Response of soil microbial communities to mixed forests of European beech and conifers: Variations with site conditions

Abstract

Tree - soil interactions depend on environmental conditions. Planting trees may strongly impact microorganisms in particular at unfavorable site conditions, compromising the functioning of soil microorganisms. To understand the effects of tree species composition on soil microorganisms, we quantified structural and functional responses of soil microorganisms to forest types across environmental conditions using substrate-induced respiration and phospholipid fatty acid analyses. Five forest types were studied including pure stands of native European beech (Fagus sylvatica), range expanding Norway spruce (Picea abies), and non-native Douglas-fir (Pseudotsuga menziesii), as well as the two conifer - beech mixtures. We found that microbial functioning strongly depends on site conditions, in particular on soil nutrients. At nutrient-poor sites, soil microorganisms were more stressed in pure and mixed coniferous forests, especially in Douglas-fir, compared to beech forests. By contrast, microbial structure and functional indicators in beech forests varied little with site conditions, likely because beech provided high amounts of root-derived resources for microbial growth. Since soil microbial communities are sensitive to Douglas-fir, planting Douglas-fir may compromise ecosystem functioning in particular at nutrient-poor sites. Overall, root-derived resources are important for determining the structure and functioning of soil microbial communities, and soil microorganisms sensitively respond to plantations containing tree species that may differ in the provisioning of these resources.