Abstract: To explore the long-term effects of straw returning on soil carbon stability in rice fields, a nine-year field experiment was conducted in a rice-wheat rotation system located in the mid-to-lower reaches of the Yangtze River. There were four treatments, including combined rice and wheat straw returning, wheat straw returning, rice straw returning, and a control (no straw returning of rice or wheat). Soil organic carbon components and physicochemical properties were measured. The results showed that combined rice and wheat straw returning significantly increased soil ammonium nitrogen and available potassium concentrations (P<0.05). Straw returning significantly increased the concentrations of soluble, particulate, and total organic carbon (P<0.05), as well as the proportion of active organic carbon (soluble and particulate) in total organic carbon (combined rice and wheat straw returning > wheat straw returning > rice straw returning > control). All straw returning treatments significantly increased microbial biomass  carbon, with the highest increase (4.71 times) in the combined rice and wheat straw returning treatment. Different straw returning treatments had varying effects on carbon conversion-related enzyme activities. Specifically, combined rice and wheat straw returning had the greatest impact on β-1,4-glucosidase and β-1,4-xylosidase activities. There was a significant relationship between active organic carbon concentration and β-1,4-glucosidase and β-1,4-xylosidase activities. Overall, long-term straw returning, especially the combined rice and wheat straw returning, significantly increased soil microbial biomass, enzyme activity, and organic carbon concentration, but reduced the stability of soil organic carbon.

Key words: straw returning, rice-wheat rotation system, organic carbon grouping, enzyme activity