Croplands are vital ecosystems for human well-being and provide important ecosystem services such as crop yields, retention of nitrogen and carbon storage. On large (regional to global)-scale levels, assessment of how these different services will vary in space and time, especially in response to cropland management, are scarce. We explore cropland management alternatives and the effect these can have on future C and N pools and fluxes using the land-use-enabled dynamic vegetation model LPJ-GUESS (Lund–Potsdam–Jena General Ecosystem Simulator). Simulated crop production, cropland carbon storage, carbon sequestration and nitrogen leaching from croplands are evaluated and discussed. Compared to the version of LPJ-GUESS that does not include land-use dynamics, estimates of soil carbon stocks and nitrogen leaching from terrestrial to aquatic ecosystems were improved.
Our model experiments allow us to investigate trade-offs between these ecosystem services that can be provided from agricultural fields. These trade-offs are evaluated for current land use and climate and further explored for future conditions within the two future climate change scenarios, RCP (Representative Concentration Pathway) 2.6 and 8.5. Our results show that the potential for carbon sequestration due to typical cropland management practices such as no-till management and cover crops proposed in previous studies is not realised, globally or over larger climatic regions. Our results highlight important considerations to be made when modelling C–N interactions in agricultural ecosystems under future environmental change and the effects these have on terrestrial biogeochemical cycles.
Growing population along with rapidly changing dietary preferences
pose one of the key economical and environmental challenges of this century
Yield increases on existing land may be
achieved through further development of high-yielding varieties or
through further improvements in the efficiency of agricultural
practices, the latter especially in regions where gaps between
actual and potential yields are large
Due to their large areal extent, agricultural ecosystems have substantially
altered global biogeochemical cycles
However, beyond the importance of land use and land-use change for understanding
the global past and future carbon balance, other aspects of crop management
also need to be investigated on large scales since the associated environmental
effects have often been detrimental. Negative impacts have been noted for
biodiversity and water quality and for the substantial emissions of N
trace gases that affect air quality and climate, such as nitrous
oxide (N
Today's knowledge about the effects of interactions between
global nitrogen and carbon cycles in terrestrial ecosystems is largely
based on simulations with DVMs representing potential natural
vegetation
Even though applied on the local to regional scale,
land-management practices often have a large regional to global impact:
via water pollution, greenhouse gas emissions or indirect land-use change
Appropriate nutrient management can increase SOC
sequestration
In this study we employ the land-use-enabled version of a global
DVM, LPJ-GUESS (Lund–Potsdam–Jena General Ecosystem Simulator)
LPJ-GUESS
Soil C–N dynamics in LPJ-GUESS are based on the Century model
The present study uses the managed land version of the model
The allocation of C and N for the CFTs in the C–N version of
LPJ-GUESS
At present, the C–N version of LPJ-GUESS is limited to three CFTs, which are based on wheat and maize growth characteristics: a C3
crop with dynamic selection between spring and autumn sowing
(represented here by winter wheat, WW), a C3 crop with sowing
carried out in spring (spring wheat, SW) and a C4 crop (maize, MA).
Allocation for SW and WW is described in
LPJ-GUESS has been evaluated against a range of experimental and
observational data types, e.g. CO
The cropland management options implemented in LPJ-GUESS are sowing, irrigation, tillage, N application, cover crops and residue management. The latter four options are relevant for this study and will be described below.
Tillage is implemented using a tillage factor (
Fertilisers are applied as mineral N
Here we have extended the available N-fertiliser application
management options to also include manure application in the
first of the three events (DS
The amount of manure is derived using the mineral N-application rate but applying the
increase in the metabolic and structural SOM pools rather than in the mineral N pool, with a C : N of 30.
This means that 30 units of C are also added for every unit of N.
The C : N has been chosen to represent the C and N
content in manure from sources ranging from poultry waste
(C : N
Summary of simulation experiments; for abbreviations and further explanations, see text.
Cover crops are intermediate crops that are grown in-between the
main agricultural growing seasons either as
a fallow that stretches over the subsequent growing season or
within the same year
In our implementation, cover crops are grown in-between two
growing periods of the generic main crop used if the crop-free
period is longer than 15 days. At the time of sowing of
the subsequent main crop, the cover-crop biomass is added to the
soil litter pool. C and N allocation of the cover crop is done
daily, with a leaf-to-root ratio that depends on the plant water
status. In case of water stress, a functional balance response
is introduced and allocation to roots increases relative to
leaves. Cover crops are modelled as grasses, being “planted”
with an initial C mass of 0.01 kg C m
A measure to increase the soil fertility and decrease the water
loss, in particular in arid areas, is to leave the residues on
the ground after harvest
Our study is divided into two parts. In the first part we test the
ability of LPJ-GUESS to simulate present-day soil C and the yield
response to management by comparing simulated results with
data sets of soil C in crop fields, potential C sequestration after
a change in management, and global yield statistics. In the
second part of the study, we investigate the efficacy of
alternative crop management options described in
Sect.
