Effect of timber harvesting on microbial biomass fluxes in a northern Rocky Mountain forest soil

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Abstract
Microbial biomass and relative bacterial and fungal percentages were measured in organic forest soil from a Rocky Mountain site subjected to four harvesting treatments: RL, clear-cut and residue left; RR, clear-cut and residue removed; RB, clear-cut and residue burned; C, uncut control. Microbial biomass peaked in spring and fall regardless of treatment. Biomass in soil from the RB treatment was significantly (p < 0.05) less than that in soil from the other treatments most of the year, biomass did not significantly differ in soil from the RR and C treatments. During summer and winter, microbial biomass in soil from the RL treatment was significantly greater than that in soil from any other treatment, probably because of the large amount of organic residue left after harvest; moreover, this residue insulated the soil, preventing it from drying or freezing. At soil temperatures above 5.degree. C, microbial biomass correlated positively with soil moisture regardless of treatment; at soil temperatures below 2.5.degree. C, microbial biomass correlated positively with increasing soil temperature. During periods with snow cover, bacterial and fungal percentages were roughly equal regardless of treatment; during the rest of the year, bacterial percentages were high in the RL and RB treatments and low in the RR and C treatments. During periods without snow cover, bacterial and fungal percentages correlated positively with increasing soil pH; however, at near-freezing temperatures the percentage of bacteria and fungi seemed unaffected by soil pH. These findings suggest that treatments that remove a large portion of available site nutrients while reducing soil microbial activity could limit stand development.