Abstract: Chinese milk vetch (Astragalus sinicus L.) is an important green manure crop of paddy fields in southern China. Understanding the effects of planting Chinese milk vetch in combination with nitrogen fertilizer application on soil organic carbon is crucial for the optimization of fertilizer application and soil fertility management, which could provide a theoretical basis for the sustainable development of agriculture in paddy fields. There were five treatments, including no Chinese milk vetch planting in winter + no nitrogen application (CK), planting Chinese milk vetch without nitrogen application, planting Chinese milk vetch with nitrogen-reducing application, planting Chinese milk vetch with conventional-level nitrogen application, and planting Chinese milk vetch with high-level nitrogen application. The results showed that, compared with CK, soil total organic carbon and active organic carbon under the treatment of planting Chinese milk vetch alone increased by 14% and 22% respectively. For the four treatments of planting Chinese milk vetch, soil microbial biomass carbon, soil microbial entropy and carbon pool management index increased by 70%, 58%, and 8%, respectively. For the three treatments of planting Chinese milk vetch with varied nitrogen application levels, total organic carbon, active organic carbon, and soil microbial entropy increased by 8%, 15%, and 72%, respectively. In summary, as compared with fallow in winter without nitrogen application, single application of Chinese milk vetch could significantly increase soil total organic carbon and active organic carbon content. Planting Chinese milk vetch in combination with nitrogen fertilizer applications could significantly increase soil microbial biomass, microbial entropy, and soil carbon pool management index. Among all the treatments, planting Chinese milk vetch in combination with nitrogen-reducing application and conventional-level nitrogen application had better performance.

Key words: spatial distribution, diameter distribution, secondary forest, scale effect