Zhengang Wang

Soil organic carbon mobilization by interrill erosion: Insights from size fractions

Sediments mobilized by interrill erosion are often highly enriched in soil organic carbon (SOC) in comparison to source soils. This selectivity may lead to the preferential mobilization of SOC with specific properties, e.g., SOC that is especially susceptible to decomposition. This may then have important implications with respect to the role of soil erosion in the global carbon cycle. We addressed this issue by investigating the behavior of different SOC components in field rainfall simulation experiments on arable fields in loess-derived soils. We characterized the mobilization of mineral-bound organic carbon (MOC) and particulate organic carbon (POC) by interrill erosion using size fractionation and we used the C:N ratio as a tracer variable to determine the composition of the SOC in eroded sediments. MOC was found to be preferentially mobilized by interrill erosion in comparison to POC. The enrichment ratio (i.e., the ratio of the concentration of a soil constituent in the eroded sediment to its concentration in the original soil) of MOC decreased with increasing sediment concentration. The enrichment ratio of POC displayed a similar pattern to that of MOC but enrichment was less pronounced. Furthermore, sediments were found to be enriched in fine POC while they were impoverished with respect to coarse POC. The selective MOC mobilization together with the dominance of MOC in the total SOC pool in the soil explained the dominance of MOC in interrill eroded sediment. The fact that it is mainly MOC that is mobilized by interrill erosion implies that the SOC in the interrill eroded sediments is on average at least as recalcitrant than that in the source soils which may have important implications for the fate of the mobilized SOC. In order to understand the role of soil erosion in C cycling, MOC and POC need to be considered separately not only because they are chemically different but also because of their different behaviors with respect to geomorphic processes.

Predicting the long‐term fate of buried organic carbon in colluvial soils

A significant part of the soil organic carbon (SOC) that is eroded in uplands is deposited and buried in colluvial settings. Understanding the fate of this deposited soil organic carbon is of key importance for the understanding of the role of (accelerated) erosion in the global C cycle: the residence time of the deposited carbon will determine if, and for how long, accelerated erosion due to human disturbance will induce sequestration of SOC from the atmosphere to the soil. Experimental studies may provide useful information, but, given the time scale under consideration, the response of the colluvial SOC can only be simulated using numerical models which need careful calibration using field data. In this study, we present a depth explicit SOC model including soil profile evolution due to sedimentation to simulate the long-term C dynamics in colluvial soils. The SOC profile predicted by our model is in good agreement with field observations. The C burial efficiency (the ratio of current C content of the buried sediments to the original C content at the time of sedimentation) of deposited sediments exponentially decreases with time and gradually reaches an equilibrium value. This equilibrium C burial efficiency is positively correlated with the sedimentation rate. The sedimentation rate is crucial for the long-term dynamics of the deposited SOC as it controls the time that buried sediments spend at a given soil depth, thereby determining its temporal evolution of C input and decomposition rate during the burial process: C input and decomposition rate vary with depth due to the vertical variation of root distribution and soil environmental factors such as (but not limited to) humidity, temperature, and aeration. The model demonstrates that, for the profiles studied, it takes circa 300 years for the buried SOC to lose half of its C load. It would also take centuries for the SOC accumulated in colluvial soils over the past decades due to soil redistribution under mechanized agriculture to be released to the atmosphere after the application of soil conservation measures such as conservation tillage.

Constraining a coupled erosion and soil organic carbon model using hillslope‐scale patterns of carbon stocks and pool composition

A large amount of soil organic carbon (SOC) is laterally redistributed by agricultural erosion. Recent studies have shown that this leads to strong horizontal (i.e., spatial) and vertical (i.e., with soil depth) gradients in SOC stock and C pool distribution in eroding landscapes. However, the mechanisms leading to these gradients in relation to erosion and deposition are still poorly documented. In particular, the effect of the inherent properties of SOC (as controlled by the SOC pool composition) versus the effect of depth-related soil environmental condition (i.e., differences in soil humidity, temperature, aeration, etc.) on the persistence of SOC in eroding landscapes is uncertain. Nonetheless, a detailed understanding of these factors is important to correctly assess landscape-scale soil C turnover and vulnerability to disturbance from human activities. This study utilizes observational data on long-term erosion/deposition rates and C pool composition derived from soil C fractionation experiments along an eroding agricultural hillslope to constrain a coupled erosion-SOC dynamics model. The simulation results show that the data set used can result in a robust parameter estimation of a multipool C model for an eroding landscape with parameter values that are consistent with incubation experiments. A scenario analysis, where we evaluate the contribution of different processes, demonstrates that soil redistribution is essential to explain the observation that depositional locations contain more SOC in subsoils, while the SOC content of the surface layer is similar to those observed along an eroding hillslope. The spatial variability of plant production could explain some of the observed variability in SOC content, but our results suggest that the spatial variability of SOC pool composition is mainly related to soil redistribution. Finally, we suggest that environmental factors may play a more important role than the inherent properties of SOC in determining the vertical variation of SOCmineralization. This implies that depositional C stocks might be highly vulnerable to disturbance from human activities that may reconnect the buried SOC with the atmosphere.