Asymmetric responses of abundance and diversity of N-cycling genes to altered precipitation in arid grasslands

Precipitation changes exert a fundamental effect on the nitrogen (N) cycle in water-limited grasslands. Soil microbes are essential drivers of N cycle, and the rates and their stabilities of interrelated N-cycling processes are reflected by the abundance and diversity of N-cycling genes. Yet, little is known about how altered precipitation affects the genes involved in the entire N-cycling pathways.

By combining a 6-year precipitation manipulation experiment (-30%, ambient, +30%, +50%) with metagenomic sequencing, we investigated the responses of N-cycling gene abundance and diversity to altered precipitation at two soil depths (0-10 and 30-50 cm).

We found that increased precipitation enhanced the abundance of numerous key genes, leading to an acceleration of N turnover, but decreased the diversity of ammonium assimilation genes. Decreased precipitation did not reduce abundance or diversity of N-cycling genes. Most N-cycling genes showed generally consistent responses to altered precipitation in the topsoil (0-10 cm) and subsoil (30-50 cm), albeit with clear distinctions in both abundance and diversity by soil depth. These precipitation-specific responses and depth-dependent variabilities of functional genes were attributed to the distinct taxonomic composition of each N-cycling gene. Further, we quantified gross N transformation rates and found that they were well predicted by the abundance of most N-cycling genes (e.g., genes involved in ammonium assimilation and nitrification).

Our study sheds new light on the soil N cycle under precipitation alterations from the perspective of individual gene abundance and diversity, and shows that future increases in precipitation could accelerate soil N turnover in arid and semi-arid lands.