Cropland Soil Organic Carbon Research Articles

Carbon budget of Finnish croplands — Effects of land use change from natural forest to cropland

The pool of soil organic carbon (SOC) in Finnish croplands has declined during recent decades according to nation-wide soil inventories, but reasons for this trend remain unclear. We studied the possible reasons using an approach based on the Yasso07 soil carbon model. We evaluated also the suitability of this approach for estimating the pool and changes of SOC in boreal croplands at the regional scale. The simulated SOC pool declined in each of the four study regions we divided the country into over the entire study period from 1900 to 2009. During the last 35years, the mean estimate of the decrease rate varied from 0.29 to 0.36Mgha−1year−1 among the regions. The mean estimate of the SOC pool in the croplands varied from 92 to 124Mgha−1 at the end of the study period. In a sensitivity analysis, the estimates of the decrease rate slowed down by 25% or accelerated by 38% at most, whereas the pool estimates increased or decreased by 18% at most. According to our simulations and the sensitivity analysis, the SOC pool declined because croplands produced less litter than pre-cropland forests and this agricultural litter decomposed more rapidly. On the other hand, climate warming has not been a significant reason for the decline yet. Increasing carbon input to the cropland soil by applying organic manure and avoiding bare fallow are means to slow down the loss of SOC. The simulated estimates were similar to measurement-based ones available for comparison. We concluded that our approach was suitable for studying the reasons for the declining trend of SOC and the major uncertainties were caused by inexact input values.

Physical and chemical stabilization of soil organic carbon along a 500-year cultived soil chronosequence originating from estuarine wetlands: Temporal patterns and land use effects

How soil carbon is stabilized during centuries of cultivation and how this would be influenced by land use largely remain unclear. Here the relative role of physical and chemical stabilization mechanisms in agricultural soil organic carbon (SOC) accumulation were studied by fractionation of paddy/upland cropland soils along a 500-year soil chronosequence created by intermittent reclamation of estuarine wetlands. In unreclaimed wetlands, about 50% of SOC was chemically-stabilized by binding to Fe/Al oxyhydrates (mainly amorphous Fe) and 30% by unknown forms of chemical association with silt/clay particles. Physical stabilization of SOC by soil aggregation was negligible in wetlands. After conversion of wetlands to croplands, SOC rapidly declined within the first 16 years and then recovered slowly with cultivation time. Chemical mechanisms still dominated SOC stabilization processes during 500 years of cultivation, but the contribution of Fe/Al-bound and Ca-bound carbon to total SOC decreased with time. The contribution of physically stabilized carbon (i.e. microaggregate-occluded particulate organic carbon, iPOM) to SOC kept around 16% in croplands even when microaggregate contents increased from 8.83% to 30.52% between 16 and 500 years. The iPOM fraction was not closely related to microaggregate formation but to free coarse particulate organic matter, a carbon fraction indicative of inputs of plant materials. Consistently higher SOC density in paddy soils than in upland soils was observed along the chronosequence, which could be accounted for by higher contents of physical and chemical carbon fractions in paddy fields. The higher physically-stabilized carbon of paddy soils probably resulted from larger stubble return rather than from stronger soil aggregation given similar contents of microaggregates between the two cropland types. Notably, in both paddy and upland soils, carbon concentrations of intra-microaggregate silt/clay particles were consistently higher than those of free silt/clay particles. An implication was that despite the small proportion (<20% here) of physically-stabilized carbon to total SOC in croplands, soil aggregation could promote chemical SOC stabilization by creating intimate interactions between occluded carbon and soil minerals within aggregates. To conclude, during five centuries of soil cultivation, chemical carbon stabilization was the dominant mechanism controlling SOC accumulation of paddy/upland croplands but physical occlusion of carbon by aggregation might have promoted chemical stabilization of SOC.

