Abstract: Abstract: Recently, crop straws returning to field has been vigorously proposed as an effective strategy to promote agricultural sustainability in China. However, there were few studies revealing the effects of straw return on rice (Oryza Sativa) - rape (Brassica campestris) rotation systems in Southwest China, especially lacking contrastive research about different crop straws. To explore the effects of straw incorporation on crop yield and soil nitrogen (N) retention of rice-rape rotation system, a field experiment was conducted from 2013 to 2015 in Yunnan Province. Specifically, the experiment consisted of 4 treatments: No fertilizer (CK), chemical fertilizer (CF), chemical fertilizer plus maize straw (CFMS), and chemical fertilizer plus broad bean straw (CFBS). Crop yield, soil N content, soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), and some other soil physicochemical properties were determined after rice or rape harvest. The results showed that straw incorporation (CFMS and CFBS) considerably increased crop yield and total biomass compared with the CF, and the yield and biomass of rape were increased by 28.6%-62.1% and 5.2%-20.2%, respectively, and those of rice were increased by 6.2%-35.8% and 35.7%-87.4%, respectively. In particular, CFMS produced the highest crop yield (4.2-4.7 t/hm2 for rape and 11.0-12.2 t/hm2 for rice), and CFMS increased the rape and rice yield by 16.7%-17.5% and 24.5%-27.9%, respectively, compared with CFBS. In consistent with the effects on crop yield, straw incorporation effectively increased the N content of rice and rape, and CFMS and CFBS increased the N content by 14.5%-20.2% and 5.2%-9.0% for rape, respectively, and 68.3%-87.4% and 35.7%-55.0% for rice, respectively, relative to the CF. In addition, CFMS significantly increased the N content (161.0-185.3 kg/hm2 for rape and 219.0-243.1 kg/hm2 for rice) compared with CFBS (153.3-162.1 kg/hm2 for rape and 176.6-201.0 kg/hm2 for rice) (P<0.05). Furthermore, the SMBC and SMBN under straw incorporation were both increased significantly compared with CK (P<0.05), and they were ranked in such order: CFMS>CFBS>CF>CK. There was no significant difference in the soil pH value, total N, soil organic matter (SOM) and soil C/N among these 4 treatments (P>0.05) because of the relatively short experimental period (only lasting for 2 years). Generally, the soil microbial entropy and soil microbial biomass C/N (SMB C/N) could quickly respond to straw incorporation compared with the highly stable soil C/N. Nitrate N was the main component of inorganic N in soil, and soil residual N after crop was harvested was significantly decreased under straw incorporation (P<0.05), which declined by 11.6%-55.0% for CFMS and 13.7%-52.3% for CFBS compared with the CF (45.4 mg/kg). Rainfall mainly occurred during rice growing season (from May to October) in Southwest China, which had a high potential to cause N losses. However, the significant reduction of residual N in soil by straw incorporation after rape was harvested could probably lead to a lower potential of N loss in rice growing season. We conclude that the incorporation of straw into soil can increase crop yield and crop N uptake, improve soil N immobilization potential and reduce risks of N loss in the rice-rape rotation in Southwest China. Particularly, the incorporation of maize straw demonstrates greater advantages in yield increase and N retention than bean straw in actual production.