Microbial diversity and activity of disturbed soil in the northern Chihuahuan Desert

The effects of intense grazing, seasonal drought, and fire on soil microbia l diversity (substrate utilization) and activity in a northern Chihuahuan D esert grassland were measured in summer 1997, winter 1998, and spring 1998. Intense livestock grazing was initiated in winter 1995, burning occurred i n August 1994, and drought stresses were imposed from October 1994 to June 1997. Microbial diversity was inferred from the cal-bon substrate utilizati on patterns in both gram (+) and gram (-) Biolog plates. Microbial activity was estimated by the activity of selected enzymes. Neither microbial diver sity nor activity was affected by grazing. The interaction of intense grazi ng and stress sub-treatments only occurred in spring for one set of diversi ty measurements. The maximum microbial diversity and activity occurred in t he winter-drought-stress sub-plots in summer and spring. Burning reduced mi crobial diversity and most enzyme activities as compared to the control in summer and spring. Microbial diversity was also lower in summer-drought-str ess sub-plots than in thr: control in summer and spring. Microbial diversit y was highest in summer, intermediate in winter, and lowest in spring. Micr obial activity was generally higher in summer and lower in winter. it was c oncluded that substrate availability was the most important factor affectin g the diversity and activity of soil microorganisms within a season. Soil m oisture was not the factor causing differences in microbial diversity and a ctivity among the stress treatments, but it was a predictor for some microb ial responses under a particular stress.