EFFECTS OF TREE SPECIES, STAND AGE AND SOIL TYPE ON SOIL MICROBIAL BIOMASS AND ITS ACTIVITY IN A SOUTHERN BOREAL FOREST J. BAUHUS, 1 * D. PARE  2,3 and L. CO à TE  3 1 Department of Forestry, Australian National University, Canberra, ACT 0200, Australia, 2 Universite du QueÂbec aÁ MontreÂal, DeÂpartment des Sciences Biologiques, Groupe de Recherche en E  cologie ForestieÁre, C.P. 8888, Succursale Centre-ville, Montreal, Que., Canada H3C 3P8 and 3 BiodoÃme de MontreÂal, 4777 avenue Pierre-de-Coubertin, Montreal, Que., Canada H1V 1B3 (Accepted 18 August 1997) SummaryÐMicrobial 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 speci®c 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 ®r. 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 ¯oors the relative concentrations 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 speci®c microbial respiration increased, indicating decreasing C assimilation eciency. In the mineral soils, in contrast, C mic -to-C org and N mic -to-N t ratios increased with stand age. The C mic -to- N mic ratio widened with stand age in both the forest ¯oors and mineral soils, suggesting that the proportion of fungi had increased. Concentrations of microbial C and N were on average lower in forest ¯oor 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 mic -to-N t 8.6%) and aspen (C mic -to-C org 2.4%, N mic -to-N t 9.2%). Average C mic -to-N mic ratios were only slightly dierent in the forest ¯oors beneath the dierent tree species (C mic -to-N mic : conifers 8.9, birch 7.2, and aspen 8.3). In both forest ¯oors and mineral soils, average concentrations of C mic and N mic were generally higher in the clay than in the till soils, but the C mic -to- C org ratios were similar 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 speci®c microbial respiration (qCO 2 =mg CO 2 -C mg C mic À1 d À1 ) 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 in¯uence of forest management on soil organic matter quality. # 1998 Elsevier Science Ltd. All rights reserved INTRODUCTION The soil micro¯ora is a small but signi®cant com- ponent in most terrestrial ecosystems. Soil microbial activity contributes to the regulation of soil carbon storage, soil respiration and ecosystem productivity. The role of soil microbial biomass as a relatively labile nutrient pool in the cycling of C, N and P is well established (Marumoto et al., 1982b; Van Veen et al., 1987; Duxbury et al., 1989, Jenkinson and Parry, 1989). Amounts of microbial biomass are in¯uenced by soil texture and soil organic matter (SOM) quality (Wardle, 1992; Ross and Tate, 1993, Bosatta and A Ê gren, 1994; Hassink, 1994). The quantity and composition of microbial biomass is sensitive to changes in the soil chemical and physi- cal environment (Wolters and Joergensen, 1991; Wardle, 1992; Bauhus and Khanna, 1994; Beck et al., 1995). In addition, it has been shown that amounts of soil microbial C and N can be in¯u- enced by forest management (Ohtonen et al., 1992; Bauhus and Barthel, 1995; PietikaÈinen and Fritze, 1995) and increase proportionally with forest pro- ductivity (Myrold et al., 1989). From the above it has been suggested that concentrations of microbial C and N may be regarded as sensitive indicators for changes in the soil ecosystem. Normally the quan- tity and quality of the bulk of SOM changes only slowly and because of the large background value of soil C and the spatial heterogeneity of soils, changes are dicult to measure. Therefore, microbial biomass and, in particular, the ratio of microbial C and N to total organic C and N have been suggested as indicators of the state and modi®- cation of SOM (Anderson and Domsch, 1989; Wolters and Joergensen, 1991; Bosatta and A Ê gren, 1994). Soil microbial biomass is usually resource limited and thus microbial C and N concentrations are generally related to amounts of soil C and N (Wardle, 1992). The ratio of microbial C to soil or- ganic carbon has thus been used as an indicator for C availability (Anderson and Domsch, 1986; Insam Soil Biol. Biochem. Vol. 30, No. 8/9, pp. 1077±1089, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-0717/98 $19.00 + 0.00 PII: S0038-0717(97)00213-7 *Author for correspondence. 1077