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Soil lifespans and how they can be extended by land use and
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LETTER
Soil lifespans and how they can be extended by land use
and management change
D L Evans
, J N Quinton
, J A C Davies
, J Zhao
and G Govers
1
Lancaster Environment Centre, Lancaster University, Lancaster, Lancashire LA1 4YQ, United Kingdom
2
Department of Geology Engineering and Geomatics, Chang’an University, Yantalu 120, Xi’an 710054, People’s Republic of China
3
Division of Geography, Department of Earth and Environmental Sciences, KU Leuven, Leuven 3000, Belgium
E-mail:
Keywords:
soil erosion, soil formation, soil lifespan, sustainability, soil conservation, sustainable management
Supplementary material for this article is available
Abstract
Human-induced soil erosion is a serious threat to global sustainability, endangering global food
security, driving desertification and biodiversity loss, and degrading other vital ecosystem services.
To help assess this threat, we amassed a global inventory of soil erosion rates consisting of 10 030
plot years of data from 255 sites under conventional agriculture and soil conservation
management. We combined these with existing soil formation data to estimate soil sustainability
expressed as a lifespan, here defined as the time taken for a topsoil of 30 cm to be eroded. We show
that just under a third of conventionally managed soils in the dataset exhibit lifespans of
<200 years, with 16% <100 years. Conservation measures substantially extend lifespan estimates,
and in many cases promote soil thickening, with 39% of soils under conservation measures
exhibiting lifespans exceeding 10 000 years. However, the efficacy of conservation measures is
influenced by site- and region-specific variables such as climate, slope and soil texture. Finally, we
show that short soil lifespans of <100 years are widespread globally, including some of the
wealthiest nations. These findings highlight the pervasiveness, magnitude, and in some cases, the
immediacy of the threat posed by soil erosion to near-term soil sustainability. Yet, this work also
demonstrates that we have a toolbox of conservation methods that have potential to ameliorate this
issue, and their implementation can help ensure that the world’s soils continue to provide for us for
generations to come.
1. Introduction
Soils have underpinned the health and longevity of
every society. They are a critical global resource,
providing the basis of food production, a store and
filter for our water resources, the largest organic car-
bon store and a platform for development (Blum
). Pressures on the soil resource grow as food
demands rise and land degradation increases. To date,
36% of the world’s cultivable land has been farmed
(FAO
) and in many areas of the world conven-
tional plough-based agriculture is accelerating local
soil degradation (Montgomery
). The United
Nations’ Food and Agriculture Organisation (FAO)
estimates that 66% of the world’s soils suffer from
some form of degradation (Bot
et al
), with soil
erosion estimated at between 25 and 40 Pg y
−
1
glob-
ally (Quinton
et al
Rates of human-induced soil erosion are estim-
ated to outpace soil formation rates by more than an
order of magnitude (Montgomery
). The con-
sequential trajectory of soil thinning is one that, left
uninterrupted, leads to the removal of the soil cover
and the exposure of the underlying parent material.
Given that the thickness of the pedosphere is a first-
order control on soil functioning, with thicker soils
having a greater capacity for water, carbon and nutri-
ent storage (Power
et al
), the continued thin-
ning of near non-renewable soil profiles is one of the
most significant threats to soil sustainability (FAO
and ITPS
, UNCCD
). It is important that
land managers, policy makers, and society as a whole
© 2020 The Author(s). Published by IOP Publishing Ltd
Environ. Res. Lett.
15
(2020) 0940b2
D L Evans
et al
understand the timeline over which soil ecosystem
services may be severely degraded by complete loss of
topsoil.
Thus far, we have not quantified the longevity
of our remaining soil resource. Media reports (e.g.
