Soil temperature controls of microbial activity in a desert ecosystem

The Chihuahuan Desert is expected to experience an altered precipitation regime due to global warming. The Hadley GCM2 and the Canadian GCM1 have forecast two different scenarios that predict either an increase in winter or summer precipitation in this arid region. An increase in precipitation may lead to increased cloud cover, which may, in turn, affect both soil temperature and evaporation rates. In June 2006 we initiated an experiment in Big Bend National Park in a sparsely vegetated creosote bajada where we simulated increased cloud cover by using wood fiber shade cloth on the soil surface. We are examining the effects of the resulting decrease in soil temperature and evaporation on microbial nutrient dynamics. Shading decreased maximum and average soil temperatures by 9.3% and 4.3%, respectively, while minimum soil temperatures were similar between treatments from June through September. No differences were observed in levels of extractable NH₄⁺ or NO₃^- even though active microbial biomass had increased by 38% in the shaded plots. Soil organic matter was similar between treatments, while soil moisture content was 36% higher in September in the shaded plots. A reduction in soil temperature may have a greater impact on soil microbial dynamics than the predicted increase in precipitation.