Moderate topsoil erosion rates constrain the magnitude of the erosion-induced carbon sink and agricultural productivity losses on the Chinese Loess Plateau.

Despite a multitude of studies, erosion rates as well as the contribution of different processes on Chinese Loess Plateau (CLP) remain uncertain. This makes it impossible to correctly assess the impact of conservation programs and the magnitude of the erosion-induced carbon sink. We used a novel approach, based on field evidence, to reassess erosion rates on the CLP before and after conservation measures were implemented. Our results show that the current average topsoil erosion rate is 3-9 times lower than earlier estimates suggested: most sediments are mobilised by gully erosion and/or landsliding. Under 2005 conditions, the combination of topsoil erosion, gully erosion and landslides mobilised 0.81 ±0.23Gt yr^-1 of sediments and 4.77±1.96 Tg yr^-1 of soil organic carbon (SOC): the latter number sets the maximum magnitude of the erosion-induced carbon sink, which is ca. 4 times lower than other recent estimates suggest. The sediment fluxes we calculate are consistent with sediment yields measured in the Yellow River. The conservation programs implemented from the 1950s onwards reduced topsoil erosion from 0.51 ±0.13 to 0.30±0.08 Gt yr^-1 while SOC mobilisation was reduced from 7.63 ± 3.52 to 4.77 ± 1.96 Tg C. Prior to 1950, a geomorphological equilibrium existed whereby the amount of sediment and carbon exported to the Bohai sea was similar to the amount of sediment eroded on the CLP, so that the erosion-induced carbon sink nearly equalled the amount of mobilised SOC. Conservation efforts and reservoir construction have disrupted this equilibrium and most eroded sediments and carbon are now stored on land where part of the SOC may decompose, thereby potentially lowering the strength of the erosion-induced carbon sink. Despite the fact that average topsoil losses on the CLP are still relatively high, the current level of topsoil erosion on the CLP is no major threat to the agricultural productivity of the area, mainly because fertilizer application has dramatically increased since 1980. Assessing the human impact on agricultural ecosystems at larger scales requires a careful identification and quantification of the processes involved: by doing so for the CLP we have shown that current perceptions regarding the intensity of soil erosion and its effects (both negative and positive) need to be revised. [ABSTRACT FROM AUTHOR]