Arsenault
, Wong
) have repeatedly stated
that there are as little as 60 years of topsoil left,
but there appears to be no scientific basis for these
claims. Here, we provide the first scientifically robust,
globally relevant estimate of soil lifespans and the
degree to which changes in land use or manage-
ment can extend them. We define a first-order upper
physical limit on the productive lifespan of soils as
the time it would take for the uppermost 0.3 m of
soil to erode, assuming current rates of soil erosion
and soil formation remain constant. We argue that a
0.3 m surface horizon is essential for the delivery of
ecosystem services, and has been previously used to
define the thickness of topsoil which is enriched with
nutrients and organic matter (IPCC
, Nachter-
gaele
et al
, FAO
) (see
Data and Methods
).
We then employ this definition with a global data-
set (10 030 plot years of annual water erosion rates,
derived from 1103 erosion plot-based records) to
quantify typical soil productive lifespans and examine
the extent to which land use change and management
practices can extend the timescales over which soils
remain productive. This is a critical step in motivat-
ing and informing land management decisions that
secure and sustain food, water and climate services
from soils.
2. Data and methods
2.1. Data collation
A compilation of 4285 plot-based gross erosion
rates representing 10 030 plot years, were amassed
from 240 studies, comprising 255 unique locations
across 38 countries (figure
; sources are listed
in Supplementary Information (available online at
stacks.iop.org/ERL/15/0940b2/mmedia
)). These data
were obtained in a series of steps. First, previously
published erosion rate inventories were obtained
from a Web of Science search, using the search term
TS
=
(‘soil erosion’ AND ‘plot’). As part of the inclu-
sion criteria, studies had to report at least a year’s
worth of empirical soil erosion data. Papers based
on modelled data and/or those that were based on
data collected during singular erosion events were
discarded. Second, references within each published
inventory were investigated in a snowball sampling
strategy. Likewise, those that did not comply with the
inclusion criteria set out above were discarded. Sub-
sequently, data from USLE plots and a Chinese data-
set were added. To avoid biasing the analysis towards
studies with long timeseries or many replicates, we
averaged plot replicates and multi-year studies such
that all data points represent one erosion rate for
each treatment at a single location. Throughout this
paper, we use
n
to denote the number of these gross
erosion rates and
PY
to denote the number of plot
years.
The data were assigned into three categories with
respect to land management: bare, non-bare con-
ventional agriculture and conservation-based agri-
culture. The bare soils dataset was from soils that
are kept free of vegetation on experimental plots
(
n
=
62;
PY
=
470; supplementary table 1). These
are often used to gauge erosion in a worst-case scen-
ario. Whilst instances of constantly bare soil are only
likely to occur periodically (e.g. prior to crop emer-
gence), the use of bare soil data provides a worst-case
baseline against which conventional agriculture and
soil conservation practices can be assessed. The non-
bare conventional agriculture dataset includes obser-
vations from plots undergoing non-conservation
agricultural practices, including downslope cultiva-
tion, non-terraced cropland and conventional tillage
(
n
=
320;
PY
=
4816; supplementary table 1). The
conservation-based agriculture dataset comprises an
array of plots that have been subject to soil con-
servation techniques, such as land-use change and
modifications to agricultural practices (
n
=
721;
PY
=
4744; supplementary table 1).
Gross erosion rates in the study’s native units
were compiled along with details of the respect-
ive management or soil conservation practice. Addi-
tional information about the study site were noted,
including: soil type (FAO World Reference Base), tex-
tural data, mean annual precipitation, slope and loc-
ation co-ordinates. Bulk density was recorded if it
was provided; otherwise, the lower and upper val-
ues from bulk density ranges were used from accep-
ted standards for each soil texture (US Department
of Agriculture
). Cultivation notes, including
ploughing depth, mulching technique and crop spe-
cies, were recorded wherever applicable (supplement-
ary table 1). The dataset includes soils from 14 of
the 26 soil orders as recognized by the World Refer-
ence Base. All soil textures apart from ‘Silt’ and ‘Sandy
Clay’ soils are represented within this inventory (sup-
plementary figure 1), with the modal texture being
‘Silt Loam’ (
n
=
182;
PY
=
1865).
