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

Abstract

Climate change and human activities have led to the degradation of desert wetlands. Free-living diazotrophs are vital to soil nitrogen input. However, a comprehensive understanding of how soil free-living diazotrophic community and co-occurrence patterns respond to desert wetland degradation is lacking. Here, quantitative polymerase chain reaction (qPCR) and amplicon sequencing targeted on nitrogenase ( nifH ) genes and network analysis were used to investigate soil free-living diazotrophic abundance, diversity and composition and co-occurrence patterns along the wetland degradation gradient [non-degraded (ND), lightly degraded (LD), moderately degraded (MD), and severely degraded (SD) wetlands] in the southeastern of Mu Us Desert, northern China. Results showed the abundance, Shannon, Simpson, Chao1 and Ace indexes decreased ( P < 0.05) by 14.6%, 20.7%, 2.1%, 46.5% and 45.0% at SD, respectively, whereas no significant difference ( P > 0.05) was observed between ND and LD. The relative abundance of Proteobacteria generally decreased across the degradation (53.5%–19.7%), while the variation was opposite for Cyanobacteria from ND to MD (6.2%–40.1%). Soil organic carbon had the strongest linkage with abundance, diversity and composition, followed by total nitrogen, moisture and pH. The lowest network nodes, edges and density were observed at MD and SD, indicating the complexity of free-living diazotrophic networks was reduced by continued degeneration. Overall, severe desert wetland degradation more negatively affects soil free-living diazotrophic abundance, diversity and network complexity than light degradation. The degradation promotes the growth of autotrophic diazotrophs, inhibiting heterotrophic diazotrophs. These changes are mostly related to the loss of soil organic carbon.