Soil aggregates are key factors that regulate erosion-related carbon loss in citrus orchards of southern China: Bare land vs. grass-covered land

Abstract

Water erosion induces a wide variation in organic carbon (OC) loss in orchard agroecosystems due to high anthropogenic disturbance. Grass cover, a common agricultural practice, can both suppress OC transport by reducing erosive rainfall and runoff, and enhance the amount of OC in soil by increasing OC inputs. These different effects on OC loss call for a deeper understanding of the process and drivers of OC loss under grass cover during water erosion processes. Hence, we conducted a simulated rainfall experiment under bare land (BL) and grass cover in orchards to analyse the direct and indirect effects of grass cover, soil aggregate properties, runoff and sediment yield on OC loss using structural equation modelling (SEM). The results demonstrated that grass cover reduced sediment and runoff by 97.84–98.81 % and 47.28–82.12 %, respectively, and correspondingly reduced the loss of 97.21–99.24 % soil organic carbon (SOC) and 83.55–92.76 % dissolved organic carbon (DOC) compared to BL. The SEM results showed that sediment, as opposed to runoff, was the main pathway of OC loss and grass cover regulated total organic carbon (TOC) loss by varying soil aggregate properties rather than by changing erosive forces. Specifically, the total effect of the soil aggregate stability and SOC content in aggregates under grass cover on TOC loss was negative (-0.21 and -1.14, respectively). Moreover, the increasing percentage of >5 mm soil aggregates contributed to reducing SOC and DOC losses, while the increasing percentage of 0.5−0.25 mm soil aggregates aggravated SOC and DOC losses. The results from this study emphasize that grass cover is a convenient and effective measure for reducing OC loss in orchards; moreover, studying the effect of multiple-factor interactions on OC loss will provide insights and improve predictions of the global carbon cycle.