2.2. Lifespan model
To permit a valid comparison between gross erosion
rates and soil formation rates, all erosion data were
converted from their native unit to mm y
−
1
. This
involved a two-step approach whereby data in nat-
ive units were first converted into t ha
−
1
y
−
1
and
then, with the bulk density estimate, into mm y
−
1
,
following standard approaches (Montgomery
).
Bulk densities were either taken directly from original
papers or, in the case of these data being absent, a
lower and upper bulk density were estimated based on
accepted standards (US Department of Agriculture
2
Environ. Res. Lett.
15
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D L Evans
et al
Figure 1.
Number and spatial distribution of plot years for the 255 unique locations in this study. Full dataset can be found in
supplementary table 1.
). A lower and an upper gross erosion rate were
thus calculated (supplementary table 1).
A previously compiled global dataset (
n
=
264)
of soil formation rates was used in this study (Evans
et al
). This dataset comprises
10
Be-derived rates
of soil formation measured at ten unique locations
across Australia, USA, Chile and the UK, represent-
ing five different climates (according to the Köppen
classification system) and all three major rock types
(igneous, sedimentary and metamorphic) (Heim-
sath
et al
,
,
,
; Wilkinson
et al
; Dixon
et al
;
Owen
et al
; Riggins
et al
). The mean soil
formation rate was 0.053
±
0.005 mm y
−
1
. The
25th and 75th percentiles of this dataset (0.011 and
0.059 mm y
−
1
, respectively) were employed into the
lifespan model in order to provide a representation of
uncertainty and variation in the formation rate (sup-
plementary table 2).
Where gross erosion rates exceed those of soil
formation, soil profiles thin; over time, assuming
these rates remain imbalanced, the soil profile will
eventually erode beyond a critical depth required
for the delivery of ecosystem services. We argue that
the surface horizons of the soil are critical for eco-
system service delivery as they are enriched with
nutrients and organic matter, and critical for plant
growth. In line with the FAO Harmonised World Soil
Database, the World Reference Base Soil Classifica-
tion and the IPCC (IPCC
, Nachtergaele
et al
, FAO
) we set the topsoil depth as 0.3 m.
We chose a single topsoil depth as few of the plots
where the erosion rates were measured had detailed
soil descriptions. Global spatially explicit estimates
of soil depth exist, such as the International Soil
Reference Information Centre Global Soil Informa-
tion System ‘
SoilGrids
’, however, the representative-
ness of these data products for the individual sites
compiled in the erosion rate dataset is unknown
(Hengl
et al
; Shangguan
et al
). Hence,
we employ the simplest assumption and calculate
the lifespan assuming a productive 0.3 m topsoil
layer at all sites. This also provides the advantage of
allowing us to harmoniously compare the estimated
lifespans across locations, land uses and management
practices.
Following Stocking and Pain
, soil pro-
ductive lifespans (
L
) were calculated for bare, non-
bare conventional and conservation-based land use
regimes using equation (
) (supplementary table 2):
D
E
−
F
(1)
where
D
is depth (mm) (set at 300 mm),
E
is the gross
annual soil erosion rate (mm y
−
1
) and
F
is the gross
annual soil formation rate (mm y
−
1
).
For each land use and management type, calcu-
lated lifespans were pooled and an Anderson Darling
test for normality was conducted. All were found to
be non-normal, leading to the median being used as
the most appropriate measure of central tendency.
Interquartile ranges and the 5th and 95th percentiles
3
Environ. Res. Lett.
15
(2020) 0940b2
D L Evans
et al
were calculated. Percentages of lifespans <100 years
were calculated, with these percentages referring to
the number of plot studies.
Where gross erosion rates fall below those of soil
formation, the soil is thickening and equation (
no longer applies (the lifespans are indefinite). In
these instances, the net annual soil gain was calcu-
lated using the gross soil erosion and formation rates.
