Soil microbial biomass and activity of a disturbed and undisturbed shrub-steppe ecosystem

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Disturbance of shrub-steppe soils and alterations in plant cover may affect the distribution, size and activity of soil microorganisms and their ability to biogeochemically cycle essential nutrients. Therefore, the soil microbial biomass and activity and selected soil enzyme activities were determined for two arid ecosystems, an undisturbed perennial shrub-steppe and annual grassland, which was initially shrub-steppe and has been an annual grassland since the disturbance caused by farming ceased in the 1940s. Soils were sampled at 0–5 and 5–15 cm depths beneath sagebrush ( Artemisia tridentata Mutt.), bluebunch wheatgrass [ Elytrigia spicata (Pursh) D.R. Dewey] and cryptogamic soil lichen crust at the perennial site and beneath downy brome ( Bromus tectorum L.) at the annual grassland site. Soils were analyzed for physical properties, inorganic N, microbial biomass C and N, respiration and several enzymes. The soil pH and bulk density usually increased, while inorganic N, total N and total C decreased as a function of soil depth. Soil microbial biomass C and N, soil respiration and soil dehydrogenase activity were 2–15 times higher in the top 5 cm of soil than at the 5–15 cm depth regardless of plant type. Loss of this surface soil would therefore be detrimental to microbially-mediated cycling of nutrients. Surface soil (0–5 cm depth) microbial biomass C and N and soil respiration, dehydrogenase and phosphatase activity were influenced by plant type and decreased in the order B. tectorum A. tridentata = E. spicata soil crust. Spatial distribution of plant species at the shrub-steppe site resulted in “islands” of enhanced microbial biomass and activity 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 more heterogeneous in the shrub-steppe, the average across the landscape is the same as the more homogeneous annual grassland.