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●Fine roots and mycorrhizal fungi may either stimulate leaf litter decomposition by providing free‐living decomposers with root‐derived carbon, or may slow decomposition through nutrient competition between mycorrhizal and saprotrophic fungi. ●We reduced the presence of fine roots and their associated mycorrhizal fungi in a northern hardwood forest in New Hampshire, USA by soil trenching. Plots spanned a mycorrhizal gradient from 96% arbuscular mycorrhizal (AM) associations to 100% ectomycorrhizal (ECM)‐associated tree basal area. We incubated four species of leaf litter within these plots in areas with reduced access to roots and mycorrhizal fungi and in adjacent areas with intact roots and mycorrhizal fungi. ●Over a period of 608 d, we found that litter decayed more rapidly in the presence of fine roots and mycorrhizal hyphae regardless of the dominant tree mycorrhizal association. Root and mycorrhizal exclusion reduced the activity of acid phosphatase on decomposing litter. ●Our results indicate that both AM‐ and ECM‐associated fine roots stimulate litter decomposition in this system. These findings suggest that the effect of fine roots and mycorrhizal fungi on litter decay in a particular ecosystem likely depends on whether interactions between mycorrhizal roots and saprotrophic fungi are antagonistic or facilitative. 

Recent work suggests mycorrhizal fungi are important drivers of soil organic matter dynamics; however, whether this is a result of the fungi themselves or related traits of their host trees remains unclear. We evaluated how tree mycorrhizal associations and foliar chemistry influence mineral-associated organic matter (MAOM) and particulate organic matter (POM) in temperate forests of northern New England, USA. We measured carbon (C) and nitrogen (N) concentrations and C:N of three soil density fractions beneath six tree species that vary in both mycorrhizal association and foliar chemistry. We found a significant decline in the concentration of MAOM C and N with increasing foliar C:N in soil beneath tree species with arbuscular mycorrhizal (AM), but not ectomycorrhizal (ECM) fungi. The C:N of POM and MAOM was positively associated with the foliar C:N of the dominant tree species in a forest, and MAOM C:N was also higher beneath ECM- rather than AM-associated tree species. These results add to the growing body of support for mycorrhizal fungi as predictors of soil C and N dynamics, and suggest that C concentration in the MAOM fraction is more sensitive to organic matter chemistry beneath AM-associated tree species. Because MAOM decomposition is thought to be less responsive than POM decomposition to changes in soil temperature and moisture, differences in the tendency of AM- vs. ECM-dominated forests to support MAOM formation and persistence may lead to systematic differences in the response of these forest types to ongoing climate change. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Soil respiration is the dominant pathway by which terrestrial carbon enters the atmosphere. Many abiotic and biotic processes can influence soil respiration, including soil microbial community composition. Mycorrhizal fungi are a particularly important microbial group because they are known to influence soil chemistry and nutrient cycling, and, because the type of mycorrhizal fungi in an ecosystem can be assessed based on the plant species present, they may be easier than other soil microbes to incorporate into ecosystem models. We tested how the type of mycorrhizal fungi—arbuscular (AM) or ectomycorrhizal (ECM) fungi—associated with the dominant tree species in a mixed hardwood forest was related to soil respiration rate. We measured soil respiration, root biomass, and surface area, and soil chemical and physical characteristics during the growing season in plots dominated by ECM-associated trees, AM-associated trees, and mixtures with both. We found rates of soil respiration that were 29% and 32% higher in AM plots than in ECM and mixed plots, respectively. These differences are likely explained by the slightly higher nitrogen concentrations and deeper organic horizons in soil within AM plots compared with soil in ECM and mixed plots. Our results highlight the importance of considering mycorrhizal associations of dominant vegetation as predictors of carbon cycling processes. Key words: Soil respiration; Mycorrhizal fungi; Carbon; Microbial activity; CO2; Northern hardwood forest.