For the simulation of the recent historic period (1901–2006),
gridded monthly mean observations of precipitation, air
temperature and cloudiness from CRU (Climate Research Institute;
For all simulations soil C and N pools were initialised with a “spin-up” using atmospheric [CO
N atmospheric deposition was provided as decadally varying monthly
averages from the ACCMIP (Atmospheric Chemistry and Climate Model Intercomparison Project) data set
As N-fertiliser input for the croplands, data from
Land cover information was adopted from
As soil input, a soil map with fractions of clay, silt and sand from the
WISE 3.0 (Wide-field Infrared Survey Explorer, version 3) data set
To evaluate LPJ-GUESS ability to simulate soil C density and sequestration,
soil columns from croplands in the WISE 3.0 data set
In
The effect of the different management strategies considered (no-tillage (NT), manure application (MN), cover crops (CC), leaving all
residues (NR); Table
To be able to compare our results with previous estimates of
global soil C and N pools and N leaching from LPJ-GUESS
LPJ-GUESS wheat (C3) and maize (C4) yields were simulated using the
gridded N-fertiliser data set FAOSTAT,
Simulation settings used for the comparison of soil C, yields and N
leaching with different agricultural management types. For implementation of
full vs. moderate tillage, see Sect.
The total simulated production (wet weight) of all agricultural
crops (including cereals, tubers and pulses) of 2.7 Gt was
within 30 % of what is reported to the FAO: 3.5 Gt
for the period 1996–2005 (cereals, 2.12 Gt; coarse grain, 0.93 Gt; roots
and tubers, 0.28 Gt Corrected for moisture content; value from
FAOSTAT: 0.68 Gt.
Per-country comparison of simulated yields for WW (wheat) and MA (maize) against reported yields from FAO (1996–2005). Marker size indicates each country's total production. The top six producer countries of both crops are labelled with the following abbreviations: ARG – Argentina; BRA – Brazil; CAN – Canada; CHN – China; FRA – France; IND – India; MEX – Mexico; RUS – Russia; USA – United States.
Simulated soil C pools (0–1.5 m) for the selected grid
cells (Sect.
Simulated mean C sequestration following the implementation of
management over the CRU historic period on agricultural soils averaged (thick
lines) for the selected grid cells in the four climatic regions, compared to
estimates (vertical lines) from
In Fig.
The simulated relative response (%) of soil carbon to management
options (Table
Soil C pools (0–1.5 m) in four climatic regions, observed
The simulated management options resulted in an increase in
cropland soil C, for all climatic regions
(Fig.
In general, the soil C pools simulated with the managed land version of LPJ-GUESS
were slightly larger than simulated with PNV (Table
From the simulations of different cropland management options, the
management combination that yielded the largest SOC stocks for the period
1996–2005 was chosen for each grid cell (
Modelled global, total land and cropland soil C and N stocks and
total N leaching (inorganic and organic) for the time period 1996–2005,
compared to estimates from the literature. References for the studies and
explanations of how some of the values were derived can be found in the notes
of this table. See Table
Optimal carbon sequestration practice (
For the future simulations, there were changes in the optimal C
sequestration management (Table
The simulated response on
Figure
Grid cells where different management options resulted in the
highest soil carbon in 2000 (Fig.
LPJ-GUESS projections of soil N pools agree well with other estimates, although the
soil C pools are at the low end of generally reported global
estimates
When making projections on global C pools, the information on
land-use history is vital
When focusing on site data collected for croplands and grouped by four climate
regions (Table
Global-scale modelling of the impacts of specific land-management options is in its infancy, but since a number of future climate and socioeconomic scenarios highlight the importance of land-based mitigation and because of the multiple trade-offs that exist with other ecosystem services, they are of importance for future research and practical applications.
In the comparison with potential C sequestration (Fig.
The relative (%) number of cropland grid cells with a shift
regarding the management practice optimal from a carbon sequestration
perspective, comparing the highest SOC for 1996–2005 and for 2046–2055 for
RCP2.6 and 8.5. Also listed are the share of the cropland grid cells with no
change in the optimal C sequestration practice and the percentage of the
total number of grid cells that were cropland around the year 2000 in the
data set from
Another important aspect is productivity during the growing season and
the possibility of multicropping. In many tropical areas the
growing season is not limited to a short period of the year,
especially in the humid tropics where two or more crops may be
grown in sequence
Compared to other measures of global C flows, statistics on crop
production and yields are relatively accessible and encompass
relatively long time series, albeit with differing qualities between
individual countries. While yield is not a direct measure of the
net primary productivity (NPP), it is a good proxy for trends and
variability in carbon flows on croplands
Our results compare favourably with these studies for WW but less so for MA. The C–N version of our model has not yet been evaluated and parameterised against observations of maize yields, and the lower degree of agreement with data was expected.
