Movement Of Soil Organic Carbon Research Articles

Soil Organic Carbon Lateral Movement Processes Integrated Into a Terrestrial Ecosystem Model

AbstractLateral movement of soil organic carbon (SOC) induced by soil erosion and runoff changes spatial distributions of SOC, and further changes the land‐atmosphere CO2 exchange and terrestrial carbon budget. However, current ecosystem models do not or only poorly integrate the process of SOC lateral movement and cannot simulate the impacts of soil erosion on the carbon cycle. This study integrated SOC erosion and deposition processes into a process‐based ecosystem model (i.e., Integrated BIosphere Simulator (IBIS)), and separately simulated the lateral movements of dissolved organic carbon (DOC) and particulate organic carbon (POC). The model was evaluated in three river basins in Northeast China that are dominated by cropland, forest, and grassland. The results showed that the model reproduced well the production, erosion, and deposition of DOC and POC. The annual SOC lateral movement (1.34–7.22 g C m−2 yr−1) induced by erosion in the three tested basins was 0.27%–1.45% of the annual net primary production. The model developed in this study has great implications for simulating the lateral movements of SOC in terrestrial ecosystems, which can improve model performance in projecting the terrestrial carbon budget.

Estimation of Permafrost SOC Stock and Turnover Time Using a Land Surface Model With Vertical Heterogeneity of Permafrost Soils

AbstractWe developed vertically resolved soil biogeochemistry (carbon and nitrogen) module and implemented it into a land surface model, ISAM. The model captures the vertical heterogeneity of the northern high latitudes permafrost soil organic carbon (SOC). We also implemented Δ14C to estimate SOC turnover time, a critical determinant of SOC stocks, sequestration potential, and the carbon cycle feedback under changing atmospheric CO2 concentration [CO2] and climate. ISAM accounted for the vertical movement of SOC caused by cryoturbation and its linkage to frost heaving process, oxygen availability, organo‐mineral interaction, and depth‐dependent environmental modifiers. After evaluating the model processes using the site and regional level heterotrophic respiration, SOC stocks, and soil Δ14C profiles, the vertically resolved soil biogeochemistry version of the model (ISAM‐1D) estimated permafrost SOC turnover time of 1,443 years, which is about 3 times more than the estimation based on the without vertically resolved version of ISAM (ISAM‐0D). ISAM‐1D‐simulated SOC stocks for permafrost regions was 319 Pg C in the top 1 m soil depth by the 2000s, about 80% higher than the estimates based on ISAM‐0D. ISAM‐1D SOC stock and turnover time were compared well with the observations. However, the longer SOC turnover time preserves less SOC stocks due to the lower carbon use efficiency (CUE) for SOC than ISAM‐0D and thus respires more SOC than being transferred downward by cryoturbation. ISAM‐1D simulated reduced SOC sequestration (3.7 Pg C) compared to ISAM‐0D (4.8 Pg C) and published Earth system models (ESMs) over the 1860s–2000s, due to weaker [CO2]‐carbon cycle and stronger climate‐carbon cycle feedbacks, highlighting the importance of the vertically heterogeneous soil for understanding the permafrost SOC sinks.

Warming-induced global soil carbon loss attenuated by downward carbon movement.

The fate of soil organic carbon (SOC) under warming is poorly understood, particularly across large extents and in the whole-soil profile. Using a data-model integration approach applied across the globe, we find that downward movement of SOC along the soil profile reduces SOC loss under warming. We predict that global SOC stocks (down to 2m) will decline by 4% (~80Pg) on average when SOC reaches the steady state under 2°C warming, assuming no changes in net primary productivity (NPP). To compensate such decline (i.e. maintain current SOC stocks), a 3% increase of NPP is required. Without the downward SOC movement, global SOC declines by 15%, while a 20% increase in NPP is needed to compensate that loss. This vital role of downward SOC movement in controlling whole-soil profile SOC dynamics in response to warming is due to the protection afforded to downward-moving SOC by depth, indicated by much longer residence times of SOC in deeper layers. Additionally, we find that this protection could not be counteracted by promoted decomposition due to the priming of downward-moving new SOC from upper layers on native old SOC in deeper layers. This study provides the first estimation of whole-soil SOC changes under warming and additional NPP required to compensate such changes across the globe, and reveals the vital role of downward movement of SOC in reducing SOC loss under global warming.

