Fifteen years of tillage and straw returning promotes soil structure and carbon sequestration under rice–rice–oilseed rape rotation in Central China

Abstract Straw returning and tillage measures are one of the key measures to improve soil structure and fertilize soil. Different rotation systems and tillage methods (no‐tillage or conventional tillage) affect the physical structure and the C sequestration efficiency of soil. Here, we examined the response of soil organic C stocks, soil C fractions and soil structure to straw returning and tillage management based on a 15‐year rice–rice–oilseed rape rotation. Our results indicated that the soil carbon stock reached C equivalent within 10 years with straw returning and 5–6 years without straw returning. No‐tillage improve the C sequestration efficiency only in the early stage. Soil organic C (SOC) fractions significantly increased after straw returning, that is, dissolved organic carbon (DOC) increased by 10.3%–21.4%, particulate organic carbon (POC) increased by 32.0%–44.2%, light fraction organic carbon (LFOC) increased by 37.9%–61.2%, and microbial biomass carbon (MBC) increased by 7.1%–12.1%. Except for LFOC, no significant difference was observed between no‐tillage and tillage for the other SOC fractions. Straw returning significantly improved the proportion of >2 mm aggregates (+40.0%) and the SOC content of >0.25 mm aggregates. Meanwhile, straw returning with conventional tillage (CTS) enhanced the SOC content of <0.25 mm aggregates. The soil structure became irregular (anisotropy increased by 115.0%), more complex (fractal dimension increased by 10.2%) and the number of soil pores increased by 108.5% after straw returning. LFOC and MBC played important roles in promoting the changes in the soil structure and the formation of macro‐aggregates. Overall, straw returning was more effective in increasing the SOC, the accumulation of macro‐aggregates, and the number of soil macropores, as well as improving the soil structure compared with tillage under the triple upland‐paddy rotation system.