Microbial biodiversity along an arid watershed
Ziehr, Lori Lynn Herrmann
The watershed approach to long-term research can provide critical data on ecosystem processes and changes resulting from anthropogenic factors in parks and natural preserves. Over the last fifteen years, a Watershed Ecosystem Research Network has been established at eleven National Parks and preserves in the United States. The Pine Canyon Watershed at Big Bend National Park is the newest addition to the network. The watershed comprises approximately 30 square miles and encompasses five vegetation zones, from upland closed canopy forest to lowland desert sub-shrub biomes. As a component of the larger Pine Canyon Watershed Program, the objectives of this research were to determine seasonal patterns in microbial biomass and functional biodiversity and to determine the relationships between nutrient dynamics, soil characteristics, and microbial activity along the Pine Canyon Watershed at Big Bend National Park, Texas. Two 100 X 30 m belt transects, 30 m, wide, were established in each biome in November 1995 and January 1996. Along these transects, extractable ammonium and nitrate, soil organic matter content, pH, soil moisture, soil microbial biomass carbon, and microbial functional diversity (using the BIOLOG system) were measured at different dates beginning in November 1995 and ending in January 1997. Rainfall and soil temperatures were recorded throughout the year. Changes in the functional status, activity levels, and size of soil microbial communities in arid ecosystems should provide a means of assessing ecosystem function in response to climate change or anthropogenic disturbance. Upper elevational forested sites, dominated by several species of oak, juniper, and pine, had the highest amounts of microbial biomass carbon and highest microbial functional diversity along the elevational gradient. A significant interaction between site and sampling time existed for microbial biomass carbon and microbial functional diversity (p < 0.001). Soil ammonium concentrations, organic carbon content, and soil moisture were correlated most strongly with microbial biomass carbon along the elevational gradient. Ammonium concentration and soil moisture were correlated most strongly with aspects of microbial functional diversity (total utilization and substrate richness) along the elevational gradient. Within sites, soil moisture was the only parameter significantly correlated with microbial biomass carbon. Soil moisture and nitrogen concentrations were correlated significantly with microbial functional diversity within sites. Microbial biomass carbon fluctuated seasonally for most sites, being highest in the summer and lowest in the winter. Microbial functional diversity in the upper elevational forested sites did not vary seasonally, while lower desert sites did so. This suggests that the upper elevational forested sites are more stable than the lower desert sites in responding to seasonal moisture inputs and other potential disturbances. Changes in patterns of precipitation that are likely to occur from global warming may have the greatest impact on lower desert ecosystems.