Organic carbon losses by eroded sediments from sloping vegetable fields in South China

Soil Organic Carbon (SOC) is the most important component of soil. Though small, it determines soil fertility and prevents soil losses. In this study, we examined relationships between the Particle-Size Distribution (PSD) of the eroded sediment and SOC loss, and evaluated the effects of plant coverage ratios (0%, 15%, 30%, 45%, 60% and 90%), slope lengths (2 m, 4 m), fertilizer treatments (unfertilized control (CK), compound N-P-K fertilizer (CF), and organic fertilizer (OF)) on SOC loss and the SOC enrichment ratio (ERSOC) in the eroded sediments. The experimental results showed that longer slope length and lower surface cover ratios produced larger surface runoff and the eroded sediments, resulting in larger SOC losses. The average SOC loss was greatest in the OF treatment and SOC loss was mainly associated with the eroded sediment. Surface runoff, which causes soil erosion, is a selective transportation process, hence there were more clay-sized particles (< 2 mu m) and silt-sized particles (2-50 mu m) in the eroded sediments than in the original soils. SOC was enriched in the eroded sediments relative to in the original soil when ERSOC > 1. ERSOC was positively correlated with ERclay (< 2 mu m) (R (2) = 0.68) and ERfine silt (2-20 mu m) (R (2) = 0.63), and from all the size particle categories of the original soil or the eroded sediments, more than 95% of SOC was concentrated in small-sized particles (< 50 mu m). The distribution of SOC in different-sized particles of the original soil and the eroded sediment is primarily associated with clay-sized particles and fine silt-sized particles, thus we conclude that as the eroded sediment particles became finer, more SOC was absorbed, resulting in more severe SOC loss.