Regional‐scale characterization of the geomorphic control of the spatial distribution of soil organic carbon in cropland

SummaryThe heterogeneity of the spatial distribution of soil organic carbon (SOC) at the landscape scale is generally not considered in regional or national SOC dynamics models. In cropland this heterogeneity is controlled largely by topography, which influences the distribution of water, energy and sediments, and thus the SOC dynamics. Sediment redistribution rates have increased strongly since the mechanization of agriculture. The over‐simplification of landscape processes in regional models of C dynamics may add to the uncertainty in C balances. Therefore, a better characterization of the importance of landscape‐scale effects on the SOC distribution throughout a region is needed. This study characterized the relative importance of geomorphology in the SOC horizontal and vertical variability across croplands in the Belgian loess belt region. A large legacy dataset of soil horizons was exploited together with 147 recently sampled profiles. Mean SOC depth profiles for different soil types were compared. Various topographic attributes were computed from a digital elevation model, and their influence on SOC was quantified through simple linear models. Finally, SOC content was mapped at three depth layers through multiple linear models, and results were cross‐validated. The legacy dataset allowed identification of significant differences in the mean SOC profile according to texture, drainage or profile development classes. A clear relationship between SOC content and topographic attributes was demonstrated, but only for the recently sampled profiles. This may be explained by a substantial error in the location of the profiles of the legacy dataset. This study thus shows evidence that the major control of the vertical distribution of SOC is related to topography in a region where observed heterogeneities for other commonly involved factors are limited. However, the large amount of unexplained variability still limits the usefulness of the spatial prediction of SOC content, and suggests the importance of additional influencing factors.

Impacts of agricultural management and climate change on future soil organic carbon dynamics in North China Plain.

Dynamics of cropland soil organic carbon (SOC) in response to different management practices and environmental conditions across North China Plain (NCP) were studied using a modeling approach. We identified the key variables driving SOC changes at a high spatial resolution (10 km×10 km) and long time scale (90 years). The model used future climatic data from the FGOALS model based on four future greenhouse gas (GHG) concentration scenarios. Agricultural practices included different rates of nitrogen (N) fertilization, manure application, and stubble retention. We found that SOC change was significantly influenced by the management practices of stubble retention (linearly positive), manure application (linearly positive) and nitrogen fertilization (nonlinearly positive) – and the edaphic variable of initial SOC content (linearly negative). Temperature had weakly positive effects, while precipitation had negligible impacts on SOC dynamics under current irrigation management. The effects of increased N fertilization on SOC changes were most significant between the rates of 0 and 300 kg ha−1 yr−1. With a moderate rate of manure application (i.e., 2000 kg ha−1 yr−1), stubble retention (i.e., 50%), and an optimal rate of nitrogen fertilization (i.e., 300 kg ha−1 yr−1), more than 60% of the study area showed an increase in SOC, and the average SOC density across NCP was relatively steady during the study period. If the rates of manure application and stubble retention doubled (i.e., manure application rate of 4000 kg ha−1 yr−1 and stubble retention rate of 100%), soils across more than 90% of the study area would act as a net C sink, and the average SOC density kept increasing from 40 Mg ha−1 during 2010s to the current worldwide average of ∼55 Mg ha−1 during 2060s. The results can help target agricultural management practices for effectively mitigating climate change through soil C sequestration.

Amount, distribution and driving factors of soil organic carbon and nitrogen in cropland and grassland soils of southeast Germany (Bavaria)

