Rhizosphere effect and water constraint jointly determined the roles of microorganism in soil phosphorus cycling in arid desert regions

Phosphorus (P) is an important macronutrient in terrestrial ecosystems because it is involved in many biochemical reactions and limits plant growth. Existing studies have found that microbial communities regulate P cycling in soils, but little is known about biochemical mechanism. Specifically, it is not clear what factors drive P cycling in soils, and how microbial genes involved in the soil P cycling respond to water scarcity in desert ecosystems. Here, we conducted metagenomic analyses to reveal changes in microbial genes involved in the soil P cycling in an arid desert, and further evaluate microbial responses to P transformation along soil water content (SWC) gradient. The results showed that the relative abundance of P-starvation response regulation genes (phoR and phoP) exhibited an obviously changes along SWC gradient, indicating that they were affected by water constraint. Among the genes related to P uptake and transport, pstSCAB showed little change in the relative abundance while was not active toward water constraint. In contrast, P-mineralization genes (phn, ppx, and phoD) obviously increased with the decrease of SWC, reflecting that microorganism alleviated water constraint by promoting P mineralization. The "rhizosphere effect" clearly improved the role of several key genes of P cycling in the whole gene co-occurrence networks, indicating that plant rhizosphere environments promoted microbial P-assimilation capacity. The influence of SWC on microbial genes increased with the decrease of its value. pH and soil texture affected microbial genes in high water content environment, but not in lower water content environment. Inorganic phosphorus, mainly in the form of Ca10-P, played a crucial role in the adaptation of desert plants to resource limitation. Overall, microbial roles in regulating soil P cycling were jointly deter-mined by "rhizosphere effect" and water constraint in arid desert.