Changes in soil free-living diazotrophic community and co-occurrence patterns along desert wetland degradation gradient in the Mu Us Desert,northern China

Climate change and human activity have led to the degradation of desert wetlands.Free-living diazotrophs are vital for soil nitrogen input.However,a comprehensive understanding of how soil free-living diazotrophic communities and their co-occurrence patterns respond to desert wetland degradation is lacking.Here,quantitative polymerase chain reaction(qPCR),amplicon sequencing targeting nitrogenase gene(nifH),and network analysis were used to investigate the abundance,diversity,community composition,and co-occurrence patterns of soil free-living diazotrophs along the wetland degradation gradient,i.e.,non-degraded(ND),lightly degraded(LD),moderately degraded(MD),and severely degraded(SD),in the southeastern Mu Us Desert,northern China.The abundance and Shannon,Simpson,Chao 1,and ACE indexes decreased(P<0.05)by 14.6%,20.7%,2.1%,46.5%,and 45.0%,respectively,in SD wetland,whereas no significant difference(P>0.05)was observed between ND and LD wetlands.The relative abundance of Proteobacteria generally decreased(by 53.5%–19.7%)across the different degradation levels,while the relative abundance of Cyanobacteria increased(by 6.2%–40.1%)from ND to MD levels.The abundance,diversity,and community composition of diazotrophs were most strongly related to soil organic carbon,followed by total nitrogen,moisture,and pH.The least number of network nodes and edges and the lowest density were observed for MD and SD wetlands,indicating that the complexity of free-living diazotrophic networks was reduced by continued degeneration.Overall,severe desert wetland degradation affected the abundance,diversity,and network complexity of soil free-living diazotrophs more negatively than light degradation.This degradation promoted the growth of autotrophic diazotrophs and inhibited the growth of heterotrophic diazotrophs.These changes were mostly related to the loss of soil organic carbon.