The proportions of <100 year, finite, and indefin-
ite lifespans were calculated for each land use and
management type. A two proportion Z test (both
one-tailed and two-tailed at 95% confidence) was
then used to assess whether these <100 year, finite,
and indefinite lifespan proportions from conserva-
tion managed plots were significantly different to
those from bare and conventionally managed plots.
2.2.1. Limitations of the soil lifespan analysis.
The lifespans calculated here are based on a single
proxy (net erosion), which is just one form of degrad-
ation that threatens the sustainability of soils glob-
ally. There are a range of retrogressive processes
that can degrade the soil’s capacity to deliver eco-
system services in shorter time frames (Heimsath
et al
). For example, soil compaction can bring
about adverse effects on water regulation and nutri-
ent cycling, without a substantial loss of soil thick-
ness (Batey
). The soil formation and erosion rate
(and, therefore, the net annual soil loss and gain) were
assumed to remain constant over time. This does not
account for the year-to-year fluctuations observed in
long-term soil erosion studies (Martínez-Casasnovas
et al
), the potential acceleration of soil erosion
rates as precipitation intensity increases, nor the
extent to which soils may become armoured by coarse
fragments (e.g. pebble beds) in the future (Evans
et al
). Moreover, the use of constant soil formation
rates do not account for potential variations in bed-
rock weathering rates as the overlying soil mantle
progressively thins (Heimsath
et al
) or climatic
or biotic variations. In addition, the bedrock-derived
soil formation rates account neither for allochthon-
ous additions (aeolian and alluvial deposits) nor the
thickening of an organic layer on the mineral soil
surface, observed as a product of some management
practices, such as conservation tillage (Sharratt
et al
). As aforementioned, we selected a generalized
critical soil depth of 0.3 m, but in reality, depending
on the soil and the environment, the critical soil depth
threshold may be lower or higher than this. Further-
more, we do not account for any adaption of the sub-
soil: if surface erosion rates are sufficiently low, the
underlying subsoil (if it exists) could be transformed,
chemically and biologically, into topsoil (Bakker
et al
).
Assumptions were also made in the collation of
input data used in the lifespan calculation. First, the
lifespans presented here have been calculated from
erosion rates measured at the plot scale, which are
not wholly representative of the erosional processes
observed at the landscape scale, such as gully erosion
(Takken
et al
, Cerdan
et al
). Erosion plot
experiments tend to be located in areas suffering
from erosion, likely leading to bias in our calculated
lifespans. In addition, the erosion rates included in
this analysis are only for water-based erosion; the
redistribution of soil by tillage was not considered,
although this is a significant erosion process (St.
Gerontidis
et al
). Therefore, it is likely that
estimates of lifespans are overestimated for soils on
slope convexities, where tillage reduces soil depth,
and underestimated on slope concavities, where till-
age leads to soil accumulation (Van Oost
et al
Finally, there is an imbalance between the number
of observations in the soil erosion dataset and within
the inventory of soil formation: 18 940 papers were
found in Web of Science (25 March 2020) following a
search for ‘soil erosion’ compared to 2785 for the term
‘soil formation’.
3. Lifespans for bare and conventional
agricultural soils
Here, we consider soils that are devoid of vegetation
(bare) and under conventional cropping (e.g. inver-
sion tillage, seedbed preparation followed by cereal or
vegetable cropping), representing the worst-case and
business-as-usual conditions respectively. The cumu-
lative distribution of the estimated soil lifespans is
illustrated in figure
For plots kept bare, 34% of the dataset reported
lifespans of <100 years and the median lifespan for
thinning soils (
n
=
60;
PY
=
447) was 333 years. For
the bare soils that were classed as thickening (
n
=
2;
PY
=
23), the mean annual estimated soil gain was
0.03–0.04 mm y
−
1
. For soils from non-bare, conven-
tionally managed plots, 16% of the dataset (
n
=
298;
PY
=
4737) reported lifespans of <100 years, with a
median of 491 years for thinning soils, and a mean
annual estimated soil gain was 0.03
±
0.001 mm y
−
1
for thickening soils (
n
=
22;
PY
=
79). These estim-
ates demonstrate the magnitude of the threat that soil
thinning can place on relatively near-term soil sus-
tainability.
