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Harvard Forest Symposium Abstract 2009

  • Title: The role of tree species and mycorrhizal fungi on seasonal patterns of amino acid cycling in temperate forest soils
  • Primary Author: Edward Brzostek (Indiana University - Bloomington)
  • Additional Authors: Adrien Finzi (Boston University)
  • Abstract:

    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. Coupled with these seasonal measurements, we performed two experiments to investigate the roles of temperature, substrate availability, and belowground carbon allocation on amino acid production rates.

    The first experiment manipulated temperature and protein substrate availability in a factorial design. We collected soils from each tree species in April, June, and August. We measured amino acid production rates at two temperatures, field temperature at time of sampling and 23 degrees C, and two substrate levels, ambient or elevated protein availability. The second experiment utilized ingrowth mineral soil cores and organic horizon bags to manipulate belowground carbon allocation. We excluded roots from the ingrowth cores and bags using a nylon mesh that allows only the ingrowth of mycorrhizal fungi. We used a 1mm mesh core or bag as a control that allowed ingrowth of both roots and mycorrhizal fungi. These treatments were implemented in April and the experiment was terminated in September.

    Seasonal monitoring shows significant differences in the availability of amino acids across the growing season and between tree species. These differences likely result from seasonal and species driven variation in microbial activity and enzyme production. 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. The belowground C manipulation experiment reveals 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 carbon released by roots fuels amino acid production in these soils by providing the carbon capital required for microbial enzyme production.

  • Research Category: Soil Carbon and Nitrogen Dynamics