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Harvard Forest Symposium Abstract 2005

• Title: Decadal-Scale Measurements of Decadal-Cycling Forest Soil Carbon
• Primary Author: Eric Davidson (University of Maryland - Center for Environmental Science)
• Additional Authors: Kathleen Savage (Woods Hole Research Center); Sue Trumobre (University of California - Irvine)
• Abstract:

  Estimates of future changes in soil C stocks in response to changing climate, CO2 and N fertilization, insect infestations, and management are based primarily on model projections. The best regarded models partition soil organic matter into multiple pools, but the sizes and turnover times of simulated C pools are usually poorly constrained because of a paucity of long-term measurements. In the 1990s, we derived an annual belowground carbon budget and a multi-pool soil C model for the Harvard Forest in central Massachusetts based on annual estimates of litterfall, soil respiration, fine root biomass, soil C fractionation, and radiocarbon contents of soil C pools, fine roots, and C fluxes (Gaudinski et al., 2000; Fig. 1). We now have more than ten years of such measurements to further constrain this budget and its interannual and seasonal variability (Figs. 2 and 3). If our parameterizations of sizes and mean residence times of soil C pools in 1996 was correct, we now should be able to predict the radiocarbon signature of SOM, fractions of SOM, and fine root biomass in 2005 and beyond (Fig. 4).
  
  
  
  
  
  Our studies have also demonstrated considerable interannual and intra-annual variation of soil respiration processes (Savage and Davidson, 2001, 2003; Fig. 2). On average, we estimate about a 55-45 split between respiration of young C substrates (≤1 year since fixation, including root respiration and microbial decomposition of root exudates and turnover of young root and mycorrhizal tissues) and microbial decomposition of older substrates (Gaudinski et al., 2000; Fig. 1). However, this ratio likely varies as root and microbial processes respond differentially to phenology, drought, and other factors, causing temporal variation in the radiocarbon content of CO2 efflux within each year in addition to the long-term steady decline of radiocarbon content (Fig. 3).
  
  
  
  
  
  We intend to resample and fractionate soil C stocks about 10 years after our initial field work at the Harvard Forest in the mid 1990s (Gaudinski et al., 2000). By showing that we can predict how radiocarbon contents of soil C fractions change over a 10 year period, we will demonstrate increased confidence in our ability to characterize soil C pools and their dynamics, thus also conferring greater confidence in our ability to estimate how soil C will change given scenarios of changing inputs and decomposition rates (e.g., the scenario in Table 1). Our results will also provide a basis for comparing tested parameterizations of these soil C pools for the Harvard Forest to those used in simulations of temperate forest soils in global biogeochemical models that employ similar multi-pool soil C parameterizations.
  
  
  
  
  
  Gaudinski, J.B., S.E. Trumbore, E.A. Davidson, and S. Zheng. 2000. Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes. Biogeochemistry 51:33-69.
  
  
  Savage, K.E., and E.A. Davidson. 2003. A comparison of manual and automated systems for soil CO2 flux measurements: tradeoffs between spatial and temporal resolution. J. Exp. Botany 54:891-899.
  
  
  Savage, K.E., and E.A. Davidson. 2001. Interannual variation of soil respiration in two New England forests. Global Biogeochemical Cycles 15:337-350.
  
  
  

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