Microbial Biomass Levels in Barren and Vegetated High Altitude Talus Soils

Microbial biomass generally increases with organic matter accretion in soils, but little is known about the relative proportion of specific microbial functional groups that compose the biomass. We measured the biomass of two microbial functional groups in soils of a high altitude talus slope (3700 m) in the Colorado Rocky Mountains. Talus slopes are composed of boulders, with occasional patches of soil in the rock matrix. Because of the severity of the physical conditions, many soil patches are barren. Carbon inputs to barren talus soils are thought to be predominantly eolian deposition. The soils we studied all had the same parent material, aspect and climate, but ranged in soil organic matter (SOM) content from 6 to 250 g kg−1 Total soil C and N, silt, clay, and SOM all increased linearly as sand content decreased. Using the substrate-induced growth response (SIGR) assay and a most-probable number (MPN) assay, we estimated the general microbial biomass (glutamate mineralizers) and the biomass of a microbial group capable of degrading more complex carbon compounds (salicylate mineralizers). In vegetated soils, both groups were positively correlated with SOM. In barren soils, however, silt content was the best predictor of total biomass, which showed no trend at all with SOM. In contrast, the relationship between the biomass of salicylate mineralizers and SOM was the same in vegetated and barren soils, although it was not significant in barren soils. In addition, the proportion of salicylate mineralizers in the total biomass was higher in barren soils than in vegetated soils, which is possibly as a result of different carbon quality inputs to the soils. This research represents the first in-depth description of the biology and soil characteristics of barren high elevation talus soils