Primary production in semi-arid and arid environments can be water limited and nitrogen regulated. We examined short-term (hourly) and relatively long-term (monthly) responses of so microbial biomass, soil N mineralization dynamics fine root production and ectomycorrhizal colonization rates to a single water and N addition in the sand shinnery-oak ecosystem in West Texas. Microbial biomass C and N exhibited rapid response to water and N addition. However, over the short term, water decreased microbial biomass C. A low addition of nitrogen (5 g m-2) had a short-term positive effect on microbial biomass, but no long-term effect. The addition of 50 g N m-2 significantly, increased microbial biomass C, N mineralization rates, and root growth over 4 months. Ectomycorrhizal colonization was decreased at the high rates of N addition. The results indicate that in this semi-arid environment, below-ground processes in the sand shinnery-oak ecosystem are strongly limited by N availability, while water regulates the period of activity.