Effects of tree species, stand age and soil type on soil microbial biomass and its activity in a southern boreal forest

Microbial C (Cmic) and N (Nmic), the Cmic-to-organic C (Corg) and Nmic-to-total N (Nt) ratios and the specific respiration of microbial biomass were investigated in a southern boreal mixed forest. The forest stands were 50 and 124 years old and consisted of trembling aspen, paper birch and mixed conifers comprising white spruce and balsam fir. Stands were growing on soils derived either from clay (89% average clay content) or till (46% average clay content) deposits in the clay belt region of northern Quebec. In the forest floors the relative concentrations of microbial C and N and the Cmic-to-Corg and Nmic-to-Nt ratios, regarded as measures of organic matter quality, declined with stand age whereas the specific microbial respiration increased, indicating decreasing C assimilation efficiency. In the mineral soils, in contrast, Cmic-to-Corg and Nmic-to-Nt ratios increased with stand age. The Cmic-to-Nmic ratio widened with stand age in both the forest floors and mineral soils, suggesting that the proportion of fungi had increased. Concentrations of microbial C and N were on average lower in forest floor beneath conifers (Cmic-to-Corg 1.9%, Nmic-to-Nt 7.5%) than beneath the deciduous species birch (Cmic-to-Corg 2.2%, Nmic-to-Nt 8.6%) and aspen (Cmic-to-Corg 2.4%, Nmic-to-Nt 9.2%). Average Cmic-to-Nmic ratios were only slightly different in the forest floors beneath the different tree species (Cmic-to-Nmic: conifers 8.9, birch 7.2, and aspen 8.3). In both forest floors and mineral soils, average concentrations of Cmic and Nmic were generally higher in the clay than in the till soils, but the Cmic-to-Corg ratios were similar in both soil types. The average Nmic-to-Nt ratios were lower in till than in clay soils only beneath conifers. The average specific microbial respiration (qCO2=μg CO2-C mg Cmic−1 d−1) in clay soils (22) was approximately half that in till soils (41). Since the microbial parameters measured were sensitive to the factors stand age, tree species and soil type, they may have the potential to be used as indicators of the influence of forest management on soil organic matter quality.