Long-term plastic film mulching and fertilization treatments changed the annual distribution of residual maize straw C in soil aggregates under field conditions: characterization by 13C tracing

Plastic film mulching and fertilization strongly affect soil aggregation and dynamics of the total soil organic carbon (C) pool. However, there is limited information on how these agricultural management practices influence the fate and seasonal dynamics of crop residue-derived C in soil aggregates. Therefore, a better understanding of the fate of C derived from crop residues and their location in soil aggregates is crucial to improve our prediction of C sequestration and stabilization in soil. In this study, an in situ C-13-tracing technique was used to identify the dynamic distribution and accumulation patterns of crop residue-derived C in soil aggregates as affected by long-term plastic film mulching and four fertilization treatments (control, CK; nitrogen, N-2; organic manure, M-2; nitrogen and organic manure, M1N1). The fate of C-13-labeled maize straw in loam soil was studied over 360 days using in situ incubation of 0.2% equivalent dry straw incorporated into the soil and aggregate size fractionation. Soil samples were separated into four particle-size fractions [large (> 2 mm) and small (1-2 mm) macroaggregates; large (0.25-1 mm) and small (< 0.25 mm) microaggregates] by dry-sieving. Long-term (27-year) application of fertilizers significantly increased the soil organic carbon (SOC) content in brown earth compared to the no-fertilizer control at the onset of our annual study, with the order of M-2 > M-1 N-1 > N-2 > CK. Both the content of straw-derived C-13 in aggregate fractions and the proportion of C-13 in total soil samples decreased considerably from the microaggregate fractions and increased moderately in the macroaggregate fractions in a manner enhanced by plastic film mulching and by seasonal transitions, specifically from spring (day 0) to summer (day 60). In addition, the decomposition of maize straw and soil aggregation exhibited a direct correlation in which the content of C-13-SOC and mean weight diameters decreased over the course of the 360-day experiment. Our results suggest that certain amounts of straw-derived C-13 could accumulate in microaggregates, which play an important role in long-term C sequestration, while a large part of straw-derived C tends to transfer from microaggregates to macroaggregates over time as affected by long-term plastic film mulching coupled with fertilization. This study improves our understanding of the effect of plastic film mulching and different fertilization regimes on the retention and stabilization processes of straw-derived C in soil.