Agricultural soils have a high potential for sequestration of atmospheric carbon due to their volume and several promising management options. However, there is a remarkable lack of information about the status quo of organic carbon in agricultural soils. In this study a comprehensive data set of 384 cropland soils and 333 grassland soils within the state of Bavaria in southeast Germany was analyzed in order to provide representative information on total amount, regional distribution and driving parameters of soil organic carbon (SOC) and nitrogen (N) in agricultural soils of central Europe. The results showed that grassland soils stored higher amounts of SOC (11.8kgm−2) and N (0.92kgm−2) than cropland soils (9.0 and 0.66kgm−2, respectively) due to moisture-induced accumulation of soil organic matter (SOM) in B horizons. Surprisingly, no distinct differences were found for the A horizons since tillage led to a relocation of SOM with depth in cropland soils. Statistical analyses of driving factors for SOM storage revealed soil moisture, represented by the topographic wetness index (TWI), as the most important parameter for both cropland and grassland soils. Climate effects (mean annual temperature and precipitation) were of minor importance in agricultural soils because management options counteracted them to a certain extent, particularly in cropland soils. The distribution of SOC and N stocks within Bavaria based on agricultural regions confirmed the importance of soil moisture since the highest cropland SOC and N stocks were found for tertiary hills and loess regions, which exhibited large areas with potentially high soil moisture content in extant floodplains. Grassland soils showed the highest accumulation of SOC and N in the Alps and Pre-Alps as a result of low temperatures, high amounts of precipitation and high soil moisture content in areas of glacial denudation. Soil class was identified as a further driving parameter for SOC and N storage in cropland soils. In total, cropland and grassland soils in Bavaria store 242 and 134Mt SOC as well as 19 and 12Mt N down to a soil depth of 1m or the parent material, respectively.

High organic carbon stock in a karstic soil of the Middle-European Forest Province persists after centuries-long agroforestry management

Little is known on soil organic carbon (SOC) stocks in karst areas worldwide, although many of them have seen long-term application of agroforestry systems with a potential for carbon sequestration. Therefore, our study aimed to assess landscape-level SOC concentration and stock in the Silica Plateau, a part of the Slovak Karst Biosphere Reserve located in the Western Carpathians (Slovakia) with a centuries-long agroforestry record. The most represented local soil units are Chromi-Rendzic Leptosols and Chromic Cambisols with clayey loam texture, C/N ratio 9–12, and \( {\text{pH}}_{{{\text{H}}_{2} {\text{O}}}} \) 6.6–6.2 in their organo-mineral surface horizons. Mull surface humus form prevails under mixed forest stands dominated by hornbeam (Carpinus betulus L.), oak (Quercus petraea L.), and beech (Fagus sylvatica L.). A total of 2,700 soil samples were collected from 150 soil pits. Both SOC concentrations and stocks were determined for the 0–60 cm mineral soil layer. Soil stoniness was accounted for by means of electrical resistivity tomography. According to the analysis of covariance, cropland SOC concentration (0.026 g g−1) is significantly lower compared to forestland (0.040 g g−1) and pastureland (0.041 g g−1) (P < 0.01). During the period of 130 years after forest clearing, cropland SOC stock has been reduced at an exponential decay rate of ca 0.002 year−1, while the SOC stock in pastureland has increased following land use change from cropland by approximately 30% during the same period of time. Irrespective of land use history, overall SOC stock is high reaching on average 207.4 Mg ha−1, out of which 66% are stored within 0–30 cm and 34% within 30–60 cm soil layers.

Agricultural management explains historic changes in regional soil carbon stocks

Agriculture is considered to be among the economic sectors having the greatest greenhouse gas mitigation potential, largely via soil organic carbon (SOC) sequestration. However, it remains a challenge to accurately quantify SOC stock changes at regional to national scales. SOC stock changes resulting from SOC inventory systems are only available for a few countries and the trends vary widely between studies. Process-based models can provide insight in the drivers of SOC changes, but accurate input data are currently not available at these spatial scales. Here we use measurements from a soil inventory dating from the 1960s and resampled in 2006 covering the major soil types and agricultural regions in Belgium together with region-specific land use and management data and a process-based model. The largest decreases in SOC stocks occurred in poorly drained grassland soils (clays and floodplain soils), consistent with drainage improvements since 1960. Large increases in SOC in well drained grassland soils appear to be a legacy effect of widespread conversion of cropland to grassland before 1960. SOC in cropland increased only in sandy lowland soils, driven by increasing manure additions. Modeled land use and management impacts accounted for more than 70% of the variation in observed SOC changes, and no bias could be demonstrated. There was no significant effect of climate trends since 1960 on observed SOC changes. SOC monitoring networks are being established in many countries. Our results demonstrate that detailed and long-term land management data are crucial to explain the observed SOC changes for such networks.

