Long-term fertilization with high nitrogen rates decreased diversity and stability of diazotroph communities in soils of sweet potato

Sweet potato (Ipomoea batatas Lam) could produce acceptable root yield in low-nitrogen (N) soils, with substantial N uptake potentially attributed to supplies provided via biological N2-fixation by free-living diazotrophs. However, dynamics of diazotrophic communities as influenced by soil properties across the N fertilization gradients are still largely unclear. Long-term fertilization experiment under wheat-sweet potato rotation was established in an acid yellow brown soil since 2011. Soil samples were collected after sweet potato harvest (October 2018). The nitrogenase (nifH) gene real-time polymerase chain reaction (RT-PCR) and Hiseq highthroughput sequencing technologies were applied to soil samples from four N fertilizer treatments (0, 60, 120 and 180 kg ha-1). The results showed that long-term N fertilization significantly decreased abundance of the nifH gene, which was closely related to decreases in the content of available phosphorus (AP). The long-term high-N (120 and 180 kg ha- 1) fertilization dramatically altered structure of soil diazotrophic community and lead to in decreased diversity of diazotrophs, whereas low-N (60 kg ha- 1) fertilization maintained diazotrophic community diversity and stability. Compared with the low-N fertilizer inputs (0 and 60 kg ha- 1), the high rates of N fertilization (180 kg ha- 1) significantly decreased the relative abundance of nifH-harboring microorganisms, especially the phylum Cyanobacteria known as potential N2-fixers that could sustain fertility of sweet potato soils. There were negative correlations between N fertilization rates and the relative abundance of Proteobacteria, whereas the Bacteroidetes and Firmicutes showed a positive correlation. Moreover, the structural equation model (SEM) results suggested that the diazotrophic community diversity and structure were influenced mostly by soil pH rather than SOM and N forms (TN, NH4+-N and NO3- -N), and diazotrophs abundance mainly regulated by soil AP content. Our results implied that appropriate N fertilization is beneficial to sustain the stability and diversity of the diazotrophic community.