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Microbial activity in drylands is mediated by the magnitude and frequency of growing season rain events that will shift as climate change progresses. Nitrogen is often co-limiting with water availability to dryland plants, and thus we investigated how microbes important to the nitrogen (N) cycle and soil N availability varied temporally and spatially in the context of a long-term rainfall variability experiment in the northern Chihuahuan Desert. Specifically, we assessed biological soil crust (biocrust) chlorophyll content, fungal abundance, and inorganic N in soils adjacent to individuals of the grassland foundation species, Bouteloua eriopoda, and in the unvegetated interspace at multiple time points associated with an experimental monsoon rain treatment. Treatments included small weekly (5 mm) or large monthly (20 mm) rain events, which had been applied during the summer monsoon for nine years prior to our sampling. Additionally, we evaluated target plant C:N ratios and added 15 N-glutamate to biocrusts to determine potential for nutrient transport to B. eriopoda. Biocrust chlorophyll was up to 67% higher in the small weekly or large monthly rainfall regimes compared to ambient controls. Fungal biomass was 57% lower in soil interspaces than adjacent to plants but did not respond to rainfall regime treatments. Ammonium and nitrate concentrations near plants declined through the sampling period but varied little in soil interspaces. There was limited movement of 15 N from interspace biocrusts to leaves but high 15 N retention in the soils even after additional ambient and experimental rain events. Plant C:N ratio was unaffected by rainfall treatments. The long-term alteration in rainfall regime in this experiment did not change how short-term microbial abundance or N availability responded to the magnitude or frequency of events, suggesting a limited response of N availability to future climate change. 

Abstract Oceanic oxygen deficient zones (ODZs) influence global biogeochemical cycles in a variety of ways, most notably by acting as a sink for fixed nitrogen (Codispoti et al. 2001). Optimum multiparameter analysis of data from two cruises in the Eastern Tropical North Pacific (ETNP) was implemented to develop a water mass analysis for the large ODZ in this region. This analysis reveals that the most pronounced oxygen deficient conditions are within the 13°C water (13CW) mass, which is distributed via subsurface mesoscale features such as eddies branching from the California Undercurrent. Nitrite accumulates within these eddies and slightly below the core of the 13CW. This water mass analysis also reveals that the 13CW and deeper Northern Equatorial Pacific Intermediate Water (NEPIW) act as the two Pacific Equatorial source waters to the California Current System. The Equatorial Subsurface Water and Subtropical Subsurface Water are synonymous with the 13CW and this study refers to this water mass as the 13CW based on its history. Since the 13CW has been found to dominate the most pronounced oxygen deficient conditions within the Eastern Tropical South Pacific ODZ and the Peru‐Chile Undercurrent, the 13CW and the NEPIW define boundaries for oxygen minimum conditions across the entire eastern Pacific Ocean.