The Impacts of Simulated Nitrogen Deposition on Soil Microbial Communities along the Pine Canyon Watershed at Big Bend National Park, TX



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Anthropogenic nitrogen deposition in arid ecosystems is relatively low when compared to mesic systems. However, because arid ecosystems are characterized by low precipitation and high potential evapotranspiration rates in addition to nutrient limitations additional anthropogenic N deposition will likely have major effects on community structure and function of soil microorganisms relative to the effects found in mesic systems. Determining the impacts of N deposition in arid systems is critical as they cover more than 40% of the global land surface, contain a large portion of agricultural lands and are inhabited by more than 1 billion humans. This study evaluated how a simulated increase in annual N deposition and variable precipitation affected soil microbial functional diversity in a mid-elevation Sotol grassland and high-elevation Oak-Pine forest in the Chihuahuan Desert in Big Bend National Park, TX. Microbial functional diversity functional diversity parameters and monthly precipitation were measured and evaluated at both sites biannually from August 2003 to August 2006 and seasonally in the Sotol grassland in 2007. Differences in bacterial and fungal functional diversity on carbon substrates and fungal functional diversity on nitrogen substrates were estimated via Biolog microtiter plates from 2003 to 2007. Additionally, FAME analysis was used to evaluate relative proportions of soil bacteria and fungal groups during 2007. Finally, extracellular enzymes were evaluated during 2007 to evaluate the contribution of microbial enzyme activity on carbon, nitrogen and phosphorus cycling. Fungal functional diversity on carbon substrates was increased by low level nitrogen treatments in the Oak-Pine forest. However, increased nitrogen deposition did not effect carbon utilization in the Sotol grasslands. Fungal utilization of amino acids was significantly altered by increased nitrogen deposition in the Sotol grasslands. Additionally, Gram-negative bacterial community abundance and nitrogen cycling enzymes showed a seasonal response to additional nitrogen deposition in the Sotol grasslands. Seasonal influences and variable precipitation had a strong effect on most of the measured microbial community structural and functional parameters in the Sotol grasslands throughout the project. The difference in responses between the sites is largely due to differences in soil temperature, precipitation patterns and the different plant communities associated with each site.