Tillage effects on aggregate turnover and sequestration of particulate and humified soil organic carbon

The seasonal dynamics of soil organic carbon (SOC) fractions and dry aggregates were investigated to determine whether aggregate turnover rates explain the variable influence of no-till (NT) practices on SOC accrual in Illinois. Soils were collected (0-15 cm) of two 17-yr-old trials where NT and conventional tillage (CT) practices had variable effects on SOC sequestration in the O- to 30-cm depth. Soil was fractionated into loose particulate organic matter (LPOM), macroaggregate-occluded particulate organic matter (OPOM), and humified fractions (HFs) associated with microaggregates and minerals. Dry aggregate mean weight diameter (DMWD) was used to evaluate structure expressed in the field. Within-season changes in fraction concentrations indicate that C derived from LPOM and plant roots was transferred ro OPOM at both sites. At DeKalb (silty clay loam), NT practices had less influence on SOC sequestration than at Monmouth (silt loam). DeKalb HF contents increased under NT while OPOM contents were unchanged. At Monmouth, NT practices increased SOC sequestration by increasing OPOM concentrations. Site-based differences in SOC accrual were related to aggregate turnover rates, which were increased by NT practices at Monmouth but were unaffected by tillage at DeKalb. Trends in DMWD and aggregate turnover suggest that when aggregate turnover is slow, as at DeKalb, OPOM formation is limited. Slow turnover in the heavier soil permitted accumulation of HF because residues had longer contact times with particle surfaces. When aggregate turnover is faster, as in the Monmouth NT soil, there is greater chance for residue incorporation into aggregates and accumulation of OPOM. Rapid aggregate turnover limits opportunities for mineral affiliation and HF accrual. Aggregate dynamics predicted site-based differences in the form and rate of SOC accrual caused by the use of NT practices.