Soil Organic Carbon Redistribution Research Articles

The importance of accounting method and sampling depth to estimate changes in soil carbon stocks

BackgroundAs interest in the voluntary soil carbon market surges, carbon registries have been developing new soil carbon measurement, reporting, and verification (MRV) protocols. These protocols are inconsistent in their approaches to measuring soil organic carbon (SOC). Two areas of concern include the type of SOC stock accounting method (fixed-depth (FD) vs. equivalent soil mass (ESM)) and sampling depth requirement. Despite evidence that fixed-depth measurements can result in error because of changes in soil bulk density and that sampling to 30 cm neglects a significant portion of the soil profile’s SOC stock, most MRV protocols do not specify which sampling method to use and only require sampling to 30 cm. Using data from UC Davis’s Century Experiment (“Century”) and UW Madison’s Wisconsin Integrated Cropping Systems Trial (WICST), we quantify differences in SOC stock changes estimated by FD and ESM over 20 years, investigate how sampling at-depth (> 30 cm) affects SOC stock change estimates, and estimate how crediting outcomes taking an empirical sampling-only crediting approach differ when stocks are calculated using ESM or FD at different depths.ResultsWe find that FD and ESM estimates of stock change can differ by over 100 percent and that, as expected, much of this difference is associated with changes in bulk density in surface soils (e.g., r = 0.90 for Century maize treatments). This led to substantial differences in crediting outcomes between ESM and FD-based stocks, although many treatments did not receive credits due to declines in SOC stocks over time. While increased variability of soils at depth makes it challenging to accurately quantify stocks across the profile, sampling to 60 cm can capture changes in bulk density, potential SOC redistribution, and a larger proportion of the overall SOC stock.ConclusionsESM accounting and sampling to 60 cm (using multiple depth increments) should be considered best practice when quantifying change in SOC stocks in annual, row crop agroecosystems. For carbon markets, the cost of achieving an accurate estimate of SOC stocks that reflect management impacts on soils at-depth should be reflected in the price of carbon credits.

The Future of Soils in the Midwestern United States

AbstractSoil is the source of the vast majority of food consumed on Earth, and soils constitute the largest terrestrial carbon pool. Soil erosion associated with agriculture reduces crop productivity, and the redistribution of soil organic carbon (SOC) by erosion has potential to influence the global carbon cycle. Tillage strongly influences the erosion and redistribution of soil and SOC. However, tillage is rarely considered in predictions of soil erosion in the U.S.; hence regionwide estimates of both the current magnitude and future trends of soil redistribution by tillage are unknown. Here we use a landscape evolution model to forecast soil and SOC redistribution in the Midwestern United States over centennial timescales. We predict that present‐day rates of soil and SOC erosion are 1.1 ± 0.4 kg ⋅ m−‐2 ⋅ yr−‐1 and 12 ± 4 g ⋅ m−2 ⋅ yr−1, respectively, but these rates will rapidly decelerate due to diffusive evolution of topography and the progressive depletion of SOC in eroding soil profiles. After 100 years, we forecast that 8.8 (+1.9/−2.1) Pg of soil and 0.17 (+0.03/−0.04) Pg of SOC will have eroded, causing the surface concentration of SOC to decrease by 4.4% (+0.9/−1.1%). Model simulations that include more widespread adoption of low‐intensity tillage (i.e., no‐till farming) determine that soil redistribution, SOC redistribution, and surficial SOC loss after 100 years would decrease by ∼95% if low‐intensity tillage is fully adopted. Our findings indicate that low‐intensity tillage could greatly decrease soil degradation and that the potential for agricultural soil erosion to influence the global carbon cycle will diminish with time due to a reduction in SOC burial.

