOC/clay = 1/10 to 1/13 (or vice versa) were observed in both

experiments. The ley treatments at Woburn gave rates of increase of 0.34–0.40 Mg C ha

−1

year

−1

(correspond-

ing to 30–35 years). At Highfield, the loss of SOC from 1/10 to 1/13 in the bare fallow treatment was at a

rate of 4.93 Mg C ha

−1

year

−1

compared with 0.985 Mg C ha

−1

year

−1

in the arable treatment. The increase in

SOC/clay in the old grass treatment from 1/10 to 1/8 (from 1951 to 2008, 58 years) gave an overall rate of

0.317 Mg C ha

−1

year

−1

.

Figure 4. Numbers of sites that changed SOC/clay index class between the NSI samplings. Numbers of sites in

each land use: arable, 223; ley grass, 126; permanent grass, 180; woodland, 30. VG, G, M and D indicate

Very

Good

(SOC/clay ≥ 1/8),

Good

(SOC/clay 1/10–< 1/8),

Moderate

(SOC/clay 1/13–< 1/10), and

Degraded

(SOC/

clay < 1/13) classes.

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Discussion

We have found that many of the arable, ley grass and permanent grass soils declined in SOC/clay between the

NSI samplings, whereas many of the woodland soils improved. Except under woodland, many soils with initially

Very Good

and

Good

SOC/clay indices lost SOC between the samplings and moved to lower classes. Hence the

proportions of

Moderate

and

Degraded

class soils increased, especially under arable and ley grass, and to a lesser

extent under permanent grass. A previous study found that rates of SOC loss in the NSI at a given SOC stock

decreased in the order arable > ley grass > permanent grass > other (mainly woodland)

. Those authors fitted a

simple single-pool model with first order decomposition kinetics to the data. The model showed that, despite the

diversity of soils in each land use, soils with small SOC stocks tended to gain C whereas those with larger values

increasingly lost it, and there was a characteristic steady-state SOC stock for each land use under the prevailing

management at which C was neither gained nor lost. The steady-state values increased in the order arable < ley

grass ≈ permanent grass < other (mainly woodland), and the rate of gain or loss increased with the degree of

departure from the steady-state SOC stock.

Since clay concentration is a major determinant of SOC protection and stability (

Introduction

), the SOC/clay

ratio gives a more definitive separation between different managements, climates and other factors within a land

use category than SOC alone. Hence there was a clearer separation between the index values for cumulative plots

of the rate of change in SOC/clay compared with SOC alone (Fig.

). The SOC/clay ratio at which mechanisms of

Figure 5. Mean SOC concentration and SOC/clay ratios over time in the long-term ley-arable rotation

experiments at (a, c) Woburn (sandy loam) and (b, d) Highfield (silty clay loam). For standard deviations of

each treatment at each timepoint see Supplementary Tables S1, S2, and S3. Horizontal lines represent SOC/clay

index thresholds equal to 1/8 (solid), 1/10 (dashed), and 1/13 (dot-dash). The vertical line marks a change in

treatments as indicated by the “/” in the legend entry.

Alt.

alternating treatment of 3-year arable, 3-year Lucerne,

and 3-year grazed ley;

LC3

3-year ley + clover;

LN3

3-year ley + nitrogen;

LC8

8-year ley + clover;

LN8

8-year

ley + nitrogen.

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SOC stabilization are saturated is approximately 1/10

18

. Above this threshold soils are less likely to retain carbon

under given OC inputs and below it they are more likely to retain added OC.

What were the causes of the declines in SOC/clay values between the NSI samplings and the differences

between land uses? Past or continuing changes in land use and management were the major drivers and model-

ling results showed that the effects of the modest warming in the two countries between the samplings would

not have been sufficient to explain much of the results

,

9

. Likewise, atmospheric deposition of nitrogen or sul-

phur was not likely to have been a factor, at least in managed soils. There was roughly a 50% decrease in rainfall

acidity between the NSI samplings as a result of decreased sulphur emissions from coal-fired power stations,

leading to modest but widespread increases in soil pH across England and Wales

34

. However, there were no clear

relationships between the SOC changes and either the baseline soil pH or measured changes in pH between

the samplings

. Increased nitrogen deposition between the samplings may have increased SOC stocks on semi-

natural land as a result of greater net primary production

14

. More recently, atmospheric nitrogen deposition

in the UK has decreased

35

. Low uptake of reduced tillage practices over the sampling periods

36

could also be a

contributing factor to the declines in SOC/clay in arable soils.

