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Harvard Forest Symposium Abstract 2008

  • 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:

    Among the largest uncertainties for understanding responses to climate change is the potential change in soil carbon stocks. The objectives of our study are: (1) to estimate the residence times of soil carbon by remeasuring the 14C of soil C fractions that we first measured a decade ago; (2) to estimate soil C pools susceptible to increased rates of decomposition in a new soil warming experiment; (3) to estimate decadal trends in root and soil microbial respiration. To address the first objective, we resampled in August 2007 the same soil pits that Gaudinski et al, (2000; Biogeochemistry 51: 33-69) sampled in 1996. These samples are being fractionated and analyzed for radiocarbon content at UC-Irvine. These measurements will enable us to test the quantitative predictions of what the current radiocarbon content of each fraction should be if the Gaudinski et al. parameterization of a model of soil C dynamics is correct. For the second objective, we have been measuring soil 14CO2 efflux in a new warming experiment run by Serita Frey at the Harvard Forest. After four such measurements during the growing season of 2007, we have not yet detected an effect of soil warming on the 14C signature of CO2 efflux in the field (Fig. 1), but we have preliminary indications of an increase in decomposition of old soil organic matter in controlled laboratory incubations. For the third objective, we have completed our 13th year of soil respiration measurements in the footprint of the EMS tower, including a 12-year record of 14C measurements. Analysis of the first 10 years of measurements showed that mean annual soil respiration during 2000-2003 was higher than during 1995-1997, which parallels a similar trend of increasing gross primary productivity and total ecosystem respiration measured by eddy covariance at the EMS tower. However, annual soil respiration has declined during the period 2004-2006 (Fig. 2), which demonstrates the importance of long-term measurements for distinguishing between long-term trends and shorter-term anomalies. Finally, we have installed new automated chambers for soil respiration measurements at the EMS site and at the Little Prospect Hill tower site. Our analysis of half-hourly measurement of soil respiration from automated systems has revealed a significant lag between diel patterns of soil temperature and soil respiration, indicating a potential role for canopy processes affecting the diel pattern of substrate supply for belowground respiration.

  • Research Category: Forest-Atmosphere Exchange

  • Figures:
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