Human activities control the source of eroded organic carbon in lake sediments over the last 100 years: Evidence from stable isotope fingerprinting

Understanding the redistribution of soil organic carbon (OC) driven by erosion contributes to the global carbon budget and watershed management. Quantifying the sources of eroded OC in sediments provides new insights to addressing this issue. However, limited quantitative information on the factors influencing OC sources has constrained the behavior of regulating the OC exported from the watershed. Here, we explored the sources of OC in lake sediments over the last 100 years and quantified the effects of climate change and human activities on OC sources. Results from the stable isotope fingerprinting and MixSIAR model indicated that phragmites (30.34%) and croplands (21.48%) were consistently the main OC sources. Therefore, efforts should be concentrated on controlling soil erosion from cropland and managing the endogenous environment. In addition, the partial least squares pathways (PLS-PM) and random forests revealed that human activities (Path coefficients (r∂): 0.45 to 0.61) have controlled the OC sources over the last hundred years. Agricultural activities, urbanization, and land use change altered the quality of OC sources by affecting the vegetation productivity in source areas. The quantity of OC sources was also influenced by underlying surface and surface connectivity changes. The effects of climate change (temperature (r∂): −0.41 to 0.1; precipitation (r∂): 0.1 to 0.29) on OC sources have not been significant, but their long-term effects cannot be ignored. These findings provide a better insight into the origin and behavior of OC in lake sediments under global change, which is crucial for basin management and lake governance.

The potential to increase grassland soil C stocks by extending reseeding intervals is dependent on soil texture and depth

Grasslands account for ∼30% of global terrestrial carbon (C), of which most is stored in soils and provide important ecosystem services including livestock and forage production. Reseeding of temporary grasslands on a 5-year cycle is a common management practice to rejuvenate sward productivity and reduce soil compaction, but is physically disruptive and may reduce soil organic carbon (SOC) stocks. However, research to date is limited, which impacts on the ability to optimise grassland management for climate change mitigation. To determine whether extending the time interval up to 20 years between grassland reseeding can increase stable SOC stocks, a soil survey was conducted across three UK grassland chrono-sequences comprising 24 fields on contrasting soil types. We found that grassland SOC stocks (39.8–114.8 Mg C ha−1) were higher than co-located fields in arable rotations (29.3–83.2 Mg C ha−1) and the relationship with grassland age followed a curvilinear relationship with rapid SOC stock accumulation in the year following reseeding (2.69–18.3 Mg C ha−1 yr−1) followed by progressively slower SOC accumulation up to 20 years. Contrary to expectation, all grasslands had similar soil bulk densities and sward composition questioning the need for traditional 5-year reseeding cycles. Fractionation of soils into stable mineral associated fractions revealed that coarse textured grassland topsoils (0–15 cm) were near-saturated in C irrespective of grassland age whilst loam soils reached saturation ∼10 years after reseeding. Fine-textured topsoils and subsoils (15–30 cm) of all textures were under saturated and thus appear to hold the most potential to accrue additional stable C. However, the lack of a relationship between C saturation deficit and grassland age in subsoils suggests that more innovative management to promote SOC redistribution to depth, such as a switch to diverse leys or full inversion tillage may be required to maximise subsoil SOC stocks. Taken together our findings suggest that extending the time between grassland reseeding could temporarily increase SOC stocks without compromising sward composition or soil structure. However, detailed monitoring of the trade-offs with grassland productivity are required. Fine textured soils and subsoils (15–30 cm) have the greatest potential to accrue additional stable C due to under saturation of fine mineral pools.

Winter wheat cover crop increased subsoil organic carbon in a long-term cotton cropping system in Tennessee

