Influence of nitrogen and phosphorus additions on N-fixation activity, abundance, and composition of diazotrophic communities in a Chinese fir plantation.

Although biological nitrogen (N) fixation (BNF) is an important N input process in subtropical forest ecosystems, how the diazotrophic communities related to this process respond to N and phosphorus (P) inputs is largely unknown. We investigated the effects of exogenous N and/or P inputs on N-fixation activity, diazotrophic abundance and community composition using a continuous application of fertilizers over 5years experiment in a Chinese fir plantation. The fertilization regimes included control (CK), P treatment (P), low N addition treatment (N1), high N addition treatment (N2), low N and P addition treatment (N1P) and high N with P addition treatment (N2P). N-fixation activity was determined using the acetylene reduction assay (ARA). Quantitative PCR and Illumina Miseq sequencing of nifH gene were performed to analyze diazotrophic abundance and community composition, respectively. Our results showed that P addition increased N-fixation activity and nifH gene abundance by 189.07nmol CH and 1.02×10copiesg dry soil, respectively, while were reduced by 1.19nmol CH and 2.04×10copiesg dry soil when N was added. The application of P with low N (N1P) effectively alleviated the inhibitory effect of N input on N-fixation activity. N-related treatments resulted in significant decreases in operational taxonomic unit (OTU) richness and shifts in diazotrophic community structure. N-fixation activity and nifH gene abundance were strongly and positively correlated with soil pH and negatively correlated with mineral N (NH-N and NO-N) contents, while mineral N concentrations rather than soil pH appeared to be the main factor altering diazotrophic community structure. These results revealed that P addition played a positive role in regulating biological nitrogen fixation in subtropical forest ecosystems.