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Harvard Forest Symposium Abstract 2010

  • Title: Commonalities and differences among arbuscular mycorrhizal and ectomycorrhizal tree species in the controls of proteolytic enzyme activity in soil
  • Primary Author: Edward Brzostek (Indiana University - Bloomington)
  • Additional Authors: Adrien Finzi (Boston University)
  • Abstract:

    The breakdown of organic nitrogen (N) by extracellular enzymes in temperate forest soils links soil organic matter decomposition to ecosystem productivity. 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 proteolytic enzyme activity. We measured seasonal variation in enzyme activity in 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 three experiments that investigated the response of proteolytic enzymes to manipulations of temperature, substrate availability, and belowground carbon (C) allocation. In the belowground C manipulation experiment, we used ingrowth treatments to either include or exclude roots from soils in the field.

    Seasonal bulk soil proteolytic enzyme activities did not differ among species or mycorrhizal association, though the controlling factors did. The belowground C manipulation experiment reveals that ECM fine roots stimulate soil enzyme activity. This suggests that ECM tree roots exert an important control over enzyme activity in the soil by actively modifying microbial activity in the rhizosphere. In AM soils however, roots exert little effect. AM soils have narrower C:N ratios and higher rates of inorganic N cycling than ECM soils, suggesting that labile soil organic matter provides the resources required to support enzyme activity. Although the root effect was only evident in ECM soils, the seasonal patterns of temperature and substrate limitation to proteolytic enzyme activity in soils was remarkably similar among tree species and mycorrhizal associations. Temperature limitation was only expressed early in the growing season whereas substrate limitation lasted well into the growing season. Further, the Q10 of proteolysis in soils of all species was relatively low compared to other common soil processes, like soil respiration. This implies that alterations of substrate availability may be of equal or greater importance than increasing soil temperatures in regulating N cycling responses to global change.

  • Research Category: Soil Carbon and Nitrogen Dynamics