Synergistic effects of diazotrophs and arbuscular mycorrhizal fungi on soil biological nitrogen fixation after three decades of fertilization

Abstract Biological nitrogen (N) fixation (BNF) via diazotrophs is an important ecological process for the conversion of atmospheric N to biologically available N. Although soil diazotrophs play a dominant role in BNF and arbuscular mycorrhizal fungi (AMF) serve as helpers to favor BNF, the response of soil BNF and diazotrophic communities to different long‐term fertilizations and the role of AMF in diazotrophs‐driven BNF are poorly understood. Herein, a 33‐year fertilization experiment in a wheat–maize intercropping system was conducted to investigate the changes in soil BNF rates, diazotrophic and AMF communities, and their interactions after long‐term representative fertilization (chemical fertilizer, cow manure, wheat straw, and green manure). We found a remarkable increase in soil BNF rates after more than three decades of fertilization compared with nonfertilized soil, and the green manure treatment rendered the highest enhancement. The functionality strengthening was mainly associated with the increase in the absolute abundance of diazotrophs and AMF and the relative abundance of the key ecological cluster of Module #0 (gained from the co‐occurrence network of diazotrophic and AMF species) with dominant diazotrophs such as Skermanella and Azospirillum . Furthermore, although the positive correlations between diazotrophs and AMF were reduced under long‐term organic fertilization regimes, green manuring could reverse the decline within Module #0, and this had a positive relationship with the BNF rate. This study suggests that long‐term fertilization could promote N fixation and select specific groups of N fixers and their helpers in certain areas. Our work provides solid evidence that N fixation and certain groups of diazotrophic and AMF taxa and their interspecies relationship will be largely favored after the fertilized strategy of green manure.