Arid regions in southwestern US are predicted to experience increases in surface air temperatures and reduced annual rainfall (IPCC 2007). It is well established that soil moisture is an important driving variable of desert ecosystem functioning. Generally, with sufficient soil moisture, soil microbial activity is enhanced with increases in soil temperature. However, few studies have examined the role of daily soil temperature fluctuations on desert ecosystem functioning even though these ecosystems may experience large diurnal temperature fluctuations depending on the amount of vegetative cover. Here, we first examined relationships between microbial biomass and abiotic soil factors along an elevation gradient in Big Bend National Park. Soil moisture and soil organic matter increased with elevation, but soil acidity was highest at the mid-elevation sotol-grasslands. Microbial biomass was positively correlated with increases in soil moisture and soil organic matter, but showed an optimum with intermediate pH. These sites experience different soil temperature regimes. The site that experienced the highest soil temperature fluctuations and had high maximum soil temperature, a low-desert creosotebush bajada, was chosen for experimental temperature manipulation to determine which of these two temperature variables may play a more important role in microbial and nutrient dynamics. Soil temperature was altered by suspending 50% white polyethylene shade cloth onto a raised PVC frame, which allowed airflow over the soil surface to minimize alterations to the boundary layer.  

Results/Conclusions

Shading resulted in reduced daily temperature fluctuations by increasing nighttime soil temperature, decreasing diurnal soil temperature, or both. In shaded plots microbial biomass increased, while NH₄⁺ decreased. NO₃^- levels were reduced in shaded plots, but showed strongest seasonal effects (higher levels in spring). This study showed that the effects of daily and seasonal soil temperature fluctuations on belowground dynamics needs to be included in climate change predictions of C and N balance of desert ecosystems.