Estimating soil organic carbon redistribution in three major river basins of China based on erosion processes

Soil erosion by water affects soil organic carbon (SOC) migration and distribution, which are important processes for defining ecosystem carbon sources and sinks. Little has been done to quantify soil carbon erosion in the three major basins in China, the Yangtze River, Yellow River and Pearl River Basins, which contain the most eroded areas. This research attempts to quantify the lateral movement of SOC based on spatial and temporal patterns of water erosion rates derived from an empirical Unit Stream Power Erosion Deposition Model (USPED) model. The water erosion rates simulated by the USPED model agreed reasonably with observations (R2 = 0.43, P < 0.01). We showed that regional water erosion ranged within 23.3–50 Mg ha–1 year–1 during 1992–2013, inducing the lateral redistribution of SOC caused by erosion in the range of 0.027–0.049 Mg C ha–1 year–1, and that caused by deposition of 0.0079–0.015 Mg C ha–1 year–1, in the three basins. The total eroded SOC was 0.006, 0.002 and 0.001 Pg year–1 in the Yangtze River, Yellow River and Pearl River Basins respectively. The net eroded SOC in the three basins was ~0.0075 Pg C year–1. Overall, the annual average redistributed SOC rate caused by erosion was greater than that caused by deposition, and the SOC loss in the Yangtze River Basin was greatest among the three basins. Our study suggests that considering both processes of erosion and deposition – as well as effects of topography, rainfall, land use types and their interactions – on these processes are important to understand SOC redistribution caused by water erosion.

Soil organic carbon distribution in relation to terrain & land use—a case study in a small watershed of Danjiangkou reservoir area, China

Understanding the responses of soil organic carbon (SOC) distribution to terrain and land use variations is important for making corresponding measures to enhance the soil quality and its carbon storage. The objective of this study was to characterize the spatial distribution of SOC, and analyze the impacts of environment factors on spatial variation of SOC along the Danjiangkou Reservoir area, China. A village size watershed was selected to investigate the SOC distribution under different terrains and land uses. We found that SOC contents followed a log-normal distribution with arithmetic and geometric means of 6.83 g kg −1 and 6.09 g kg−1, respectively, which were moderately variable (Coefficient of Variation = 49.6%) and demonstrated a moderate spatial dependence according to the nugget ratio (37.8%). The experimental variogram of SOC was best-fitted by a spherical model after the spatial outliers had been detected and subsequently eliminated. From the spatial distribution map of SOC contents, the highest SOC content occurred in the center of the watershed, which were along the main water channel or tributaries and normally had lower altitude. While,the lower values of SOC contents were located at the edge of the watershed, such as in the northwest, southwest and southeast of the area where citrus were normally planted. This spatial distribution pattern of SOC was generally consistent with the spatial structure of terrain and land use on the unique terrain of the agriculture-based reservoir area. Therefore, taking terrain and land use type into account when estimating the spatial variation of SOC pool would increase the accuracy in prediction and modeling of SOC movement at the watershed scale.

Soil organic carbon and soil erosion – Understanding change at the large catchment scale

Soil organic carbon (SOC) is a major soil component. However, there is still much to learn regarding its spatial and temporal distribution as well as how SOC moves through the landscape. Of particular interest is how SOC movement is related to soil erosion and deposition. Here we examine the spatial distribution of SOC over two large (562 and 606 km2) catchments in relation to soil erosion and deposition. We found that the spatial distribution of SOC concentration on average is stable (over an eight year period) for the two study catchments. However, differences were found in SOC when concentrations were compared between samples collected in 2006 and 2014. The environmental tracer caesium-137 (137Cs) was used to assess erosion and deposition patterns across the study catchment and a significant relationship was found between SOC change and erosion and deposition at each sample point. That is, locations with an increase in SOC corresponded with an increase in 137Cs concentration (depositional sites) while locations with a decrease in SOC corresponded with a decrease in 137Cs concentration (erosional sites). A Monte-Carlo assessment confirmed these results. The driver of the SOC change and soil movement corresponds with the largest rainfall event (since 1969) in the area. The results suggest that SOC can be translocated by significant rainfall events. The findings provide insight into how catchments may respond to stronger and more frequent storm events.

Storage and spatial patterns of organic carbon of soil profiles in Guangdong Province, China

Regional soil organic carbon (SOC) investigations play an important role in building knowledge of the global soil C cycle system. The purpose of the present study was to estimate soil organic carbon (SOC) storage for different soil types and land uses in Guangdong Province, China. The results showed that the total SOC storage in the study area was 1.25 Pg, of which 0.41 Pg SOC was in A horizon soils (mean depth 17.0cm), 0.51 Pg SOC was in the B horizon (mean depth 29.5cm) and 0.33 Pg SOC was in the C horizon (mean depth 48.9cm). SOC storage in Ferrallisols was approximately 0.976 Pg for the total soil profile, accounting for 78.1% of total SOC storage. Forest soils were the main SOC pool by land use, accounting for approximately 80.3% of total SOC storage. Regardless of soil type and land use, subsoil was the primary SOC storage location in the study area. The SOC contents of the upper soil horizon were closely related to the SOC contents of the lower soil horizon, possibly suggesting that there is movement of SOC from the surface soil to lower horizons. Because of soil degradation and erosion, approximately 13.3 Tg SOC entered the surrounding water, accounting for 3.2% of the SOC storage of A horizon soils, and approximately 20.9 Tg SOC was redistributed in surface soils of the study area each year.

