Mycorrhizal associations of trees have different indirect effects on organic matter decomposition

1. Organic matter decomposition is the main process by which carbon (C) is lost from terrestrial ecosystems, and mycorrhizal associations of plants (i.e. arbuscular mycorrhizas (AM) and ectomycorrhizas (ECM)) may have different indirect effects on this loss pathway. AM and ECM plants differ in the soil decomposers they promote and the quality of litter they produce, which may result in different patterns of organic matter decomposition, and hence, soil C loss. 2. To determine how mycorrhizal associations indirectly affect decomposer activity, we collected soils and litters from four AM and four ECM tree species from a mixed-deciduous temperate forest for a field and laboratory study. We first characterized in situ patterns in soil chemistry and soil microbial biomass among these eight tree species. We then conducted a microcosm experiment with mineral soils, leaf litter and fine roots originating from these tree species, where we reciprocally crossed litters and soils, and quantified the rate of heterotrophic respiration over a 140-day laboratory incubation. 3. In natural forest conditions, AM tree soils contained lower total C and microbial biomass C:N relative to ECM tree soils. In our microcosm experiment, AM soils supported greater heterotrophic respiration than did ECM soils. The addition of AM litter stimulated respiration more than did ECM litter, owing to the lower C:N of AM litter. Matching the mycorrhizal identity of litter and soil resulted in a difference in total respiration, such that combinations of AM litters with AM soils lost more C than did combinations of ECM litters with ECM soils. 4. Synthesis. Our findings demonstrate that AM and ECM trees have differing indirect effects on soil decomposer activity through the decomposers they cultivate and/or the quality of organic matter they produce. Mycorrhizal differences in litter quality accentuate these effects on soil C loss and may explain patterns in soil C dynamics in terrestrial ecosystems.