Long term responses of microbial biodiversity to changes in precipitation in a Chihuahuan desert grassland: Implications towards understanding effects of global climate change



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Global climate models predict increased temperature and precipitation variability in arid regions throughout southwestern North America within the next century, resulting in fewer rain events of greater magnitude, and longer periods between rain events. Longer inter-pulse periods coupled with increased air temperature will greatly reduce soil moisture availability. Although many studies have addressed the short-term impacts of precipitation variability on soil microbial and biogeochemical response patterns in arid ecosystems, few efforts have directly assessed the role of precipitation-pulse shifts in regulating long-term functional- or structural soil-microbial community responses over more than a couple of years. This research examined soil microbial and edaphic responses to climate model predictions of 25% increased seasonal rainfall applications over a 7-year period between 2002-2008 to determine long-term soil microbial responses to climate change with respect to variable rainfall in a Chihuahuan Desert sotol grassland at Big Bend National Park. We hypothesized that over time, these minor but realistic increases in moisture 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 fatty acid methyl ester (FAME) techniques. Microbial community functional responses to precipitation were characterized via carbon substrate utilization and enzymic activity. Our results strongly suggested that seasonal soil moisture timing and magnitudes regulate soil microbial ecosystem functionality. These results further suggested that minor shifts in the magnitude if rainfall patters are capable of altering soil microbial community dynamics in this desert grassland at Big Bend National Park. 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 and functional responses emerged during the fourth year of this study, as the relative abundances of saprophytic fungi, AM fungi, and gram negative bacteria, and soil exoenzymes β-Glucosidase (responsible for cellulose degradation) and Phosphodiesterase (responsible for phosphorus mineralization) displayed elevated levels in the summer + winter (SW) watering plots during this period. Interestingly, once the microbial response occurred in 2005, the changes in microbial community structure and function remained throughout the duration of the study (2006-2008) irrespective of annual ambient rainfall amounts received over the succeeding three years. This change in the microbial structure and function suggests that different components of the soil-microbial community may provide similar ecosystem function, but differ in response to seasonal temperature and precipitation. As soil microbes encounter altered precipitation amounts and timing along with increased soil temperatures predicted for this region, the ability of the soil microbial community to maintain functional resilience across the year may be reduced in this Chihuahuan Desert ecosystem.

This dissertation won 2nd Place in the Texas Tech University Outstanding Thesis and Dissertation Award, Biological Life Sciences, 2011.

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