Changes in soil prokaryotic communities and nitrogen cycling functions along a groundwater table drawdown gradient in desert wetlands

Desert wetlands are evolving into deserts by groundwater table (GWT) drawdown. However, the changes in microbial communities and functions during the GWT drawdown are unclear, which hinders the predictive power of biogeochemical processes across the desertification. Here, 16S rRNA gene sequencing, PICRUSt2 and qPCR were used to investigate soil prokaryotic diversity, composition and nitrogen cycling gene abundance at four vegetation types [flooded swamp (FS), drained swamp (DS), desert grassland (DG), and bare sandy land (BS)] along a GWT decline gradient in the Mu Us Desert, northern China. Results showed that prokaryotic Shannon and Chao1 indexes were significantly reduced at BS than those at FS (p < 0.05). Whereas no significant difference was observed between FS, DS and DG (p > 0.05). Distinct shifts in community composition were found along the GWT decline gradient. The dominant taxa gradually changed from obligate anaerobes and eutrophic microbes to facultative anaerobes, and finally to aerobic, oligotrophic and drought-tolerant microbes. Soil moisture was the most important factor in regulating the communities. In addition, GWT drawdown inhibited the relative abundance of genes involved in nitrogen fixation, assimilatory nitrite reduction, and nitrate oxidation, but enhanced the relative abundance of genes related to denitrification, assimilated nitrate reduction, ammonia oxidation and ammonification. Thus, GWT drawdown inhibits nitrogen input potential and exacerbates nitrogen loss potential. These results help in understanding the succession characteristics of desert wetland desertification.