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Mycorrhizal fungi are important drivers of soil organic matter dynamics, but it can be difficult to isolate the effects of the fungi themselves from co-varying traits of their host trees. For example, many trees with an evergreen leaf habit associate with ectomycorrhizal (ECM) fungi, while many deciduous tree species associate with arbuscular mycorrhizal (AM) fungi. Because leaf habit influences the quantity and quality of organic matter inputs to soil, it is often an important factor in soil carbon and nitrogen dynamics, and thus can mask the effects of mycorrhizal fungi on soil organic matter processes. We evaluated how tree mycorrhizal associations and leaf habit separately influence the amount and composition of mineral-associated organic matter (MAOM) and particulate organic matter (POM) in forest soils in New Hampshire and Vermont, USA. We measured carbon (C) and nitrogen (N) concentrations and C:N ratios of three soil density fractions beneath six tree species that vary in mycorrhizal association and leaf habit. We found lower concentrations of MAOM C and N beneath evergreen vs. deciduous trees, but only for tree species associating with AM fungi. Further, MAOM C:N was higher beneath evergreen trees and beneath trees with ECM fungi rather than AM fungi. These results add to the growing body of support for mycorrhizal fungi as mediators of soil organic matter dynamics, suggesting that the MAOM fraction is more sensitive to leaf habit beneath AM-associated versus ECM-associated trees. 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- and 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. 

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.