The analysis also shows that all regions of the
world have a predominance of thinning soils (table
finite lifespans), with soils with <100 years present
on all continents. The location of sites with lifespans
<100 years include some of the wealthiest nations
(Australia, Brazil, China, Italy, Mexico, Spain and the
USA), as defined by gross domestic product (World
Bank
) demonstrating the pervasiveness of this
issue.
Our analysis suggests that a sizeable proportion
of our soil resource under conventional management
practices have lifespans <100 years comparable to
human timescales (16% with lifespans <100 years).
This is not in line with the popular claim that there
4
Environ. Res. Lett.
15
(2020) 0940b2
D L Evans
et al
Figure 2.
Cumulative distribution of soil lifespans (
n
=
929;
PY
=
9168 for thinning and
n
=
174;
PY
=
862 for thickening soils)
and annual soil gain for bare soils (grey), non-bare conventionally managed soils (blue) and conservation management (green).
Bold lines on the cumulative distribution chart represent median lifespans; dotted lines represent the lower and upper bounds of
lifespans when the uncertainties in soil formation and soil erosion data are taken into account (see
Methods
). The shaded red box
highlights the lifespans <100 years. Full dataset can be found in supplementary table 2.
are 60 years of global topsoil left (e.g. Arsenault
,
Wong
). Our analysis suggests that this claim
may be too alarmist given that 79% of the worst-
case bare soils had lifespans longer than 60 years, and
that rises to >90% for conventionally managed soils.
However, it should also be noted that these lifespans
were spread across five orders of magnitude and were
heavily dependent on the management in operation
and the local environmental conditions at each site.
Hence, quoting a single lifespan estimate does not
effectively acknowledge the range of lifespans that
exist under a number of different land management
practices.
4. Extending soil lifespans by changing
land use and agricultural practice
The lifespans of soils under conservation manage-
ment, including changes both in land use and agricul-
tural practice, were estimated using measured erosion
rates (
n
=
721;
PY
=
4744) drawn from 201 plot-
based studies. These plots include those adopting
conservation-based agricultural practices (e.g. con-
tour cultivation) and different types of land use,
including forest and grassland. In the majority of
studies, forest and grassland sites were established
prior to the commencement of the study, although
some studies were not explicit in this regard. Gross
soil erosion rates were lower than soil formation in
21% of cases (
n
=
150;
PY
=
760), leading to a net
annual soil gain (figure
). Compared with the bare
and conventionally managed plots, the proportion of
thickening soils from the conservation management
plots was significantly greater (
p
< 0.05) with these
soils more than twice as likely to be thickening rather
than thinning.
Pooling all data for plots managed with conserva-
tion practices, 7% of lifespans were <100 years. This
represents a statistically significant reduction of more
than half in the proportion of lifespans <100 years
compared with the conventionally managed plots in
the dataset (
p
< 0.05; figure
). The distribution of
lifespans for the plots under conservation manage-
ment was shifted towards longer lifespans when com-
pared to conventionally managed soils. For conser-
vation managed soils, 48% of the estimated lifespans
exceeded 5000 years compared to 23% for conven-
tional agriculture, and 39% exceeded 10 000 years
compared to 18% for conventionally managed soils.
4.1. Land use change
Analysing by land use suggests that land use change
towards a forested site would be the most effective
land use change for extending soil lifespans (figure
). The shortest lifespan in the forested dataset was
420 years, compared to 16 years for cropland soils. In
50% of cases, gross erosion rates in forests fell below
those of soil formation, promoting soil thickening
(0.032
±
0.000 mm y
−
1
). This proportion of thick-
ening soils was significantly greater than that for bare
plots (15 times greater) and cropland plots (six times
greater) (
p
< 0.05). These findings concur with sim-
ilar work that has concluded that croplands erode, on
5
Environ. Res. Lett.
