Dynamics of soil aggregate-associated carbon as influenced by simulated tillage and runoff

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.