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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.

来源平台：SOIL USE AND MANAGEMENT