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Harvard Forest Symposium Abstract 2012

  • Title: Seasonal plasticity in the temperature sensitivity of microbially-mediated fluxes in soils of three temperate forest types
  • Primary Author: John Drake (Boston University)
  • Additional Authors: Adrien Finzi (Boston University); Marc-Andre Giasson (Boston University); Kim Spiller (Boston University)
  • Abstract:

    The temperature sensitivity of the decomposition of soil organic matter strongly affects projections of future climate, given the strong effects of decomposition on atmospheric [CO2] and soil nutrient supply for plant production. However, estimates of the temperature sensitivity of decomposition have been highly controversial and depend on many factors, such as organic matter quality, abiotic factors governing substrate supply, and seasonal changes in microbial composition or function. Here, we test three hypotheses regarding possible temporal variation in the temperature sensitivity of five microbially-mediated soil fluxes (net N mineralization, net nitrification, proteolysis, maximum potential proteolysis, and microbial respiration) across forests in the Prospect Hill tract of Harvard Forest dominated by eastern hemlock (Tsuga Canadensis), white ash (Fraxinus americana), or a combination of red oak (Quercus rubra) and red maple (Acer rubrum).



    We sampled soils from these forests from the summer of 2010 though the fall of 2011, and measured the temperature-sensitivity of the five fluxes using lab incubations at 4, 10, 17, 23, 30, and 35oC. The three hypotheses were: (1) the temperature sensitivity of soil fluxes is constant across a seasonal cycle, (2) temperature sensitivity is thermally plastic, such that temperature sensitivity declines with increasing soil temperatures, and (3) there is distinct seasonal plasticity, such that temperature sensitivities vary strongly across seasons but not in a way that is directly correlated with soil temperature.



    Each of the hypotheses received support depending on the process under question and the metric of temperature sensitivity (i.e. Q10 vs. R10). Generally, Q10 values were invariant in time (supports hypothesis 1), while strong seasonal plasticity was observed in R10 values (supporting hypothesis 3). Large seasonal changes in R10 values were observed following snowmelt, suggesting that a temperature-independent seasonal change in microbial function or composition strongly affects spring soil N cycling. In particular, proteolytic capacity strongly declined during the spring following snowmelt and then recovered in the summer, while N mineralization followed the opposite pattern. Additionally, there were strong differences between the forest types related to the magnitude of soil N fluxes and the degree of observed seasonal plasticity. The ‘extravagant’ N-cycling ash forests with weak plant-microbe linkages typically had high flux rates across all processes and a larger degree of seasonal plasticity, relative to the ‘conservative’ N-cycling hemlock forests or the intermediate oak forests with stronger plant-microbe linkages. In summary, we will likely need to understand the seasonal biology of soil microbes to accurately predict seasonal patterns of decomposition in Harvard Forest.

  • Research Category: Soil Carbon and Nitrogen Dynamics