Measuring soil organic carbon in croplands at regional scale using airborne imaging spectroscopy

Conventional sampling techniques are often too expensive and time consuming to meet the amount of data required in soil monitoring or modelling studies. The emergence of portable and flexible spectrometers could provide the large amount of spatial data needed. In particular, the ability of airborne imaging spectroscopy to cover large surfaces in a single campaign and to study the spatial distribution of soil properties with a high spatial resolution represents an opportunity for improving the monitoring of soil characteristics and soil threats such as the decline of soil organic matter in the topsoil. However, airborne imaging spectroscopy has been generally applied over small areas with homogeneous soil types and surface conditions. Here, five hyperspectral images acquired with the AHS-160 sensor (430 nm–2540 nm) were analysed with the objective to map soil organic carbon (SOC) at a regional scale. The study area, covering a surface of ∼ 420 km 2 and located in Luxembourg, is characterized by different soil types and a high variation in SOC contents. Reflectance data were related to surface SOC contents of bare croplands by means of 3 different multivariate calibration techniques: partial least square regression (PLSR), penalized-spline signal regression (PSR) and support vector machine regression (SVMR). The performance of these statistical tools was tested under different combinations of calibration/validation sets (global and local calibrations stratified according to agro-geological zones, soil type and image number). Under global calibration, the Root Mean Square Error in the Predictions reached 5.3–6.2 g C kg − 1 . Under local calibrations, this error was reduced by a factor up to 1.9. SOC maps of bare agricultural fields were produced using the best calibration model. Two map excerpts were shown, which display intra- and inter-field variability of SOC contents possibly related to topography and land management.

Spatial patterns and effects of soil organic carbon on grain productivity assessment in China

AbstractIn this paper, we present an assessment of the content and effects of cropland soil organic carbon (SOC) on grain productivity at the national scale in China using a Web‐based Land Evaluation System. Homogeneous 5 km × 5 km grid data sets of climate, crop, soil and management parameters were created and grain production in 2005 was simulated. Attempts were made to incorporate SOC into the land evaluation procedure and to quantify the potential effects of SOC deficiency on grain productivity. Results were statistically analysed and the modelling approach was validated. National cropland SOC maps were generated. At the national scale, the cropland SOC content averaged 1.20, 0.58, 0.41, 0.31 and 0.26% for the five 20‐cm sections consecutively from the surface downwards. At the regional scale it tended to decline slightly from northeast (1.63%) to southwest (1.11%). On average, 64% of grain yield was lost due to SOC deficiency for the humid provinces and 7% for the arid and sub‐arid ones. Soil management options are suggested based on the simulation results.

Assessing causes of recent organic carbon losses from cropland soils by means of regional-scaled input balances for the case of Flanders (Belgium)

Several recent reports on cropland soil organic carbon (SOC) stock changes throughout Europe indicate a general continuing loss of SOC from these soils. As most arable soils in Europe are not in an equilibrium situation because of past changes in land-use and management practices, shifts in both have been suggested to drive this decline of SOC stocks. A lack of data has prevented the unambiguous verification of the contribution of these factors to SOC loss. First, this study focused on recent evolutions in management options for SOC sequestration in Flanders and showed that despite such practices have increased since 1990, their current contribution is still limited. Strikingly, their expansion is at odds with the reported general losses of SOC (−0.48 t OC ha−1 year−1 on average). We used very detailed datasets of livestock numbers, N-application rates and cropping surfaces to calculate regional shifts in input of effective OC from animal manure application, cereal straw incorporation and crop residue incorporation which amounted to −0.094, −0.045 and −0.017 t OC ha−1 year−1, respectively. Shifts in management were identified to have potentially brought about but a third of the recent loss of SOC in the study area, although for central West-Flanders and the Eastern border of Flanders larger impacts of management were observed. This study suggests other influences such as land-use change and climate change to be involved as well. We estimated that another 10%–45% of the loss of SOC could potentially be attributed to land-use changes from grassland to cropland during the 1970–1990 period and about 10% to the observed temperature increase. While being a regional-scaled case study, these findings may be relevant to other European regions in particular (Denmark, The Netherlands, North-West Germany, Brittany and the North-West of France, the Po-valley in Italy and parts of England), with similar climate and intensity of agriculture, and where comparable trends in farming management may well have taken place.

