A mechanistic understanding of nutrient movement associated with the erosion process is required to formulate precision soil conservation measures. We explored the response of surface soil nutrients and soil organic carbon (SOC) fractions to tillage erosion and water erosion. Tillage and water erosion rates were estimated by the directional tillage erosion model and revised universal soil loss equation, respectively. One hundred and twelve surface soil samples (0-20 cm) were collected from a sloping farmland (3.6 ha) in the Mollisols region of China. Soils were analyzed for total nitrogen (TN), total phosphorus (TP), total SOC, particulate organic carbon (POC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC). Results showed that no significant relation between TN and tillage or water erosion rates exists at any slope position. The TP distribution is more affected by water erosion than tillage erosion. Water erosion also played a greater role in controlling distribution of DOC than tillage erosion, whereas POC distribution was more sensitive to tillage erosion. In addition, we observed a contrasting relationship between MBC and water erosion for the mild erosion (r = −.43, P < .05) vs. intense erosion scenario (r = .38, P < .05). This shift indicates a possible dual role of microbes in SOC cycling associated with water erosion: mild erosion (averaged 17.4 t ha −1 yr −1 ) depletes microbial biomass and contributes to SOC mineralization, whereas intense

Tillage practices and water erosion are the most important anthropogenic and natural processes, respectively, driving soil organic C turnover in agricultural land. The aim of this study was to explore the responses of soil organic C (SOC) turnover to tillage and runoff by comparing the variation of soil aggregate-associated organic C (AOC) and intra-aggregate particulate organic C (iPOC) under simulated tillage and runoff conditions. Soil samples were collected from a native vegetation land with no cultivation history in the Mollisol region of Northeast China. After a series of simulated tillage (ST) and simulated runoff (SR) treatments, the samples were incubated for 30 d and then separated through 2-, 1-, 0.25-, and 0.053-mm sieves by wet-sieving to obtain different aggregate size fractions. Each aggregate fraction was subsequently shaken for 18 h in 0.5% hexametaphosphate to get different intra-aggregate particle size fractions. The proportion of the fractions and their AOC and iPOC were determined. The ST treatment promoted the reaggregation of macroaggregates (>2 mm) by accelerating the turnover of their coarse iPOC (0.25-2 mm), leading to a lower concentration of AOC. Runoff transformed larger aggregates (>0.25 mm) to smaller particles (<0.25 mm) without catalyzing C turnover. Fine iPOC (0.053-0.25 mm) could serve as an indicator for AOC (>1 mm) dynamics, especially associated with tillage operations. Our findings highlight the different influences of tillage and runoff, and the negative effect of tillage on SOC dynamics.