Effects of Aggregation: How Significant Are They to Potentially Skew Slope-Scale Carbon Balances?

During slope-scale erosion events, soil fractions and the associated soil organic carbon (SOC) will be transported away from eroding sites mainly by overland flow. Eroded soil will be either gradually re-deposited along hillslopes or further transferred to river systems. However, the re-distribution of eroded SOC during transport is not always uniform, but very often affected by preferential deposition. Under given flow conditions, the site of SOC deposition depends on the transport distances of sediment particles where the SOC is stored. Very often, soil and SOC erosion risk is assessed by applying the mineral particle specific SOC distribution observed either from eroding sites or colluvial depositional sites. However, soil is not always eroded as dispersed mineral particles, but mostly in form of aggregates. The aggregates possibly have distinct settling velocity from individual mineral particles, which may considerably change the transport distance of the associated SOC. Yet, little has been known about the potential effects of aggregation onto the movement and mineralizing susceptibility of eroded SOC.

A simulated rainfall was applied to two soils of different texture, structure and SOC content. The generated sediments were fractionated by a settling tube apparatus according to their likely transport distances. The long-term mineralization potentials of the fractionated sediments were measured for 50 days. The results show: 1) the re-deposition of eroded SOC into the terrestrial system was increased up to 64% compared to the likely re-distribution suggested by the mineral particle specific SOC distribution. This indicates that using mineral particle size distribution and the associated SOC as in current erosion models would under-estimate terrestrial deposition of SOC. 2) Over 50 days, the mineralization of the sediment from the silty loam was doubled when compared to the original silty loam. The mineralization of the sediment from the silty clay roughly equaled that from the original Movelier soil. This requires investigation on the potential effects of different degrees of aggregation onto the fate of eroded SOC. Overall, the results observed from this study suggest that soil erosion is more likely to be a source of atmospheric CO₂.