Microbial biomass generally increases with organic matter acrretion in soil
s, 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, w
ith 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 s
oils we studied all had the same parent material, aspect and climate, but r
anged in soil organic matter (SOM) content from 6 to 250 g kg(-1). Total so
il C and N, silt, clay, and SOM all increased linearly as sand content decr
eased. Using the substrate-induced growth response (SIGR) assay and a most-
probable number (MPN) assay, we estimated the general microbial biomass (gl
utamate mineralizers) and the biomass of a microbial group capable of degra
ding more complex carbon compounds (salicylate mineralizers). In vegetated
soils, both groups were positively correlated with SOM. In barren soils, ho
wever, silt content was the best predictor of total biomass, which showed n
o trend at all with SOM. In contrast, the relationship between the biomass
of salicylate mineralizers and SOM was the same in vegetated and barren soi
ls, although it was not significant in barren soils. In addition, the propo
rtion of salicylate mineralizers in the total biomass was higher in barren
soils than in vegetated soils, which is possibly as a result of different c
arbon quality inputs to the soils, This research represents the first in-de
pth description of the biology and soil characteristics of barren high elev
ation talus soils.