Anthropogenic nitrogen (N) deposition in arid ecosystems is relatively low when compared to mesic systems. However, because arid ecosystems are characterized by low precipitation and high potential evoaptranspiration rates (PET) 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 (2 and 4 times the background of 3.9 kg/ha) and variable precipitation affected soil microbial functional diversity (FD) 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 (FD) parameters and monthly precipitation were measured and evaluated at both sites biannually from August 2003 to August 2006. Differences in bacterial and fungal FD on carbon (C) substrates and fungal FD on nitrogen (N) substrates were estimated via substrate utilization profiles from Biolog microtiter plates.

Results/Conclusions

In the Oak-Pine forest site, Fungal FD on C substrates was increased by low level N treatments but not in response to the high level N treatment. Variable precipitation and N deposition had a synergistic effect on microbial functional diversity in the mid-elevation grasslands site. The high level N treatments differed significantly from controls when coupled with high seasonal precipitation. This difference was due primarily to increases in fungal FD on N substrates. During periods of average seasonal rainfall sites microbial function did not differ between treatments. Bacterial FD did not respond to either N treatments or precipitation in either site.The overall impact of N deposition on fungal FD in the Oak-Pine forest site is likely due to the higher precipitation and the more diverse litter makeup of the site when compared to the grasslands. Microbial dependence on precipitation in more arid systems is illustrated by the lack of response to N treatments with low precipitation amounts in the grasslands system. With continuing increases in global N deposition and GCM models showing major changes in precipitation patterns for the Chihuahuan Desert, the results of this study suggest a potential for major changes in arid system microbial function.