Changes in land use and management will tend to shift SOC stocks towards new steady states, characteristic

of these changes. This may cause gains of carbon in soils with small SOC stocks, or losses in soils with large

stocks. There have been large changes in land use and management across England and Wales since the Second

World War

37

. The following changes in particular will have affected SOC stocks: conversion of grassland and

natural vegetation to crops after the war

38

; widespread improvements in land drainage

39

; greater use of mineral

fertilizers

40

; greater animal stocking rates

41

; and, in general, the adoption of more uniform management practices

across both countries

37

. Similar changes in land use and management affecting SOC stocks have taken place in

other countries around the world and continue to do so

42

,

43

. Ambitions to increase SOC stocks globally need to

be seen in the light of this.

The results of the long-term experiments illustrate the value of normalising for soil clay concentration. Sandy

soils, such as those at Woburn, should not be expected to reach the same SOC concentrations as those with

higher clay concentration, such as at Highfield. By quantifying SOC/clay ratios, the value of the SOC for soil

properties can be assessed across contrasting soil types. Despite the differences in clay concentration between

Woburn and Highfield (approximately 125 g kg

−1

), the arable treatments levelled off at a similar SOC/clay ratio

of approximately 1/16. By contrast, the bare fallow treatment at Highfield had a similar SOC concentration to

the arable treatment at Woburn (approximately 10 g kg

−1

) but had a much smaller SOC/clay ratio. Therefore,

the bare fallow had a greater deficit of carbon and was degraded structurally and perhaps in other functions.

This would not be apparent by comparing SOC values alone. Ley treatments had a moderating effect on SOC

loss on conversion from grassland at Highfield and improved the soil from the

Degraded

class to near the

Good

threshold at Woburn. Results of the Woburn organic manuring experiment

—on the same site as the experiment

presented here but with different treatments—largely agreed with these differences in index classes with SOC/

clay for arable < 1/13 (~ 1/14), and ley > 1/13. Farmyard manure inputs gave greater increases in SOC/clay, but

at unrealistically high rates. Combinations of these managements, such as through ley rotations with manure

applications during arable rotations, might provide sustainable means to increase the rates and stability of SOC

gains. Whilst the 8-year leys could achieve higher SOC/clay, they did not stabilise higher than the 3-year leys.

Key steps for soil management are soil testing to identify if a soil is

Degraded

(or better) followed by monitor-

ing how management decisions impact the soil. Soil clay concentration is not expected to change, except with

extreme abuse, so requires less frequent monitoring. Once clay concentration has been mapped, SOC can be

monitored periodically to assess changes in the index. It is important that clay concentration is mapped accurately

at the scale of interest, as shown by the large variability in the standard deviations of SOC/clay in the Woburn

data (Supplementary Table S1). We expect that with more precise clay concentration data, as for Highfield, the

mean SOC/clay results would be similar but with variability better accounted for.

Table 4. Carbon stock differences between SOC/clay thresholds for Woburn and Highfield soils. Carbon

stock calculated to 25 cm depth.

a

Dashes indicate not applicable.

b

Typical of arable management in the two

experiments.

Site

SOC/clay Carbon stock at SOC/clay (t C ha

−1

)

Difference in carbon

stocks between

SOC/clay ratios

(Mg C ha

−1

)

SOC/clay

a

1/8

1/10 1/13

Woburn

1/8

65.0

–

1/10

52.0

13.0 –

–

1/13

40.0

25.0 12.0

–

1/16

b

32.5

32.5 19.5

7.5

Highfield

1/8

92.0

–

1/10

73.6

18.4 –

–

1/13

56.6

35.4 17.0

–

1/16

b

46.0

46.0 27.6

10.6

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Irrespective of how far soil carbon stocks might be increased globally to sequester atmospheric CO

2

, in many

parts of the world soils needs to be restored to levels needed for resilience to the effects of climate change and

continued provision of functions. Management practices for increasing SOC concentrations are well known

and include management of manures and crop residues, use of cover crops and leys, and the various methods

known as “Regenerative Agriculture”

44

. The long-term experiments reported here show that normalising for clay

concentration allows more meaningful comparisons of management effects on SOC, and that our index SOC/

clay ranges are of an appropriate magnitude for assessing changes over time. Very high SOC/clay ratios > 1/8

are unrealistic for all land uses, as shown in the various treatments in the long-term experiments and in the NSI

results for the different land uses (Fig.