Soil organic carbon loss and selective transportation under field simulated rainfall events.

The study on the lateral movement of soil organic carbon (SOC) during soil erosion can improve the understanding of global carbon budget. Simulated rainfall experiments on small field plots were conducted to investigate the SOC lateral movement under different rainfall intensities and tillage practices. Two rainfall intensities (High intensity (HI) and Low intensity (LI)) and two tillage practices (No tillage (NT) and Conventional tillage (CT)) were maintained on three plots (2 m width × 5 m length): HI-NT, LI-NT and LI-CT. The rainfall lasted 60 minutes after the runoff generated, the sediment yield and runoff volume were measured and sampled at 6-min intervals. SOC concentration of sediment and runoff as well as the sediment particle size distribution were measured. The results showed that most of the eroded organic carbon (OC) was lost in form of sediment-bound organic carbon in all events. The amount of lost SOC in LI-NT event was 12.76 times greater than that in LI-CT event, whereas this measure in HI-NT event was 3.25 times greater than that in LI-NT event. These results suggest that conventional tillage as well as lower rainfall intensity can reduce the amount of lost SOC during short-term soil erosion. Meanwhile, the eroded sediment in all events was enriched in OC, and higher enrichment ratio of OC (ERoc) in sediment was observed in LI events than that in HI event, whereas similar ERoc curves were found in LI-CT and LI-NT events. Furthermore, significant correlations between ERoc and different size sediment particles were only observed in HI-NT event. This indicates that the enrichment of OC is dependent on the erosion process, and the specific enrichment mechanisms with respect to different erosion processes should be studied in future.

Comparison of vertical transport of 137Cs and organic carbon in agricultural cracking soils

The movement of soil organic carbon (SOC) down the soil profile has been compared to that of the environmental tracer 137Cs at two neighbouring field sites sharing the same land management history but comprising clay soils with different cracking characteristics (cracking black Vertisol and a red Luvisol). A finite element model (FEM) simulation of the vertical transport of SOC and 137Cs was developed for each site which accommodates the differing spatial and temporal trends of input and decay of the two species. From these models the diffusion and convection coefficients which best describe the movement of 137Cs at each site were determined. Both convection and diffusion coefficients were found to be substantially higher in cracking Vertisol soils (DCs=721mm2/yr, VCs=−0.84mm/yr) than in the neighbouring Luvisol soils (DCs=94mm2/yr, VCs=0mm/yr). Finally the 137Cs transport coefficients determined for each site were used in modelling the SOC profile. The excellent match between predicted and observed SOC profiles suggests that transport of the SOC and 137Cs down the soil column at the Luvisol site follows the same pathways. While the match between predicted and observed SOC profiles at the Vertisol site was weaker this was concluded to be more likely due to the impact of extensive soil cracking which is not explicitly accounted for in the SOC FEM rather than the result of the use of 137Cs transport coefficients.

Application of RothPC-1 to soil carbon profiles in cracking soils under minimal till cultivation

This study examines soil organic carbon (SOC) profiles in two neighbouring field sites (‘Dog’ and ‘Shed’ paddocks) which share a similar land use history but are characterised by different soil types (euchrozem and vertosol). SOC inventories to 30cm depth at the two sites were similar however enhanced transport of SOC in the cracking vertosol soils of the Shed paddock produced a significantly flatter SOC profile and higher whole profile SOC inventories at this site. No clear relationship was observed between clay content and SOC distribution at either sample site. Modelling of SOC turnover using RothPC-1 yielded a satisfactory fit to the observed SOC profiles however the soil moisture and temperature values calculated within the model were found to be at odds with 5year field observations reported from the site. When field measured values for the moisture and temperature were substituted into the model the fit to the observed SOC profiles was significantly poorer indicating that the purely convective transport approach used to model SOC movement within RothPC-1 may not be sufficient to model vertical SOC transport in cracking soils. The results highlight the need for a cautious approach when using RothPC-1 in arid, cracking soil environments.