Estimations of soil organic carbon storage in cropland of China based on DNDC model

Loss of soil organic carbon (SOC) can cause soil degradation, which may not only undermine soil productivity, but may also-affect environmental health. In China, a huge amount of crop residues is regularly removed from the fields, and therefore China's agriculture depends on high levels of chemical fertilizer inputs. This paper aims to estimate the SOC storage in Chinese cropland, identify its changing trends under current cropping systems, and finally put forward some strategies to keep the SOC in balance. A computer simulation model of carbon and nitrogen biogeochemistry in agro-ecosystems (DeNitrification and DeComposition or DNDC) was applied to predict SOC dynamics in the upper (0–30 cm) soil layer of agricultural ecosystems at national scale. Data on climate, soil properties, cropping systems, acreage, and management practices at county scale were collected from various sources and integrated into a GIS database to support the model runs. The model results revealed (1) the total SOC storage in croplands in China is about 3968 Tg C; and (2) SOC is lost at a rate of 78.89 Tg C/year. The highest losses of SOC occur in the northeastern provinces. Chinese cropland soils release 186 Tg C as carbon dioxide into the atmosphere, and receive only 68 Tg C from crop residues annually. Considering the potential of global warming, SOC loss in cropland could be a serious contributor. Strategies to reduce the loss of SOC in Chinese cropland are proposed based on DNDC model runs for a number of scenarios under different management practices.

Projected changes in mineral soil carbon of European croplands and grasslands, 1990-2080.

We present the most comprehensive pan-European assessment of future changes in cropland and grassland soil organic carbon (SOC) stocks to date, using a dedicated process-based SOC model and state-of-the-art databases of soil, climate change, land-use change and technology change. Soil carbon change was calculated using the Rothamsted carbon model on a European 10 × 10' grid using climate data from four global climate models implementing four Intergovernmental Panel on Climate Change (IPCC) emissions scenarios (SRES). Changes in net primary production (NPP) were calculated by the Lund-Potsdam-Jena model. Land-use change scenarios, interpreted from the narratives of the IPCC SRES story lines, were used to project changes in cropland and grassland areas. Projections for 1990-2080 are presented for mineral soil only. Climate effects (soil temperature and moisture) will tend to speed decomposition and cause soil carbon stocks to decrease, whereas increases in carbon input because of increasing NPP will slow the loss. Technological improvement may further increase carbon inputs to the soil. Changes in cropland and grassland areas will further affect the total soil carbon stock of European croplands and grasslands. While climate change will be a key driver of change in soil carbon over the 21st Century, changes in technology and land-use change are estimated to have very significant effects. When incorporating all factors, cropland and grassland soils show a small increase in soil carbon on a per area basis under future climate (1-7 t C ha-1 for cropland and 3-6 t C ha-1 for grassland), but when the greatly decreasing area of cropland and grassland are accounted for, total European cropland stocks decline in all scenarios, and grassland stocks decline in all but one scenario. Different trends are seen in different regions. For Europe (the EU25 plus Norway and Switzerland), the cropland SOC stock decreases from 11 Pg in 1990 by 4-6 Pg (39-54%) by 2080, and the grassland SOC stock increases from 6 Pg in 1990 to 1.5 Pg (25%) under the B1 scenario, but decreases to 1-3 Pg (20-44%) under the other scenarios. Uncertainty associated with the land-use and technology scenarios remains unquantified, but worst-case quantified uncertainties are 22.5% for croplands and 16% for grasslands, equivalent to potential errors of 2.5 and 1 Pg SOC, respectively. This is equivalent to 42-63% of the predicted SOC stock change for croplands and 33-100% of the predicted SOC stock change for grasslands. Implications for accounting for SOC changes under the Kyoto Protocol are discussed.

