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 (NO3--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 center dot 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 NO3--N were the main environmental factors driving changes in abundance, activity, and population composition of soil N-fixing bacteria under ST.