The effect of aggregation onto the fate of eroded carbon

The effect of soil erosion on the global Carbon (C) cycle is subject of an intense debate. The controversy is mostly due to the lack of understanding of the fate of transported C from the source of erosion to the eventual site of deposition. The fate of eroded carbon is strongly influenced by the settling velocities of the eroded fractions and the corresponding transport distance and environment at the site of deposition. Some erosion/deposition models already include the settling velocity of particles to evaluate the sediment transport distance and selective redistribution after erosion. However, the settling velocities in these models are either based on grain size (e.g. the EUROSEM model) or an arbitrary number of size classes (e.g. the WEPP model and the Hairsine-Rose model). In reality, most soil is eroded as aggregates, or at least aggregates are present in the sediment, which affects settling velocity and C content of the eroded soil. Without considering the effects of aggregation on sediment movement and C content, the extent of mineralization of deposited C as well as the transfer of organic C to aquatic systems is likely to be incorrect. To identify the effect of aggregation on the fate of eroded C, a rainfall simulation was carried out on two soils of distinct texture and structure. The eroded sediments were then fractionated by a settling tube apparatus according to their likely transport distance after erosion. The sediment of each class was incubated for 50 days to monitor the respiration rate. Weight, total organic carbon (TOC) of the sediment in each class were also measured. The distribution of C across our settling velocity classes indicates that most of the eroded C was incorporated in coarse aggregates that would have been deposited after short transport distances. This portion likely to be deposited across the landscape carried 58.8 to 88.2% of the total organic C stock in the eroded sediment and released about 55.4 to 81.9% of the total sediment CO2 emission. The fine sediment likely transferred into rivers contained 3.6 to 7.4% of the total organic C stock and produced about 6.1 to 17.3% of the total CO2 emission. As a consequence, if C erosion and mineralization had been solely based on grain size and associated C, the potential release of CO2 during transport would have been underestimated. Future research should account for the effects of transport processes in a real field environment.