Soil carbon stocks of Jordan and projected changes upon improved management of croplands

Inventories of carbon stocks and projected changes at national scale are needed in the context of the Framework Convention on Climate Change (UNFCCC), but uncertainties in the necessary soil data remain large. Soil organic carbon (SOC) and inorganic carbon (SIC) reserves in Jordan were estimated using a revised 1:500 000 soil and terrain database, providing baseline data. The 95% confidence limits for the median, total stock of carbon to 1 m depth were estimated at 1877–1896 Tg C (Tg = 10 12 g or million tonnes). About 7% thereof is organic, reflecting the predominantly arid conditions in the Kingdom. Many cultivated areas have been degraded by inappropriate management and overgrazing. An empirical approach that considers spatial data on climate, soil conditions and land cover was used to estimate the ecologically and technically feasible gains in SOC in Jordan's croplands over 25 years of sustained, adapted management. The projected gains for the reference scenario were estimated at 25–124 Gg C (Gg = 10 9 g or thousand tonnes). This would correspond to an estimated annual mitigation potential of < 0.5% of Jordan's CO 2–C emissions from fossil fuels and land use change for 1990, the baseline year for Kyoto Protocol reporting. This exploratory study suggests that follow-up studies that use dynamic C-models should focus on scenarios for the conservation and rehabilitation of Jordan's degraded rangelands.

Assessment of soil organic carbon in semi-arid Sudan using GIS and the CENTURY model

Using the UNFCCC as a basis, and the objectives of estimating soil organic carbon (SOC) changes during the period 1900–2100, a spatially explicit database of climate, land cover and soil texture was compiled for a 262,000 km 2 region in semi-arid Sudan. The area is characterized by low input cultivation of millet, sorghum and sesamé combined with livestock grazing. By integrating the database with the CENTURY ecosystem model, we were able to estimate historical, current and future pools of SOC as a function of land management and climate. The SOC (upper 20 cm) decrease from 1900 to 2000 was estimated to be 6·8 Mt and the maximum potential carbon sink (SOC increase) for the period 2000 to 2100 was estimated to be 17 Mt. Cropland and grassland lost 293 and 152 t SOC km −2 respectively whereas the savannahs gained 76 t SOC km −2 from 1900 to 2000. The SOC sequestration scenario simulated during 2000–2100 recovered 94, 84 and 75 t km −2 for cropland, grassland and savannah respectively. In addition to climate and soils, cropping intensity, fallow periods, fire frequency and grazing intensity also influence cropland SOC variation. Grassland and savannah SOC variations depend on grazing intensity and fire return interval. Land management may affect future amounts of SOC in semi-arid areas thereby turning them from sources into sinks of carbon. SOC estimates were reasonably consistent with measurements ( r 2=0·70, n=13).

Pool Size and Mean Age of Stable Soil Organic Carbon in Cropland

AbstractMost modern surface soils are believed to contain a pool of very stable soil organic carbon (SOC) with a mean age (MA) that may reach several thousand years. The remaining SOC pools are much younger, perhaps no older than a few decades. The SOC can thus be divided into two major classes: very old (stable pool) and relatively young (active pool). Because active and stable SOC play very different roles in soil organic‐matter dynamics and nutrient cycling, it is desirable to know the pool size and turnover time of the two fractions in an ecosystem. Unfortunately, none of the physical or chemical fractionation methods can satisfactorily separate active from stable SOC. The pool size and MA of the stable SOC pool in the surface soil of the Morrow Plots (University of Illinois) and Sanborn Field (University of Missouri) were determined using the concept of management‐induced differential decomposition of active SOC in a pair of soil samples and the principle of radiocarbon dating. Pool size and MA of the stable SOC in the surface soil (0–20 cm) was estimated at 11.0 g/kg and 2973 yr for the Morrow Plots and 7.3 g/kg and 853 yr for Sanborn Field, respectively.

Pool Size and Mean Age of Stable Soil Organic Carbon in Croplands

Pool Size and Mean Age of Stable Soil Organic Carbon in Croplands