Long-term cover-cropping and no-tillage practices can facilitate soil organic carbon (SOC) accumulation in agroecosystems for soil health and climate mitigation benefits. However, the contribution of these conservation management practices to SOC gain from the subsoil layers is not extensively studied. To understand this knowledge gap, it is essential to determine the distribution of total SOC and SOC fractions in response to management practices across the soil profile. Therefore, this study was conducted by leveraging a 40-year replicated field experiment in a continuous cotton (Gossypium hirsutum) system. The management treatments examined included three cover crop treatments: 1) hairy vetch, HV (Vicia villosa); 2) winter wheat, WW (Triticum aestivum L.), and 3) no cover crop, NC; and two tillage treatments: 1) chisel-tillage, CT (10 cm deep) and 2) no-tillage, NT. Soil samples were collected from four depths (0–5, 5–10, 10–30, and 30–60 cm) and analyzed for total SOC and C fractions such as microbial biomass (MBC), particulate organic matter (POM-C), and mineral-associated organic matter (MAOM-C). Results showed that the profile-scale (0–60 cm) SOC stock was greater for WW (33.7 Mg ha −1) compared to HV (29.1 Mg ha −1) and NC (24.4 Mg ha −1), while the tillage effect was non-significant (28 Mg ha −1 for NT and 29.3 Mg ha −1 for CT). The NT increased SOC in the top 5-cm layer, which was reflected in the concentrations of POM-C and MBC. Below the tillage depth (10–30 cm), SOC accrual was greater under CT (10.4 Mg ha −1) than NT (8.9 Mg ha −1), despite similar profile-scale SOC stocks. Further analysis revealed that total SOC in topsoil and subsoil were driven by POM and MAOM-related parameters, respectively. We also observed a limited accumulation of C in POM fraction (0.2–6.1 g C kg−1 soil in tilled topsoil and 0.01–0.71 g C kg−1 soil in subsoil) with a weak relationship with total SOC concentration (topsoil slope = 0.29, subsoil slope = 0.13). Contrastingly, accumulation of C in MAOM fraction was greater (2.7–12.7 g C kg−1 soil in topsoil and 0.6–5.4 g C kg−1 soil in subsoil), and it was strongly related to total SOC concentration than POM-C, especially in the subsoil (topsoil slope = 0.43, subsoil slope = 0.71). Growing deep-rooted winter wheat cover crop increased SOC in the subsoil, indicating the need for climate, soil, and ecosystem-specific management practices for profile-scale net SOC accumulation and redistribution in agroecosystems.

Influence of Liming Intensity on Fractions of Humified Organic Carbon in Acid Soil: A Case Study

Soil pH influences the composition and activities of microbial communities, which are driving the transformations of soil organic carbon (SOC). However, the effect of liming on SOC sequestration remains controversial. Evaluation of the effect of liming on SOC and humified carbon fractions was one of the most important tasks of this work. Studies were carried out in the long-term field experiment using slaked-lime rates of 0.5 or 2.0, depending on the soil hydrolytic acidity. It was determined that liming did not increase SOC content, but led to redistribution of SOC. The amount of mobile humic acids in the soil decreased while the intensity of liming increased. When liming at the 2.0 rate, the lowest HA1 level (0.251 g kg−1 C) and highest amount of HA2 (0.21 g kg−1 C) were found. It could be stated that soil liming, as a measure, allows the stabilization of HA and FA by the cation bridging mechanism. Liming resulted in slower mineralization of SOC and at the same time it also resulted in carbon sequestration in the form of valuable humic acids. An important consistent pattern of carbon transformation was identified in this study and can be applied in selecting of the most suitable combination of agricultural practices, promoting carbon sequestration and soil fertility.

Soil fauna drives vertical redistribution of soil organic carbon in a long-term irrigated dry pine forest.

Summer droughts strongly affect soil organic carbon (SOC) cycling, but net effects on SOC storage are unclear as drought affects both C inputs and outputs from soils. Here, we explored the overlooked role of soil fauna on SOC storage in forests, hypothesizing that soil faunal activity is particularly drought‐sensitive, thereby reducing litter incorporation into the mineral soil and, eventually, long‐term SOC storage.In a drought‐prone pine forest (Switzerland), we performed a large‐scale irrigation experiment for 17 years and assessed its impact on vertical SOC distribution and composition. We also examined litter mass loss of dominant tree species using different mesh‐size litterbags and determined soil fauna abundance and community composition.The 17‐year‐long irrigation resulted in a C loss in the organic layers (−1.0 kg C m−2) and a comparable C gain in the mineral soil (+0.8 kg C m−2) and thus did not affect total SOC stocks. Irrigation increased the mass loss of Quercus pubescens and Viburnum lantana leaf litter, with greater effect sizes when meso‐ and macrofauna were included (+215%) than when excluded (+44%). The enhanced faunal‐mediated litter mass loss was paralleled by a many‐fold increase in the abundance of meso‐ and macrofauna during irrigation. Moreover, Acari and Collembola community composition shifted, with a higher presence of drought‐sensitive species in irrigated soils. In comparison, microbial SOC mineralization was less sensitive to soil moisture. Our results suggest that the vertical redistribution of SOC with irrigation was mainly driven by faunal‐mediated litter incorporation, together with increased root C inputs.Our study shows that soil fauna is highly sensitive to natural drought, which leads to a reduced C transfer from organic layers to the mineral soil. In the longer term, this potentially affects SOC storage and, therefore, soil fauna plays a key but so far largely overlooked role in shaping SOC responses to drought.

