Long-term straw return increases biological nitrogen fixation by increasing soil organic carbon and decreasing available nitrogen in rice–rape rotation

Background Straw return may affect the soil nitrogen (N) balance by changing soil carbon (C) and N components and contents. To understand how straw return affects the soil N fixation capacity, the relationship between biological N fixation and changes in soil C and N contents at different depths need to be examined. Methods Soil samples were collected at 20-cm intervals from 0 to 100 cm after five years of a rice-rape rotation system with straw return (ST) and without (CK). Organic C and inorganic N contents, potential N fixation rate, and abundance and population composition of N-fixing bacteria were analyzed at different soil depths. Results Compared with CK, ST increased soil organic C (SOC), particulate organic C (POC), mineral-associated organic C (MOC), and total N (TN) but decreased nitrate–N (NO 3 − -N). Straw return also increased the abundance of N-fixing bacteria from 2.9- to 18-fold at depths from 0 to 40 cm and increased the potential N fixation rate at 0–20 cm by 25%. The annual biological N fixation rates at 0–60 cm by soil N-fixing bacteria were estimated to be approximately 0.13–2.70 and 0.12–2.16 kg·ha −1 in ST and CK, respectively. Long-term ST significantly affected the composition of N-fixing bacteria at depths from 0 to 40 cm. Conclusions Long-term straw return can increase the abundance of the N-fixing bacteria and effectively alter the structure of the N-fixing bacterial community. SOC, POC, TN, and NO 3 − -N were the main environmental factors driving changes in abundance, activity, and population composition of soil N-fixing bacteria under ST.