Background/Question/Methods: Global climate models predict increased temperature and precipitation variability across arid regions in North America within the next century, resulting in fewer rain events of greater magnitude. Longer inter-pulse periods along with increased temperature will reduce soil moisture availability. While many studies have addressed the short-term impacts of precipitation - pulse variability in arid ecosystems on soil microbial and biogeochemical response patterns, few efforts have directly assessed the role of precipitation-pulse shifts in regulating long-term soil-microbial responses over more than a couple of years. This research examined soil microbial and edaphic responses to GCM predictions of 25% increased seasonal rainfall applications over a 7-year period between 2002-2008 to determine long-term soil responses to climate change with respect to variable rainfall in a Chihuahuan Desert grassland at Big Bend National Park. We hypothesized that over time, these minor but realistic increases in moisture additions would produce a measurable accumulative change in microbial, biogeochemical, and edaphic properties. To characterize the soil microbes in this desert grassland, community structure was classified as bacterial (gram-negative, gram-positive, and actinomycetes) and fungal (saprophytic fungi and arbuscular mycorrhiza) categories using FAME techniques. Microbial community functional responses to precipitation were characterized via carbon substrate utilization and enzymic activity.

Results/Conclusions: Our results strongly suggested that long-term, increased precipitation applications alter soil microbial community dynamics in this desert grassland. Over time, increases in moisture (25% additions based on climate change predictions) produced cumulative changes in soil microbial, biogeochemical, and edaphic properties. Microbial community structural responses emerged during the third year of this study, as the relative abundances of saprophytic fungi, arbuscular mycorrhizal fungi, and gram-negative bacteria were higher with supplemental watering. The enzyme β-Glucosidase (responsible for cellulose degradation) and the enzyme Phosphodiesterase (responsible for phosphorus mineralization) also displayed elevated levels with supplemental watering during this period. Furthermore, soil chemistry was highly responsive to increased moisture, and immediately responded to supplemental watering treatments. These observed changes in soil parameters suggest that different components within the soil-microbial community provide similar ecosystem functional contributions but differ in response to seasonal temperature and precipitation. As soils microbes encounter less-frequent precipitation events coupled with elevated soil temperatures in this ecosystem as predicted for this region, the ability of soil microbial communities to maintain functional resilience may be reduced in this Chihuahuan Desert grassland ecosystem.