Uncertainty in soil carbon accounting due to unrecognized soil erosion

The movement of soil organic carbon (SOC) during erosion and deposition events represents a major perturbation to the terrestrial carbon cycle. Despite the recognized impact soil redistribution can have on the carbon cycle, few major carbon accounting models currently allow for soil mass flux. Here, we modified a commonly used SOC model to include a soil redistribution term and then applied it to scenarios which explore the implications of unrecognized erosion and deposition for SOC accounting. We show that models that assume a static landscape may be calibrated incorrectly as erosion of SOC is hidden within the decay constants. This implicit inclusion of erosion then limits the predictive capacity of these models when applied to sites with different soil redistribution histories. Decay constants were found to be 15-50% slower when an erosion rate of 15tsoilha(-1) yr(-1) was explicitly included in the SOC model calibration. Static models cannot account for SOC change resulting from agricultural management practices focused on reducing erosion rates. Without accounting for soil redistribution, a soil sampling scheme which uses a fixed depth to support model development can create large errors in actual and relative changes in SOC stocks. When modest levels of erosion were ignored, the combined uncertainty in carbon sequestration rates was 0.3-1.0t CO2 ha(-1) yr(-1) . This range is similar to expected sequestration rates for many management options aimed at increasing SOC levels. It is evident from these analyses that explicit recognition of soil redistribution is critical to the success of a carbon monitoring or trading scheme which seeks to credit agricultural activities.

Dual roles of tillage erosion in lateral SOC movement in the landscape

The facts that the global carbon budget cannot be currently balanced and current estimates of agricultural sources and sinks may be inaccurate, may be linked to unaccounted‐for erosion‐induced changes in soil organic carbon (SOC). A closed landscape with field banks and an open landscape without field banks were selected from two sites located in Jianyang County, Sichuan Province, and Zhongxian County, Chongqing Municipality, respectively. In these landscapes, the role of tillage and water erosion was examined using measurements of soil redistribution in relation to 137Cs radionuclide depth‐stratigraphy, to elucidate the mechanism of SOC depth distribution in the soil profile and resultant stocks in agricultural landscapes of terraced field systems. Changes in the 137Cs inventory at different landscape positions depend on both 137Cs concentrations of individual subsample layers (5‐cm depth) and the vertical extent of 137Cs depth distribution in the terrace system with field banks, while the changes are only associated with the vertical extent of 137Cs depth distribution in the terrace system without field banks because of similar 137Cs concentrations of individual subsample layers. The profile shape of SOC depth distribution exhibits notable differences between the upper and lower parts of the terrace in systems with field banks, but no apparent differences were found in the systems without field banks and the SOC profile shape is similar to that of the upper part of the terrace in systems with field banks. It is suggested that SOC depth distribution in these two types of terraced field systems is controlled by different soil erosion patterns. Tillage erosion playing a dominant role in the process of soil erosion within a landscape can increase SOC stocks. However, SOC depletion takes place in situations where the two processes of tillage and water erosion are both important and tillage erosion acts as a delivery mechanism for water erosion. We conclude that tillage erosion plays a dual role: enhancing carbon storage at depositional positions, and accelerating carbon depletion when combined with water erosion within the same landscape.

SOIL EROSION AND SIGNIFICANCE FOR CARBON FLUXES IN A MOUNTAINOUS MEDITERRANEAN-CLIMATE WATERSHED

In topographically complex terrains, downslope movement of soil organic carbon (OC) can influence local carbon balance. The primary purpose of the present analysis is to compare the magnitude of OC displacement by erosion with ecosystem metabolism in such a complex terrain. Does erosion matter in this ecosystem carbon balance? We have used the Revised Universal Soil Loss Equation (RUSLE) erosion model to estimate lateral fluxes of OC in a watershed in northwestern Mexico. The watershed (4900 km2) has an average slope of 10 degrees +/- 9 degrees (mean +/- SD); 45% is >10 degrees, and 3% is >30 degrees. Land cover is primarily shrublands (69%) and agricultural lands (22%). Estimated bulk soil erosion averages 1350 Mg x km(-2) x yr(-1). We estimate that there is insignificant erosion on slopes < 2 degrees and that 20% of the area can be considered depositional. Estimated OC erosion rates are 10 Mg x km(-2) x yr(-1) for areas steeper than 2 degrees. Over the entire area, erosion is approximately 50% higher on shrublands than on agricultural lands, but within slope classes, erosion rates are more rapid on agricultural areas. For the whole system, estimated OC erosion is approximately 2% of net primary production (NPP), increasing in high-slope areas to approximately 3% of NPP. Deposition of eroded OC in low-slope areas is approximately 10% of low-slope NPP. Soil OC movement from erosional slopes to alluvial fans alters the mosaic of OC metabolism and storage across the landscape.