15
(2020) 0940b2
D L Evans
et al
T
ab
le
1.
Glo
bal
dist
rib
u
tio
n
o
f
finit
e,
lif
espans
<100
year
s,
and
infinit
e
lif
espans
fo
r
fi
ve
re
gio
ns.
A
ll
Data
Finit
e
Lif
espans
Lif
espans
<100
year
s
Infinit
e
Lif
espans
n
PY
M
edian
lif
espan
(y
ear
s)
n
PY
%
o
f
datase
t
in
the
re
gio
n
n
PY
%
o
f
datase
t
in
the
re
gio
n
N
PY
%
o
f
datase
t
in
the
re
gio
n
M
ean
so
il
gain
(mm
y
−
1
)
N
o
rth
A
me
rica
217
2487
583
181
2346
83
39
755
18
36
141
17
0.033
S
ou
th
A
me
rica
201
3221
7101
147
3019
73
24
134
12
54
203
27
0.031
sub-S
ahar
an
A
fr
ica
105
1219
671
100
1198
95
17
212
16
5
21
5
0.035
E
ur
o
p
e
and
M
idd
le
East
107
963
6270
83
689
78
5
24
5
24
274
22
0.032
A
sia
and
A
ust
ralia
473
2140
1397
418
1917
88
40
162
9
55
223
11
0.031
a
C
o
mp
rises
bar
e,
no
n-bar
e
co
n
ve
nt
io
nal,
and
co
nse
rv
at
io
n
land
use
manag
eme
nt.
b
F
ul
l
data
can
b
e
found
in
sup
ple
me
ntar
y
tab
le
2.
6
Environ. Res. Lett.
15
(2020) 0940b2
D L Evans
et al
Figure 3.
(a) Distribution of lifespans <100 years (
lowermost bars
), finite (>100 years;
middle bars
) and infinite (thickening;
uppermost bars
) lifespans for bare, conventional and conservation-based management (
n
=
1103;
PY
=
10 030). (b) Soil lifespans
for conventional (blue) and soil conservation (green) management practices (
n
=
929;
PY
=
9168), excluding those soils that are
thickening. (c) Net annual soil gain for conventional (blue) and conservation-based (green) management practices (
n
=
174;
PY
=
862). Dotted bars denote incidences where either annual soil gain did not occur or there were not enough data. All lifespans
in the dataset were calculated by accounting for the uncertainty in soil formation and erosion rates (see
Methods
). Boxplots
represent the interquartile range of these pooled lifespans; error bars refer to the 5th and 95th percentiles, within each
management shift category. A description of management practices can be found in supplementary table 3. Full dataset can be
found in supplementary table 2.
average, more than an order of magnitude faster than
forest soils (Zhao
et al
).
The analysis suggests that land use change from
bare or conventional cropland to grassland would be
similarly effective in lengthening soil lifespans. For
grassland plots, we found that 2% of lifespans were
<100 years, a 17-fold reduction in the proportion
of <100 year lifespans compared to plots kept bare
(
p
< 0.05) and a seven-fold reduction in comparison
to cropland soils. In 37% of cases, gross soil erosion
rates fell below those of soil formation, promoting soil
thickening (0.032
±
0.000 mm y
−
1
). This represents
an 11-fold increase in the occurrence of thickening
soils (
p
< 0.05) compared to bare soils, and a four-
fold increase (
p
< 0.05) compared to cropland soils.
4.2. Changing agricultural practices
Without reducing the area of land designated for agri-
culture, we found that cover cropping was an effective
method of extending lifespans. No plot in the cover-
crop dataset had lifespans of <100 years (figure
(a)).
