Rainfall pulses increased short-term biocrust chlorophyll but not fungal abundance or N availability in a long-term dryland rainfall manipulation experiment

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. This study 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, biological soil crust (biocrust) chlorophyll content, fungal abundance, and inorganic N were measured in soils adjacent to individuals of the grassland foundation species, Bouteloua eriopoda, and in the unvegetated interspace at multiple time points associated with experimental monsoon rainfall treatments. Treatments included 12 small weekly (5 mm) or 3 large monthly (20 mm) rain events, which had been applied during the summer monsoon for nine years prior to this study. Additionally, target plant C:N ratios were measured, and 15N-glutamate was added to biocrusts to determine potential for nutrient transport to B. eriopoda. Biocrust chlorophyll was up to 67% higher in the small weekly and large monthly rainfall treatments compared to ambient controls. Fungal biomass was 57% lower in soil interspaces than adjacent to plants but did not respond to rainfall treatments. Ammonium and nitrate concentrations near plants declined through the sampling period but varied little in soil interspaces. There was limited movement of 15N from interspace biocrusts to leaves but high 15N retention occurred 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 rain events at the end of the growing season, suggesting a limited response of N availability to future climate change.