A Landscape Evolution Modeling Approach for Predicting Three‐Dimensional Soil Organic Carbon Redistribution in Agricultural Landscapes

AbstractSoil erosion diminishes agricultural productivity by driving the loss of soil organic carbon (SOC). The ability to predict SOC redistribution is important for guiding sustainable agricultural practices and determining the influence of soil erosion on the carbon cycle. Here, we develop a landscape evolution model that couples soil mixing and transport to predict soil loss and SOC patterns within agricultural fields. Our reduced complexity numerical model requires the specification of only two physical parameters: a plow mixing depth, Lp, and a hillslope diffusion coefficient, D. Using topography as an input, the model predicts spatial patterns of surficial SOC concentrations and complex 3D SOC pedostratigraphy. We use soil cores from native prairies to determine initial SOC‐depth relations and the spatial pattern of remote sensing‐derived SOC in adjacent agricultural fields to evaluate the model predictions. The model reproduces spatial patterns of soil loss comparable to those observed in satellite images. Our results indicate that the distribution of soil erosion and SOC in agricultural fields can be predicted using a simple geomorphic model where hillslope diffusion plays a dominant role. Such predictions can aid estimates of carbon burial and evaluate the potential for future soil loss in agricultural landscapes.

Tracer elements revealed the soil organic carbon sources in a dam-controlled watershed

The dynamics of the soil organic carbon (SOC) pool and the mechanisms that underpin those dynamics are at the frontier of terrestrial carbon (C) cycling research. Soil erosion seriously affects the migration and redistribution of SOC, driving a critical part of the C cycle. In this study, soil samples were collected from a 14.69 m sediment profile formed between 1960 and 2017, which was located in a small agricultural catchment on the Loess Plateau in northwest China. Nuclear tracer elements (137Cs, 210Pb) and stable C isotopes (δ13C) from organic matter were used to quantitatively identify the SOC sources in different siltation stages. The method used assumes that the contribution of a source is proportional to the elemental concentration in that source. SOC in soil samples from different erosion source areas (farmland, forest land, grassland, shrub land and ditch wall) were compared against the deposit in the check dam. The concentration of SOC in the sediments of the check dam was between 1.21 g kg−1 and 4.03 g kg−1. Nearly half of the SOC in the check dam sediments was traced back to the ditch wall. Farmland contributed 24.8 %, while shrub land contributed the least SOC at 0.4 %. SOC erosion attributed to farmland decreased gradually, while the relative contribution from the ditch wall increased. Within the agricultural catchment area, there was a net migration of SOC from the various land cover types (the source) to the sedimentary area behind the dam (the sink). These results indicate that a dynamic balance exists between redistribution and heterotopic storage of SOC during sedimentation. This study provides an opinion for sediment source identification and the soil C cycle in watersheds on the Loess Plateau.

Impacts of Landscape Evolution on Heterotrophic Carbon Loss in Intensively Managed Landscapes

Soil respiration that releases CO2 into the atmosphere roughly balances the net primary productivity and varies widely in space and time. However, predicting its spatial variability, particularly in intensively managed landscapes, is challenging due to a lack of understanding of the roles of soil organic carbon (SOC) redistribution resulting from accelerated soil erosion. Here we simulate the heterotrophic carbon loss (HCL)—defined as microbial decomposition of SOC—with soil transport, SOC surface redistribution, and biogeochemical transformation in an agricultural field. The results show that accelerated soil erosion extends the spatial variation of the HCL, and the mechanical-mixing due to tillage further accentuates the contrast. The peak values of HCL occur in areas where soil transport rates are relatively small. Moreover, HCL has a strong correlation with the SOC redistribution rate rather than the soil transport rate. This work characterizes the roles of soil and SOC transport in restructuring the spatial variability of HCL at high spatio-temporal resolution.

