DECOMPOSITION, MICROBIAL COMMUNITY STRUCTURE, AND EARTHWORM EFFECTS ALONG A BIRCH-SPRUCE SOIL GRADIENT

To quantify C and N mineralization and the response of soil organisms in soil mixtures under controlled conditions, I set up a laboratory so il replacement series with soil from a pure spruce and a pure birch st and. To examine the role of a key species on process rates, the earthw orm Aporrectodea caliginosa was added to half of the microcosms. Carbo n mineralization and microbial biomass increased with the proportion o f birch soil along the experimental mixtures, while N mineralization d ecreased. Microbial community structure (estimated by examining the ph ospholipid fatty acid pattern) changed linearly along the soil series. Carbon mineralization, microbial biomass, and microbial community str ucture in soil mixtures could therefore be predicted from the patterns in pure birch and spruce soils. However, the mineralization of N was lower in mixtures than predicted from the pure soils. Earthworms incre ased C and N mineralization, decreased microbial biomass, and modified microbial community structure, but they required at least 25% birch c ontent in the soil mixture to be active throughout the experiment. The results suggest that microbial community structure and respiration ar e predictable in soil mixtures, but that earthworms exhibit thresholds and may respond in a nonlinear manner. Possible mechanisms behind the observed C and N mineralization pattern were explored with a simple m odel in which N mineralization was linked to the C how through microbe s. The model suggests that nitrogen concentration of assimilated subst rate may have been lower in birch soil than in spruce soil, although t he opposite was true for bulk soil. Alternatively, the rate of inorgan ic nitrogen losses, through abiotic fixation to organic matter and/or gaseous losses, may have been higher in birch than in spruce soil.