EFFECTS OF TREE SPECIES, STAND AGE AND SOIL TYPE ON SOIL MICROBIAL BIOMASS AND ITS ACTIVITY IN A SOUTHERN BOREAL FOREST

Microbial C (C-mic) and N (N-mic), the C-mic-to-organic C (C-org) and N-mic-to-total N (N-t) ratios and the specific respiration of microbia l biomass were investigated in a southern boreal mixed forest. The for est 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 c lay content) or till (46% average clay content) deposits in the clay b elt region of northern Quebec. In the forest floors the relative conce ntrations of microbial C and N and the C-mic-to-C-org and N-mic-to-N-t ratios, regarded as measures of organic matter quality, declined with stand age whereas the specific microbial respiration increased, indic ating decreasing C assimilation efficiency. In the mineral soils, in c ontrast, C-mic-to-C-org and N-mic-to-N-t ratios increased with stand a ge. The C-mic-to-N-mic 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 (C-mic-to-C-org 1.9%, N-mic-to-N-t 7. 5%) than beneath the deciduous species birch (C-mic-to-C-org 2.2%, N-m ic-to-N-t 8.6%) and aspen (C-mic-to-C-org 2.4%, N-mic-to-N-t 9.2%). Av erage C-mic-to-N-mic ratios were only slightly different in the forest floors beneath the different tree species (C-mic-to-N-mic: conifers 8 .9, birch 7.2, and aspen 8.3). In both forest floors and mineral soils , average concentrations of C-mic and N-mic were generally higher in t he clay than in the till soils, but the C-mic-to-C-org ratios were sim ilar in both soil types. The average N-mic-to-N-t ratios were lower in till than in clay soils only beneath conifers. The average specific m icrobial respiration (qCO(2) = mu g CO2-C mg Cmic-1 d(-1)) in clay soi ls (22) was approximately half that in till soils (41). Since the micr obial parameters measured were sensitive to the factors stand age, tre e species and soil type, they may have the potential to be used as ind icators of the influence of forest management on soil organic matter q uality. (C) 1998 Elsevier Science Ltd. All rights reserved.