Vegetation Restoration Alleviated the Soil Surface Organic Carbon Redistribution in the Hillslope Scale on the Loess Plateau, China

The redistribution of soil organic carbon (SOC) in response to soil erosion along the loess slope, China, plays an important role in understanding the mechanisms that underlie SOC’s spatial distribution and turnover. Consequently, SOC redistribution is key to understanding the global carbon cycle. Vegetation restoration has been identified as an effective method to alleviate soil erosion on the Loess Plateau; however, little research has addressed vegetation restoration’s effect on the SOC redistribution processes, particularly SOC’s spatial distribution and stability. This study quantified the SOC stock and pool distribution on slopes along geomorphic gradients in naturally regenerating forests (NF) and an artificial black locust plantation (BP) and used a corn field as a control (CK). The following results were obtained: 1) vegetation restoration, particularly NF, slowed the migration of SOC and reduced the heterogeneity of its distribution effectively. The topsoil SOC ratios of the sedimentary area to the stable area were 109%, 143%, and 210% for NF, BP, and CK, respectively; 2) during migration, vegetation restoration decreased the loss of labile organic carbon by alleviating the loss of dissolved organic carbon (DOC) and easily oxidized organic carbon (EOC). The DOC/SOC in the BP and NF increased significantly and was 13.14 and 17.57 times higher, respectively, than that in the CK (p < 0.05), while the EOC/SOC in the BP and NF was slightly higher than that in the CK. A relevant schematic diagram of SOC cycle patterns and redistribution along the loess slope was drawn under vegetation restoration. The results suggest that vegetation restoration in the loess slope, NF in particular, is an effective means to alleviate the redistribution and spatial heterogeneity of SOC and reduce soil erosion.

Relationship between the slope microtopography and the spatial redistribution pattern of soil organic carbon under water erosion

Soil erosion affects the dynamics of soil organic carbon (SOC) by redistributing SOC within the slope. The spatial redistribution of SOC was closely related to the slope of microtopography erosion. Although the mechanism of this process is poorly understood, it has the potential to influence the C balance of terrestrial ecosystems. This research aimed to study the relationship between the slope microtopography and the spatial redistribution pattern of SOC and to explain the spatial variation in SOC via water erosion. An observation was conducted on the runoff plot to quantify the change in the SOC content (0 to 1 cm depth) based on the 40 soil sample sites before and after erosion. A microtopographical digital elevation model was built by using a three-dimensional laser scanner, which was used for the automated measurement of point-by-point elevation on the slope surface before and after rainfall events. A generalized additive model and a geostatistical approach were combined to explain the relationship between the slope microtopography and the loss of SOC. Results showed that the slope microtopography significantly changed after erosion, and all the microtopographic indices in post-erosion were higher than those in pre-erosion. The SOC content decreased after erosion, and the spatial redistribution of SOC was closely related to the intensity of erosion. The spatial variability of SOC was reduced by water erosion. Moreover, the relief amplitude was the key factor that affected the spatial distribution of SOC pre- and post-erosion. The relief amplitude and net loss volumes significantly influenced the change in SOC.

