Rainfall amount and timing jointly regulate the responses of soil nitrogen transformation processes to rainfall increase in an arid desert ecosystem

Climate models predict greater rainfall will occur in the arid and semiarid regions of Northwest China, where nitrogen (N) cycling is particularly sensitive to changes in rainfall regimes. Yet, how increasing rainfall regulates soil N transformation processes in these water-limited regions is still not well understood. We conducted a manipulative experiment in a desert ecosystem in Northwest China, whereby we simulated five different scenarios of future rain regimes (natural rains plus 0%, 25%, 50%, 75% and 100% of the local mean annual precipitation) each month from May to September in 2009. We examined in situ net N mineralization and soil N availability in both vegetated and bare soils, as well as leaf litter N release for the dominant shrub species Nitraria tangutorum monthly after each rain addition. We found that increased water availability via the simulated rain addition significantly decreased total net N mineralization rates over the growing season in both vegetated and bare soils. A larger amount of litter N was released after rain addition in vegetated soils, which could contribute to the higher concentrations of inorganic N in vegetated soils compared to bare soils. Furthermore, we found that the responses of soil N transformation processes to rain additions showed great seasonality, and thus both rainfall amount and timing jointly regulate the responses of soil N transformation processes to rainfall increase under future rainfall scenarios in this arid desert ecosystem. Over the growing season, rainfall addition reduced soil inorganic N concentrations but favored plant N uptake and microbial N immobilization. We suggest that the cycling of N will be greatly changed under future rainfall regimes, which may have consequences for ecosystem stability and functioning in this "N-conserving" desert ecosystem