In 25% of cases, gross soil erosion rates fell below
those of soil formation, promoting an estimated net
annual soil gain of 0.031
±
0.001 mm y
−
1
. The pro-
portion of thickening soils from cover crop plots was
significantly greater than that of conventional man-
agement practices (
p
< 0.05). These results accord
with previous work showing the importance of cover
cropping in reducing erosion (Nyakatawa
et al
Verstraeten
et al
, Gyssels
et al
Whilst cover cropping is one of the most effect-
ive agricultural practices for lengthening soil pro-
ductive lifespans, a suite of additional practices both
prior to and during the cropping season can extend
these further. In our dataset, conservation tillage led
to a two-fold reduction in the proportion of cases
with lifespans <100 years compared to conventional
tillage, although this was not statistically significant
(
p
> 0.05). Moreover, we found that zero tillage prac-
tices also substantially lengthened soil lifespans, with
a significant reduction in the proportion of <100 year
lifespans compared to conventional tillage (
p
< 0.05)
(figure
(b)).
For cultivation direction, we found that con-
tour cultivation compared to downslope practices
brought about an extension of the soil lifespan, and
7
Environ. Res. Lett.
15
(2020) 0940b2
D L Evans
et al
a significant reduction in the proportion of <100 year
lifespans (
p
< 0.05), with 7% of the former having
lifespans of <100 years compared to 37% for the lat-
ter. For 6% of the contour cultivation dataset, soil
thickening led to an estimated net annual soil gain of
0.032
±
0.001 mm y
−
1
. On particularly steep gradi-
ents, contour cultivation may not be practical and
the most effective conservation-based management
strategy may be terracing. For terraced soils, 2% of the
dataset reported lifespans of <100 years, which com-
pared to downslope cultivation represents a 23-fold
reduction in lifespans <100 years (
p
< 0.05). However,
terraces can lead to a reduction in the cropping area
(Herweg and Ludi
), although some have argued
that the risers are potentially cultivable (Sahoo
et al
) and their success as a soil conservation measure
is fundamentally dependent on their maintenance.
5. Decision making at the site scale
Underpinning the variability in the efficacy of soil
conservation management across different regions
are local, site-specific variables including climatic,
topographic and pedological factors (see supple-
mentary information for a detailed analysis of the
effects of precipitation, slope, and soil texture on soil
lifespans) and it is important that these are taken into
consideration when planning soil conservation meas-
ures. The decision about which conservation practice
is most likely to be more effective in sustaining soils at
a given location is also dependent on an array of social
and economic factors. For example, land use change
to either forest or grassland would not be appropri-
ate if agricultural activity is displaced to more erosive
locations, and the selection of specific agricultural
practices is also likely to be partly determined by the
social context, financial and resource capability of the
land manager in question. Issues such as the duration
of land tenancies and the existing policies, incent-
ives and advisory services provided at a regional and
national level will influence the adoption of soil con-
serving approaches. In addition, the approaches to
sustain, improve, or restore soil health in the short-
term may not always be the most effective strategies
of protecting the soil from erosion. Our dataset repor-
ted 25 instances where soil lifespans are shorter than
25 years. In ten of these cases, the soils had been
subject to conservation management practices, and
in seven of these, fallowing had been adopted. Some
researchers have found that fallowing for a period
after agriculture is necessary to restore soil aggrega-
tion and biological health (Aboim
et al
, Zeleke
et al
). In water-limited environments, leaving
crop residues or stubble on otherwise uncultivated
soil is one of the ways of increasing soil moisture prior
to the sowing of the subsequent crop. Whilst the soil
is not wholly bare during the fallowing period, the
sparseness of vegetation cover can lead to increased
erosion. Our analysis showed that, for the majority of
cases, fallowing increased soil erosion rates by more
than an order of magnitude compared to those during
the cropping period. This demonstrates that incor-
porating a protective cover into the fallowing period
is essential to minimize soil erosion. Furthermore, the
land use changes and agricultural practices presented
in this paper vary in their ‘establishment time’: the
time for a selected conservation management regime
to be set up, launched and become effective. For
soils with lifespans <100 years, and especially the 25
instances in our dataset where lifespans are shorter
than 25 years, it could be argued that the most effect-
ive management decision is to adopt a strategy with
a short establishment time. In this context, conver-
sion to grass, cover cropping and/or contour cultiv-
ation may be most appropriate, as the establishment
time for these strategies is in the order of months to
a year. By contrast, the planting of trees to convert
cropland to forest incurs a significant lag time whilst
forests mature.
