Effects of Altered Precipitation on Biological Soil Crusts, Fungi, and Soil Nitrogen Availability at the Monsoon Rainfall Manipulation Experiment (MRME) in the Sevilleta National Wildlife Refuge, New Mexico (2016)

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.