Fallow associated with autumn-plough favors structure stability and storage of soil organic carbon compared to continuous maize cropping in Mollisols

Background and aims Aggregate formation and stability of soil organic carbon (SOC) differ in different farming systems, probably due to differences in effects of tillage and residue management. This study used a 24-year field experiment to compare the effects of continuous maize cropping and natural fallow on aggregate formation and SOC storage in various aggregate-size classes and density fractions of a Chinese Mollisol. Methods Soils collected from the upper 0.2-m layer were wet-sieved into four aggregate-size classes (> 2, 0.25-2, 0.053-0.25 and < 0.053 mm) which were then fractionated into light, occluded and mineral C fractions. The concentrations of SOC and natural C-13 abundance of each fraction in bulk soil and the aggregate classes were determined. Results Continuous maize cropping decreased the proportion of macro-aggregates (> 0.25 mm) and increased that of micro-aggregates (< 0.25 mm) compared to the initial value while the opposite was observed in the natural fallow system. The fallow system generally had greater SOC concentration in the occluded fraction, higher proportion of newly-derived C as % total SOC in the light fraction and greater contribution of total residue C to new C in macro-aggregates and light fractions compared to the continuous maize system. Furthermore, the fallow system resulted in shorter turnover time of SOC than the continuous maize system. Conclusions Natural fallow associated with autumn-plough improved soil structural stability and SOC storage while continuous maize cropping with residue removal decreased SOC sequestration and soil aggregate stability.