Microbial controls on symbiotic and free-living N2 fixation in subtropical Pueraria lobata communities, southwest China

Biological N2 fixation (BNF) is a major pathway of external nitrogen input to terrestrial ecosystems. Nevertheless, the relative effects of diazotrophic community and soil properties on symbiotic (SNF) or free-living (FNF) N2 fixation remain poorly understood. Here, 20 sites of kudzu (Pueraria lobata) communities, which were at the early succession stage following agricultural abandonment, were selected across a karst area in southwest China. BNF rates were determined using the acetylene reduction assay calibrated with the 15N2 fixation method. The average rate was 17.81 ± 2.17 mg N g−1 day−1 nodule for SNF and 45.36 ± 5.15 ng N g−1 day−1 soil for FNF. Diazotrophic communities were dominated by Bradyrhizobium at the genus level in both nodule and soil. According to structural equation modeling, the relative abundances of two diazotrophic genera (Desulfovibrio and Geobacter) were the strongest explanatory variable for the variation of SNF rates with the second strongest variable being nifH gene abundance. Soil water content and available phosphorus indirectly affected SNF via their effects on diazotrophic community composition. In contrast, soil FNF rates were most pronouncedly affected by the availability of vanadium and iron, followed by soil water content, available phosphorus, and the relative abundances of two diazotrophic genera, i.e., Burkholderia and Cupriavidus. Our findings therefore help to improve the understanding of the relationship between BNF rates and their associated diazotrophic communities, and hence benefit a better prediction of BNF under global change.