The impact of precipitation change on nitrogen cycling in a semi-arid ecosystem

Climatic change is altering ecosystem structure and function, especially in the southwestern United States where trees are near their physiological water stress threshold. In piñon-juniper (Pinus edulis-Juniperus monosperma; PJ) woodlands, increased drought is causing differential mortality of piñon resulting in an ecosystem that is becoming juniper dominated. Using a precipitation manipulation, we assessed how both increased and decreased precipitation altered ecosystem function beneath piñon and juniper. We predicted that changes in precipitation would alter nitrogen (N) availability and mineralization at the site. Further, we predicted that these responses would differ beneath piñon and juniper crowns due to plant-level differences in transpiration and N uptake in response to drought. We found minimal interactions between tree species and the precipitation treatments on N cycling. However, across all years measured, soil nitrate decreased with increasing soil volumetric water content; a pattern that is likely due to reduced turnover in dry plots. In contrast, potential soil net-nitrogen mineralization was reduced in water removal plots relative to water addition plots indicating that nitrogen cycling rates were slower under drought. Tree type also influenced nitrogen dynamics in this woodland. Across all 4 years, soil N availability and potential soil net-mineralization rates were higher in soils beneath piñon relative to juniper across all treatments. Interestingly, the observed shifts in N cycling were not reflected in the abundance of N in microbial biomass or in ammonia-oxidizing bacteria, which are responsible for nitrification. The observed patterns may be due to increased N leaching from the soil during periods of increased rainfall or due to decreased microbial activity or plant N uptake when conditions are dry. The effect of precipitation change on N cycling may have long-term consequences on the plant community in this semi-arid ecosystem. Nitrogen concentrations are highest in the soil when water availability is low, thus when N concentrations are high, plants and microbes are relatively inactive and unable to use this resource.