Cultivation increased soil potential denitrification rates by modifying denitrifier communities in desert-oasis ecotone

Oases soils in northwestern China are used widely for agricultural production, but low soil moisture and fertility necessitate high volumes of irrigation and fertilization, with significant losses of water via evaporation and nitrogen via denitrification. The dynamics of denitrifying communities and their responses to potential denitrification rate (PDR) in continuously irrigated oases remain unclear. In this study, we examined the dynamics of nirK and nirS denitrifying communities in three distinct areas, an old oasis field (OOF, 54 years of cultivation), a young oasis field (YOF, 20 years) and an adjacent uncultivated sandy land (USL, 0 years), and used the partial least squares path model (PLS-PM) to predict how and to what extent soil properties and denitrifying communities may be responsible for changes in PDR. Our findings indicate that cultivation, compared with the USL treatment, improved soil structure and fertility and increased the abundance and diversity of denitrifying microbes, resulting in a further elevation of soil PDR in YOF and OOF. Additionally, our analysis highlights the potential dominance of the nirK gene in denitrification. PLS-PM revealed that soil chemical properties and microbial biomass indirectly affected soil PDR by regulating the abundance and diversity of nirK and nirS genes. Conversely, soil physical properties had a direct negative impact on PDR. Alterations in PDR were, in part, attributed to changes in abundance, richness and beta-diversity, but not correlated with changes in alpha-diversity. Notably, the standardized total effect demonstrated that the denitrifier community exhibited greater responsiveness to changes in PDR than did soil properties. Overall, our findings suggest that denitrifying communities may play a more important role than soil properties in PDR, and an increased understanding of denitrifying communities allows PDR prediction during conversion of oasis to cultivated land.