Effects of human activities on soil organic carbon redistribution at an agricultural watershed scale on the Chinese Loess Plateau

Human activity is thought to perturb the dynamic change of organic carbon (OC), but its impact on the transportation and redistribution of erosion-induced OC is still poorly understood. Here, we use multi-source field sampling data and long-term hydrological observation data to clarify the transportation and redistribution of OC in an agricultural watershed on the Loess Plateau. The endmember mixing model based on the radiocarbon isotopes (14C) and a budget equation were used to assess the composition and budget of erosion-induced OC in a 187 km2 watershed. The results showed that compared with the period of agricultural activities (1960−1969), soil conservation activities in 1970−1999 and 2000−2019 reduced soil erosion by 31.5 % and 75.4 %, respectively. Additionally, land use changes significantly reduced the mobilized OC, from 14,370 ± 1966 (1960−1969) to 3311 ± 431 Mg C yr−1 (2000−2019). Check dam construction led to the mobilized OC was effectively buried at a rate of 2125 ± 478 and 1420 ± 282 Mg C yr-1 in periods of 1970−1999 and 2000−2019, respectively. Further radiocarbon isotopes analysis shows that the land use changes and check dam construction effectively reduced the output of ancient petrogenic organic carbon (OCpetro) and young biospheric organic carbon (OCbio) from the watershed, which has a positive impact on the regulation of atmospheric CO2 level. Our results emphasize the positive effects of soil conservation activities on terrestrial carbon sequestration, which are of great significance to clarify the effects of soil erosion on the carbon cycle.

Relationship Between the Composition of Soil Aggregates and the Distribution of Organic Carbon Under Long-Term Abandoned Restoration

Soil aggregates are important carriers of soil organic carbon (SOC) accumulation, and play an important role in the evaluation of soil structure and quality. Natural recovery can promote change in soil aggregate structure and quantity via the redistribution of SOC in the aggregates. Natural restoration from farmland is an important vegetation restoration model on the Loess Plateau. The changes in soil aggregate structure and soil carbon stock after natural restoration have received extensive attention. However, little is known about the continuous study of soil changes on the abandoned grassland during the recovery process. Therefore, to understand how SOC accumulates in the process of natural recovery and quantitatively analyze the contribution of aggregates to the total soil carbon pool, we selected four abandoned grasslands of different restoration ages on the Loess Plateau, China, and studied the changes in soil structure, soil total organic carbon (TOC), soil C:N, soil aggregate distribution, soil aggregate stable index (mean weight diameter, MWD; geometric mean diameter, GMD), and aggregate-associated SOC changes as well as their correlations from 0-20 cm and 20-40 cm soil layers in abandoned grasslands. In addition, we calculated the contribution of aggregates with different sizes to soil TOC stock. The results showed that:① natural restoration increased the macroaggregate amount, MWD, and GMD, but decreased the amount of microaggregate and silt-and clay-sized fractions. There are significant differences in the distribution and stability of aggregates between different soil layers; the promotion effect of the surface was higher than that of the subsurface soils. ② In the 42 years after abandoning recovery, soil TOC stock, macaggregate-and mesaggregate-associated SOC stock increased significantly, and varied with soil depth and years of abandonment (1.92 times, 10.2 times, and 3.61 times). In contrast, micaggregate-associated SOC stock decreased significantly, and silt-and clay-sized fractions-associated SOC stock showed no distinct change. In addition, natural restoration promoted the ratio of C:N; nevertheless, the ratio of C:N under the surface showed a reduced phenomenon after 42 years of abandonment. ③ The improvement in soil TOC stock depends primarily on changes in the macaggregate-associated organic carbon stocks, which account for 80% of macaggregate, and the significant increase in the amount of macaggregate is the main reason for the high contribution.The results of our study suggest that natural restoration is conducive to the accumulation of soil organic carbon, and improvement in soil structure and stability. Macroaggregate is the key factor in soil organic carbon accumulation and soil structure improvement in the process of natural restoration.

Soil Organic Carbon Redistribution and Delivery by Soil Erosion in a Small Catchment of the Yellow River Basin

