Changes in Soil Rhizobia Diversity and their effects on the Symbiotic Efficiency of Soybean Intercropped with Maize
It has been established that maize/soybean intercropping can improve nitrogen use efficiency. However, few studies have addressed how maize/soybean intercropping affects nitrogen-fixing bacterial diversity and N fixation efficiency of intercropped soybean. In this study, nitro-gen-fixing bacterial communities, N fixation efficiency, and their relationships with soil proper-ties under three nitrogen fertilization application rates(N0 0kg/ha,N1 40kg/ha,N2 80kg/ha) were explored through field experiments. Nitrogen fixation and nitrogen-fixing bacteria diversity were assessed using 15N natural abundance, Illumina high-throughput sequencing, and nifH(nitrogen fixation) gene copies quantification in the rhizosphere soil of intercropped soybean. The results showed that nitrogen application rates significantly decreased the nitrogen-fixing bacteria diversity, nitrogen fixation efficiency, and nifH gene copies in the rhizosphere soil. Ni-trogen fixation efficiency, nodule number, and dry weight of intercropped soybean were highest in the N0 treatment, and nitrogen fixation was the highest in the N1 treatment. The nitrogen fixing efficiency and nodule number of soybean increased by 69%, 59%, 42% and 10%, 22%, 21%, re-spectively, under the three N levels of N0, N1 and N2 compared with monocultures. Intercrop-ping under N0 and N1 treatments significantly increased the soybean nitrogen-fixing bacteria di-versity compared with monocultures. There was a significant positive correlation between soil nifH gene copies and N fixation efficiency and a negative correlation with soil available nitrogen. Bradyrhizobium abundance in soybean rhizosphere soil decreased significantly with the increase in nitrogen application rates and was significantly correlated with soil AN (alkali-hydrolyzable nitrogen) and pH content in the soybean rhizosphere. These results provide new insights to study the dynamics of soil nitrogen fixing bacteria and nitrogen fixing mechanism in intercropping system under different nitrogen supply levels that can be exploited to enhance nitrogen use effi-ciency in soybean/maize intercropping systems.