Effects of straw return on soil aggregates and carbon balance in farmland under conservation tillage

　　Conservation tillage can improve soil aggregate structure. However, a systematic understanding is lacking of how it influences carbon sequestration in farmland ecosystems and of the internal relationships among soil aggregate structure, soil organic carbon, and ecosystem carbon sequestration. Using a long-term experiment, we investigated the effects of conservation tillage on soil aggregate stability, organic carbon sequestration, and the carbon balance of a maize farmland ecosystem. Five treatments were compared: whole straw random mulch (WSM), crushed mulch (CSM), whole straw strip mulch (SSM), whole straw strip mulch + plastic film mulch (FSDM), and straw non-return (CK). The results indicate that all conservation tillage treatments, and WSM in particular, significantly increased the proportion of large aggregates (≥0.25 mm), the mean weight diameter (MWD), and the geometric mean diameter (GMD) in the 0-30 cm soil layer by 5.27%-10.91%, 13.76%-17.92%, and 16.67%-29.23%, respectively. With the exception of FSDM, which reduced organic carbon content and stocks in the 20-30 cm layer, the conservation tillage treatments increased organic carbon content and stocks. Under WSM, organic
　　carbon content and stocks in the 0-30 cm layer rose by 14.77%-18.79% and 17.84%-20.77%, respectively. Across treatments, aggregates sized 1-2 mm and 0.25-1 mm contributed the largest shares to total soil organic carbon, accounting for 28.64%-44.53% and 30.31%-38.97%, respectively. The WSM treatment also reduced the rate at which organic carbon declined with increasing soil depth. Carbon balance analysis shows that all conservation tillage treatments converted the farmland ecosystem into a carbon sink, with net ecosystem productivity carbon sequestration capacity (C_NEP) rising by 5.21% to 10.27% relative to CK, and the WSM treatment produced the highest C_NEP. Partial least squares analysis indicates that conservation tillage primarily enhances soil total organic carbon by increasing both the organic carbon content and the stability of aggregates, thereby improving ecosystem carbon balance. In conclusion, unordered mulching with whole corn stalks (WSM) is the most effective tillage practice in this area for improving soil quality and increasing carbon sequestration capacity.