Soil diazotrophs sustain nitrogen fixation under high nitrogen enrichment via adjustment of community composition

ABSTRACT Biological nitrogen (N) fixation, an important pathway of N, inputs from the atmosphere to Earth’s ecosystems, is well demonstrated to decline under N input. However, it remains unclear why N fixers sustain N fixation in many forests under high atmospheric N deposition. To address this knowledge gap, we analyzed the response of the diazotroph community to low N loads (short-term and low N addition; 3-year N addition at the rates of 25–50 kg N ha −1 year −1 ) vs high loads (chronic and high N addition; 9-year N addition at the rate of 150 kg N ha −1 year −1 ) in forest soils using high-throughput sequencing. Rates of N fixation decreased under low and high N loads (by 13%–27% and 10%–12%, respectively). Richness and alpha diversity (ACE and Chao1) of the soil diazotroph community decreased under low but not high N loads. Approximately 67.1%–74.4% of the nifH gene sequences at the OTU level overlapped between the control and low N loads, but only 52.0%–53.6% of those overlapped between the control and high N loads, indicating a larger shift of diazotroph community composition under high N loads. Low N loads increased soil NH 4 + concentrations, which decreased diazotroph community richness, diversity, and N fixation rates, whereas the increased soil NH 4 + concentrations under high N loads did not have negative impacts on the structure and function of the diazotroph community. These findings indicate that diazotrophs sustain N fixation under high N deposition via adjustment of their community composition in forest soils. IMPORTANCE This study examined the changes in soil diazotroph community under different loads of simulated N deposition and analyzed its relationship with N fixation rates in in five forests using high-throughput sequencing. The magnitudes of N fixation rates reduced by low N loads were higher than those by high N loads. Low N loads decreased richness and diversity of diazotroph community, whereas diazotroph community structure remained stable under high N loads. Compared with low N loads, high N loads resulted in a less similarity and overlap of nifH gene sequences among the treatments and a larger adjustment of diazotroph community. Low N loads increased soil NH4+ concentrations, which decreased diazotroph community richness, diversity, and N fixation rates, whereas the increased soil NH4+ under high N loads did not have negative impacts on diazotroph community structure and N fixation. Based on these findings, it is urgently needed to incorporate the loads of N deposition and the composition of diazotroph community into terrestrial N-cycling models for accurate understanding of N inputs in forest ecosystems.