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

Abstract

To quantify C and N mineralization and the response of soil organisms in soil mixtures under controlled conditions, I set up a laboratory soil replacement series with soil from a pure spruce and a pure birch stand. To examine the role of a key species on process rates, the earthworm Aporrectodea caliginosa was added to half of the microcosms. Carbon mineralization and microbial biomass increased with the proportion of birch soil along the experimental mixtures, while N mineralization decreased. Microbial community structure (estimated by examining the phospholipid fatty acid pattern) changed linearly along the soil series. Carbon mineralization, microbial biomass, and microbial community structure 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 increased C and N mineralization, decreased microbial biomass, and modified microbial community structure, but they required at least 25% birch content in the soil mixture to be active throughout the experiment. The results suggest that microbial community structure and respiration are 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 model in which N mineralization was linked to the C flow through microbes. The model suggests that nitrogen concentration of assimilated substrate may have been lower in birch soil than in spruce soil, although the opposite was true for bulk soil. Alternatively, the rate of inorganic nitrogen losses, through abiotic fixation to organic matter and/or gaseous losses, may have been higher in birch than in spruce soil.