Microbial population dynamics in an extreme environment: controlling factors in talus soils at 3750 m in the Colorado Rocky Mountains

High elevation talus soils are extreme environments for life. They are pertinent to the hydrology and biochemical cycling of high elevation ecosystems that supply drinking water to major metropolitan areas, and are undergoing change as a result of N deposition. As biological systems, they are virtually unstudied. In order to gain a basic understanding of these important systems, we studied the seasonal timing of C inputs and microclimate, and how they corresponded to microbial biomass dynamics, in vegetated and unvegetated soils from a high altitude talus slope at 3750 m in the Colorado Front Range. The soil microclimate was described by soil moisture and temperature measurements; C inputs were estimated with measures of eolian dust inputs and photosynthetically active radiation (PAR). The biomass of different microbial functional groups (glutamate and salicylate mineralizers adapted to different temperatures) was estimated seasonally over 3 years. We found that the soil microclimate can be divided into three distinct seasons: Winter, with free water and temperature between -2.9 and 0 oC; spring, characterized by wet isothermal soils (0 C); and summer, characterized by hot (mean 10.3 ?C; range 0-29.3 ?C) dry soils occasionally wetted by precipitation. The highest OM inputs to unvegetated soils occurred in spring; PAR only reached soils in the summer. In unvegetated soils, the biomass of glutamate mineralizers (GM) was highest in spring and summer. In contrast, the GM of vegetated soils had highest biomass in the winter when plants were senesced. In both unvegetated and vegetated soils, salicylate mineralizers (SM) had greater biomass levels in the summer when temperatures were highest.