We also compared historical global crop yields with numbers found in FAOSTAT.
Yields in the early 1960s were similar (ca. 1.5 t ha
The modelling approach taken here to represent all crops globally
with three CFTs introduces an uncertainty into the estimates of
global food production and thus also into the carbon cycle.
We expect that this would be most prominent for crops whose growing seasons,
water requirements, or physiology differ substantially from the functional
types used here, e.g. regions where rice (south-east Asia) or tubers (Africa)
are grown over a large section of harvested area.
In many rice-producing regions, a second growing season is
often present each year
Global estimates of N leaching from terrestrial ecosystems are
uncertain Derived by scaling their
average 0.47 g N m Derived from the leaching-associated N
Due to the rising human population, changing lifestyles, as well
as a number of – sometimes conflicting – policies related to,
e.g., climate change mitigation, agriculture, conservation or
water regulation, the demand for resources from land ecosystems is
increasing and also constantly changing. In order to achieve,
ultimately, a sustainable use of natural resources, there is a need
to identify strategies that minimise degradation and wastage of
resources while still addressing society's growing needs for
land-based ecosystem services including agricultural production.
To this end, information on the trade-offs implicit in different
management strategies but also possible win-win situations is of
high value. In our analysis we attempted to compare three
important parameters related to ecosystem functioning (yield, C
uptake and N leaching) in terms of how different forms of crop
management may be expected to influence their relative patterns of
change. From our results (Fig.
All the implemented management options targeting carbon benefits
resulted in a net increase in simulated soil carbon
(Fig.
Absence of residue removal was positive for soil carbon as well
as for yields because of the higher litter input. Similar
responses of enhanced C storage (up to 30 %) and
increased yields (10–30 %) to residue
management have also been found for, e.g., maize and soybean in the
US
Manure (
By contrast with moderate tillage, complete absence of tillage
resulted in enhanced soil C, with only small to moderate yield
reduction and a small reduction in N loss through leaching.
Depending on the regional climate and N-fertiliser applications,
reductions in crop productivity by up to 0.5 t ha
The initial difference between
We have presented a global model analysis highlighting effects of alternative crop management strategies for a range of core ecosystem processes and the services derived from them, related to interactions of climate change and land-use change.
Our large-scale approach based on the simplifying assumption of uniform management across regions does not faithfully represent actual conditions but instead allows the influence of different management actions to be evaluated and geographical differences to be highlighted. The model is equipped to perform simulations with more detailed (country-scale or regional) management, and it can thus be used in applications addressing questions of the environmental impact from, for instance, policies or trends relating to agricultural intensification or extensification or climate mitigation.
Results demonstrate that the effects of management on cropland can be beneficial for carbon and nutrient retention without risking (large) yield losses. Nevertheless, effects on soil carbon are small compared with extant stocks in natural and semi-natural ecosystem types and managed forests. While agricultural management can be targeted towards sustainable goals, from a climate change or carbon sink perspective avoiding deforestation or reforestation constitutes a far more effective overall strategy for maintaining and enhancing global carbon sinks. However, enhanced carbon storage in agricultural soils could also be seen as a surrogate for enhanced soil structure and reduced erosion having additional (non-climate) environmental benefits.
In
The relative relationships of daily assimilate allocation to the
organs are described by Eq. (
The parameters for the factors
CFT-specific parameters of specific leaf area (SLA), minimum C : N value
of the leaves and the amount of the total N that is applied at the different
developmental stages (DS), where DS
Input used for the simulations over the historic and scenario RCP2.6
and 85 periods:
Global simulated mean crop production using the GCM climate (blue – RCP2.6; red – RCP8.5) compared to statistics from FAOSTAT (black).
Climatic regions, as defined in Sect.
This study contributes to the Strong Research Environment Land-Use Today and Tomorrow funded by the Swedish Research Council FORMAS (Contract No. 211-2009-1682), to the Strategic Research Areas BECC and MERGE, and to projects within the Lund University Centre for Studies of Carbon Cycle and Climate Interactions (LUCCI). A. Arneth and T. A. M. Pugh acknowledge support from the European Commission in the FP 7 projects OPERAS (grant no. 308393) and LUC4C (grant no. 603542). M. Lindeskog was funded by the Mistra Swedish Research Programme for Climate, Impacts and Adaptation. Edited by: S. Dekker