AbstractSoil erosion modulates the atmospheric CO2 level by affecting the redistribution of young biospheric organic carbon (OCbio) and ancient petrogenic organic carbon (OCpetro) in different order streams. However, the fate of soil organic carbon (SOC) in low‐order stream systems is still uncertain due to the complex influences of terrain, land uses, and anthropogenic disturbances. Here, we used the geochemical properties fingerprinting method and a radiocarbon‐based two‐member mixing model to clarify the source, budgets, and composition of SOC in a low‐order stream catchment in the Yellow River basin. The results showed that the primary source of SOC in the catchment was agricultural activity area, which accounted for 68.5%. After vegetation restoration, the SOC delivery rate and loss rate decreased from 0.38 ± 0.06 to 0.26 ± 0.04 and 0.19 ± 0.06 to 0.14 ± 0.04 t·ha−1·yr−1, respectively. The SOC mobilized by soil erosion in the source area was 1,085.8 ± 170.5 t from 1969 to 2015, and the SOC lost during the transport process was 552.9 ± 170.1 t. The lost carbon was dominated by OCbio, whereas the loss of OCpetro was minimal. Soil erosion can mobilize the OCpetro sequestered in terrestrial ecosystems and deliver it to a river system if it is not intercepted. The oxidation of OCpetro during long‐distance fluvial transport may represent a significant carbon source for atmospheric CO2 on a geological timescale. This study contributes to our understanding of the carbon sink/source issue and quantifies the SOC budgets in stream systems.

CE-DYNAM (v1): a spatially explicit process-based carbon erosion scheme for use in Earth system models

Abstract. Soil erosion by rainfall and runoff is an important process behind the redistribution of soil organic carbon (SOC) over land, thereby impacting the exchange of carbon (C) between land, atmosphere, and rivers. However, the net role of soil erosion in the global C cycle is still unclear as it involves small-scale SOC removal, transport, and redeposition processes that can only be addressed over selected small regions with complex models and measurements. This leads to uncertainties in future projections of SOC stocks and complicates the evaluation of strategies to mitigate climate change through increased SOC sequestration. In this study we present the parsimonious process-based Carbon Erosion DYNAMics model (CE-DYNAM) that links sediment dynamics resulting from water erosion with the C cycle along a cascade of hillslopes, floodplains, and rivers. The model simulates horizontal soil and C transfers triggered by erosion across landscapes and the resulting changes in land–atmosphere CO2 fluxes at a resolution of about 8 km at the catchment scale. CE-DYNAM is the result of the coupling of a previously developed coarse-resolution sediment budget model and the ecosystem C cycle and erosion removal model derived from the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model. CE-DYNAM is driven by spatially explicit historical land use change, climate forcing, and global atmospheric CO2 concentrations, affecting ecosystem productivity, erosion rates, and residence times of sediment and C in deposition sites. The main features of CE-DYNAM are (1) the spatially explicit simulation of sediment and C fluxes linking hillslopes and floodplains, (2) the relatively low number of parameters that allow for running the model at large spatial scales and over long timescales, and (3) its compatibility with global land surface models, thereby providing opportunities to study the effect of soil erosion under global changes. We present the model structure, concepts, limitations, and evaluation at the scale of the Rhine catchment for the period 1850–2005 CE (Common Era). Model results are validated against independent estimates of gross and net soil and C erosion rates and the spatial variability of SOC stocks from high-resolution modeling studies and observational datasets. We show that despite local differences, the resulting soil and C erosion rates, as well as SOC stocks from CE-DYNAM, are comparable to high-resolution estimates and observations at subbasin level. We find that soil erosion mobilized around 66±28 Tg (1012 g) of C under changing climate and land use over the non-Alpine region of the Rhine catchment over the entire period, assuming that the erosion loop of the C cycle was nearly steady state by 1850. This caused a net C sink equal to 2.1 %–2.7 % of the net primary productivity of the non-Alpine region over 1850–2005 CE. This sink is a result of the dynamic replacement of C on eroding sites that increases in this period due to rising atmospheric CO2 concentrations enhancing the litter C input to the soil from primary production.

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.

