SOIL MICROBIAL BIOMASS AND ACTIVITY OF A DISTURBED AND UNDISTURBED SHRUB-STEPPE ECOSYSTEM

Disturbance of shrub-steppe soils and alterations in plant cover may a ffect the distribution, size and activity of soil microorganisms and t heir ability to biogeochemically cycle essential nutrients. Therefore, the soil microbial biomass and activity and selected soil enzyme acti vities were determined for two arid ecosystems, an undisturbed perenni al shrub-steppe and annual grassland, which was initially shrub-steppe and has been an annual grassland since the disturbance caused by farm ing ceased in the 1940s. Soils were sampled at 0-5 and 5-15 cm depths beneath sagebrush (Artemisia tridentata Nutt.), bluebunch wheatgrass [ Elytrigia spicata (Pursh) D. R. Dewey] and cryptogamic soil lichen cru st at the perennial site and beneath downy brome (Bromus tectorum L.) at the annual grassland site. Soils were analyzed for physical propert ies, inorganic N, microbial biomass C and N, respiration and several e nzymes. The soil pH and bulk density usually increased, while inorgani c N, total N and total C decreased as a function of soil depth. Soil m icrobial biomass C and N, soil respiration and soil dehydrogenase acti vity were 2-15 times higher in the top 5 cm of soil than at the 5-15 c m depth regardless of plant type. Loss of this surface soil would ther efore be detrimental to microbially-mediated cycling of nutrients. Sur face soil (0-5 cm depth) microbial biomass C and N and soil respiratio n, dehydrogenase and phosphatase activity were influenced by plant typ e and decreased in the order B. tectorum > A. tridentata = E. spicata > soil crust. Spatial distribution of plant species at the shrub-stepp e site resulted in ''islands'' of enhanced microbial biomass and activ ity underneath the shrubs and grasses when compared to the interplant areas covered with soil crust. When plant cover was used to compute a landscape estimate of soil microbial biomass C and N for the perennial shrub-steppe and the annual grassland, similar values were obtained. This indicates that while the distribution of microorganisms may be mo re heterogeneous in the shrub-steppe, the average across the landscape is the same as the more homogeneous annual grassland.