Relatively little attention has been given to the processes that control the production and subsequent availability of amino acids in temperate forest soils. We examined how differences in soil organic matter chemistry and mycorrhizal association between temperate forest tree species of the Northeastern US lead to variation in amino acid cycling. We measured amino acid cycling throughout the growing season in soils from single tree species plots located in both the Harvard Forest, MA and the Pisgah State Forest, NH. These plots are dominated by either arbuscular mycorrhizal associated trees, Acer saccharum and Fraxinus americana, or ectomycorrhizal associated trees, Tsuga canadensis and Fagus grandifolia. We performed experiments to investigate the roles of temperature and substrate availability, and also belowground carbon (C) allocation on amino acid production rates.
Results from the temperature and substrate manipulation experiment demonstrate that early in the growing season amino acid production is limited by a significant interaction between temperature and substrate availability. This interaction fades throughout the growing season and surprisingly, in August, production rates are only limited by temperature. These findings show a strong seasonal control on the factors limiting amino acid production and shed light on the response of amino acid cycling to global change. Experimental manipulations of belowground communities revealed that only in tree species with high carbon-to-nitrogen ratios and ectomycorrhizal fungal associations, T. canadensis and F. grandifolia, did the exclusion of roots from soil lead to a reduction in amino acid production. This result suggests that C released by roots fuels amino acid production in these soils by providing the C capital required for microbial enzyme production.