Effects of past land use on soil organic carbon changes after dam construction

A large-scale and high-dropout water-level-fluctuation zone (WLFZ) was formed by the construction of cascaded large dam in the southwest hydropower base of China. The original land use patterns changed significantly by the formation of the WLFZ, which possibly affected the soil organic carbon (SOC) stocks and local carbon cycle. However, little is known about the effects of land uses before dam construction on the SOC stocks in the WLFZ. To address this, we chose the Nuozhadu mega reservoir built in 2014 in the upper Mekong River basin as our study area. We established five sampling transects around the reservoir and sampled them in 2015. Each transect contained three vertical sampling lines representing three land-use types: natural forest, farmland, and rubber plantations. SOC stocks were determined in the WLFZ and in the infralittoral reference zone (IRZ), and the interactions among SOC, land uses, soil erosion, inundation, and other soil physicochemical properties were analyzed to establish the factors controlling SOC in the WLFZ. The average soil organic carbon density (SOCD) was not significantly different between the WLFZ and IRZ, but it differed significantly among the three types of land uses. The SOC stocks in natural forests of the WLFZ and IRZ were significantly greater than those in the other land use types. The SOCD decreased with increasing elevation in the natural forest and rubber plantation in the WLFZ, while no significant trend was found in farmland. These results indicated that the construction of a large dam in southwestern China may not lead to significant changes in regional SOC stocks, but it can result in the redistribution of SOC in the large-scale and high-dropout WLFZ. Land use before dam construction can affect the background concentrations of SOC, soil physicochemical properties, and soil erosion rate in the WLFZ, which controlled the redistribution of SOC.

Redistribution of Soil Organic Carbon Induced by Soil Erosion in the Nine River Basins of China

AbstractGlobal soil erosion redistributes a large amount of soil organic carbon (SOC) and is potential to significantly change the terrestrial carbon budget. However, there are large uncertainties in the redistribution of SOC within the terrestrial and aquatic ecosystems. Based on two national survey data sets on soil erosion and sediment measurements from hydrological stations, this study estimated the redistribution of sediment and SOC in the nine river basins of China during 1995–1996 and 2010–2012. Over these two periods, 3.55–4.50 Pg of soil and 68.42–77.32 Tg C of SOC were eroded each year. For the SOC budget, on average 57% and 47% of the eroded SOC was deposited over land, 25% and 44% was deposited in the channel, and 18% and 8% was delivered into the sea during 1995–1996 and 2010–2012, respectively. Compared with the corresponding magnitudes during 1995–1996, the eroded SOC, the SOC deposited over land, and the SOC discharged into the sea decreased during 2010–2012, and only the SOC deposited into the river channel increased (from 19.5 to 30.1 Tg C yr−1). The changes in SOC deposition in the channel of the Yangtze River and the Yellow River basins demonstrate that the influence of human activities is extensive. Our results show that the erosion‐induced redistribution of SOC alters the carbon budget of China.

Distribution of carbon and nitrogen along hillslopes in three valleys on Herschel Island, Yukon Territory, Canada

Thermokarst results from the thawing of ice-rich permafrost and alters the biogeochemical cycling in the Arctic by reworking soil material and redistributing soil organic carbon (SOC) and total nitrogen (TN) along uplands, hillslopes, and lowlands. Understanding the impact of this redistribution is key to better estimating the storage of SOC in permafrost terrains. However, there are insufficient studies quantifying long-term impacts of thaw processes on the distribution of SOC and TN along hillslopes. We address this issue by providing estimates of SOC and TN stocks along the hillslopes of three valleys located on Herschel Island (Yukon, Canada), and by discussing the impact of hillslope thermokarst on the variability of SOC and TN stocks. We found that the average SOC and TN 0–100 cm stocks in the valleys were 26.4 ± 8.9 kg C m-2 and 2.1 ± 0.6 kg N m-2. We highlight the strong variability in the soils physical and geochemical properties within hillslope positions. High SOC stocks were found at the summits, essentially due to burial of organic matter by cryoturbation, and at the toeslopes due to impeded drainage which favored peat formation and SOC accumulation. The average carbon-to‑nitrogen ratio in the valleys was 12.9, ranging from 9.7 to 18.9, and was significantly higher at the summits compared to the backslopes and footslopes (p < 0.05), suggesting a degradation of SOC downhill. Carbon and nitrogen contents and stocks were significantly lower on 16% of the sites that were previously affected by hillslope thermokarst (p < 0.05). Our results showed that lateral redistribution of SOC and TN due to hillslope thermokarst has a strong impact on the SOC storage in ice-rich permafrost terrains.