Organic matter decomposition is the main process by which carbon (C) is lost from terrestrial ecosystems, and mycorrhizal symbioses of plants (i.e., arbuscular mycorrhizae (AM) and ectomycorrhizae (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, and therefore, plant mycorrhizal identity may contribute to patterns in organic matter decomposition, and hence, soil C loss. To determine how mycorrhizal symbioses indirectly affect decomposer activity, we targeted four AM and four ECM tree species from a mixed-deciduous temperate forest for a field and lab study. We first characterized soil chemistry and soil microbial biomass for these eight tree species in situ. 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 lab incubation.
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, soils originating from AM trees respired more C than those from ECM trees. The addition of AM litter stimulated respiration more than did ECM litter, and this response was explained by litter C:N. 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 and ECM soils. Our findings demonstrate that AM and ECM trees have differing indirect effects on decomposer activity through quality of their litter and in the organic matter and/or decomposers they cultivate in soils. Such effects may help explain patterns in soil C dynamics in terrestrial ecosystems.