6. Conclusions
In this paper, we have presented the first broad quant-
itative estimates of the productive lifespans of soils
and the degree to which these may be extended by
land management change. By compiling globally dis-
tributed soil erosion and soil formation rate data and
applying the soil lifespan concept, we can contribute
the following conclusions.
First, an assessment of soil lifespans using soil
loss rates measured from non-bare soils under con-
ventional management systems suggest that, under a
worst-case scenario, 93% were thinning and 16% had
lifespans <100 years. At these sites, soil erosion is a sig-
nificant threat to the soil’s capacity to grow food, sup-
port ecosystems, store and regulate water, cycle car-
bon and nutrients and thus to the overall functioning
of the soil system.
Second, we have shown that land use and land
management practices can extend the soil lifespan
and, in many cases, promote the onset of soil thick-
ening. Only 7% of the conservation plot dataset
had lifespans <100 years, with 48% of the estimated
lifespans exceeding 5000 years and 39% exceeding
10 000 years. The estimates for forested and grass-
land sites suggest that conversion to these land uses
would be most effective in achieving both of these
outcomes, closely followed by the introduction of
cover cropping. However, given the need to meet the
growing demand for food, cover cropping is argu-
ably the more attractive option. A suite of additional
strategies to extend soil lifespans and promote annual
soil gain include conservation and zero till prac-
tices, contour cultivation and terracing. In general,
conservation practices extend soil lifespans and may
promote soil thickening, increasing the potential for
water, carbon and nutrient storage, and thereby soil
conditions which could enhance crop yields.
8
Environ. Res. Lett.
15
(2020) 0940b2
D L Evans
et al
Third, we have shown that there is a wide distri-
bution of soil lifespans globally, encompassing five
orders of magnitude, partly reflective of an extens-
ive variation in the underlying driving variables such
as climate, slope and soil texture, which in turn can
influence the efficacy of soil management techniques.
However, soils with human-scale lifespans shorter
than 100 years are present in all of the observed
regions, including many of the world’s wealthiest
nations. This clearly demonstrates that soil erosion
is one of the most critical threats to soil sustainab-
ility globally, and that urgent action worldwide by
land managers, policy makers and society is imper-
ative to prevent the collapse of soil ecosystem service
provision.
Acknowledgments
We wish to thank all those who assisted us in our
efforts to compile the dataset, particularly Khasi-
jah Binti Muhamad, Jianlin Zhao, Calvin Rose,
Bofu Yu and Mark Nearing. The statistical advice
of Professor Murray Lark and Dr Victoria Janes-
Bassett is also gratefully acknowledged. We also wish
to thank the anonymous referees for their very
constructive feedback on the paper. This work was
supported by the Biotechnology and Biological Sci-
ences Research Council and the Natural Environ-
ment Research Council through a Soils Training and
Research Studentships (STARS) grant (Grant Num-
bers NE/M009106/1). STARS is a consortium con-
sisting of Bangor University, British Geological Sur-
vey, Centre for Ecology and Hydrology, Cranfield
University, James Hutton Institute, Lancaster Uni-
versity, Rothamsted Research and the University of
Nottingham.
Author contributions
D L E, J N Q. and J A C D designed research; D L E
performed research; D L E, J N Q and J A C D analysed
data and D L E wrote the paper with contributions
from J N Q, J A C D, J Z, and G G
Competing Interests
The authors declare no competing interests
Data Availability
The authors declare no restrictions on the availability
of materials or information.
ORCID iD
D L Evans
https://orcid.org/0000-0002-4484-7874
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