Split application of phosphorus fertilizer in Chinese milk vetch-rice rotation system reshapes soil diazotrophic community structure

Chinese milk vetch (CMV) is the most popular leguminous green manure in green manure-rice rotation system in southern China. Although bacteria that work in symbiosis with CMV fix the majority of the nitrogen (N) in agroecosystems, diazotrophs are crucial to the N-cycle and contribute to the plant available N pool. The present study's objectives were to examine the impacts of moving partial phosphorus (P) fertilizer from the rice season to the CMV season in a CMV-rice rotation system on soil nutrients content, soil enzyme activity and stoichiometry, and diazotrophic community structure at the CMV full blooming stage. The treatments included control, no any fertilizers application with winter fallow-rice rotation, and P0, P1, P2, and P3, which meant 0, 1/3, 2/3, and 3/3, respectively, of the full dose P fertilizer addition (60 kg ha-1 P2O5) in one rotation system were applied in the CMV season and the remaining were applied in the rice season. Comparing with P0, the treatments of P1, P2, and P3 increased CMV dry biomass and rice yield from 2018 to 2021 seasons; Also, with these treatments, the soil total N (TN), organic matter (OM), and available P (AP) contents were improved by 34%-49%, 43%-55%, and 110%-130%, respectively. Alkaline phosphatase and L-leucine aminopeptidase activities were reduced significantly under the P1 treatment, indicating that P fertilizer application reduced the mineralization of organic nutrients and reduced P limitation to soil microorganisms. The treatments of P1, P2, and P3 increased the relative abundance of Frankia (1.5%-2.7%) and Mesorhizobium (0.4%-3.5%). In addition, the correlation analysis showed that Frankia, Skermanella, and Mesorhizobium were positively correlated with TN, OM, AP, ammonium-N, and nitrate-N contents. In conclusion, P fertilizer split application in the CMV-rice rotation system reshaped the soil diazotrophic community structure, with increased diazotrophic abundance positively correlated with soil nutrients contents.