Influence of Parent Material on Soil Microbial Communities in a Range of Ponderosa Pine Forests.

Parent material exerts indirect control over soil microbial communities through its influence on soil pH and mineral composition. However, the ways in which soil microbial community parameters vary as a function of parent material are poorly understood. We sampled a lithosequence of four parent materials (rhyolite, granite, basalt, limestone) under Pinus ponderosa to examine the effects of soil mineralogy and acidity on soil microbial biomass and mineralization rates. Three soil profiles were examined on each parent material and analyzed by X-ray diffraction, CEC, pH, selective dissolution, C and N content, and ¹³C stable isotope signature. Samples of both surface and subsurface soils from each of the four parent materials were incubated for 40 days. Microbial biomass was measured by chloroform fumigation extraction on days 5, 15, and 40 of the incubation. Microbial communities from incubated soils were compared on the basis of specific metabolic activity, biomass, δ¹³C of respired CO₂ and cumulative amount of C mineralized over the course of the incubation. We found that mineralization rates and their associated controlling factors varied both with parent material and with depth. Surface soils had the highest mineralization rates and also showed the strongest correlations with soil parameters. We found that in surface soils substrate quality and not mineral variables was the greatest determining factor in mineralization rates. Total soil organic C content, ¹³C content, and C/N ratio accounted for 95% of the variation in cumulative mineralized C in surface soils. Factors controlling biomass and δ¹³C of respired CO₂ differed between surface and subsurface soils, but also varied with incubation time as substrate quality and N availability changed.