). Arable soils had smaller SOC/clay ratios than permanent grassland

and woodland soils.

Based on our results, we suggest suitable SOC/clay targets of > 1/13, > 1/10, and > 1/8 for arable, ley grass,

and permanent grass and woodland soils, respectively. The net changes in SOC/clay between the NSI samplings

for land uses other than woodland suggest that further decreases may have taken place, but the median SOC/

clay in each sampling for each land use was still above these thresholds (Fig.

). Timescales for achieving targets

will depend on land use history, the availability of organic matter amendments, the length of ley rotations if

employed, and practices such as the use of cover crops and reduced tillage.

In general, soils across England and Wales—and by extension managed soils in other temperate regions—have

SOC concentrations below optimal levels due to sub-optimal management. The arable soils in the long-term

experiments maintained a SOC/clay ratio ≈ 1/16, but could likely achieve SOC/clay ≥ 1/13 with improved man-

agement. On these soils, increasing SOC/clay to 1/13 could result in an increase of 6–9 Mg C ha

−1

in the carbon

stock. If this were to increase to SOC/clay = 1/10 that becomes 18–26 Mg C ha

−1

, which we have shown could be

achievable with frequent or well managed ley rotations in approximately 40 years (Fig.

10. 1038/ s41598- 022- 09101-3

).

A SOC/clay of 1/10 is proposed as an approximate limit for SOC protection by clay

18

, and we have shown

that soils with SOC/clay > 1/8 are more likely to lose SOC, depending on land use. The results support the use

of the SOC/clay ratio to normalise between different soils and inform management decisions to maintain SOC

where possible, and increase it where needed.

Conclusions

The SOC/clay ratio index provides a simple method to normalise SOC values across soils and to assess changes

in SOC status over time. The NSI results showed that SOC/clay ratios declined between the NSI samplings under

arable, ley grass and permanent grass, especially in soils that initially had large SOC/clay ratios. In arable land the

proportion of

Degraded

soils increased between the samplings, even though a large proportion were already in

that class. The results of the long-term experiments showed the value of normalising for soil clay when assessing

SOC management practices. Similar management practices resulted in similar SOC/clay ratios on the two soils

with contrasting clay concentration, despite SOC concentrations that differed by a factor of two. Realistic long-

term targets for SOC/clay ratios differ between land uses: they are > 1/13, > 1/10, and > 1/8 for arable, ley grass,

and permanent grass and woodland soils, respectively. Our conclusions are based on results for a wide range of

soils across Northern Europe. While soils with different mineralogies may behave differently, for example, highly

weathered soils of the humid tropics or volcanic ash soils, the basic SOC/clay index tested here should be useful

in similar temperate regions globally.

Data availability

The NSI dataset is held by Cranfield University and accessed via LandIS (

www. landis. org. uk

). Data for the

Rothamsted ley-arable (Highfield) and Woburn ley-arable experiments can be obtained via the Electronic

Rothamsted Archive (era.rothamsted.ac.uk).

Received: 16 December 2021; Accepted: 16 March 2022

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Acknowledgements

JMP is grateful for a PhD studentship through the Rothamsted-Cranfield Soil Agricultural Research and Inno-

vation Accelerator. We thank the Lawes Agricultural Trust and Rothamsted Research for data from the e-RA

database. The Rothamsted Long-term Experiments National Capability is supported by the BBSRC and the Lawes

Agricultural Trust. We thank the e-RA data managers, and Andy Gregory for access to data and information on

for the Controller of HMSO 2021. This work was supported by Rothamsted Research; Cranfield University; the

Biotechnology and Biological Sciences Research Council (BBSRC) of the United Kingdom [BBS/E/C/000I0310,

BBS/E/C/000 J0300]; and the Lawes Agricultural Trust.

Author contributions

All authors: conceptualization, writing—review & editing. J.M.P.: methodology, software, formal analysis, investi-

gation, data curation, writing—original draft, visualisation, project administration. K.D.S.: methodology, super-

vision. S.P.M.: supervision, funding acquisition. G.J.D.K.: supervision. K.L.H.: methodology, software, formal

analysis. S.M.H.: supervision, project administration, funding acquisition.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary Information The online version contains supplementary material available at

https:// doi. org/

.

Correspondence and requests for materials should be addressed to G.J.D.K.

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