P.I.(s): Susan E. Trumbore -- University of California, Irvine
Co-I(s): Eric A. Davidson -- Woods Hole Research Center; Jennifer Harden, Eric Sundquist -- USGS
Objectives: To combine measures of carbon inventories of soils, 14C content of soil atmospheres, and rates of soil respiration to estimate the rates of carbon accumulation and turnover in soils of each of the major vegetation types of the boreal forests at the Southern and Northern Study Areas. 14C measurements of physically and chemically fractionated soils will be used to partition soil organic matter into pools that turn over on annual, decadal-centennial, and millennial time scales. The understanding of soil carbon dynamics gained from these estimates will be tested against evidence from chronosequence studies which document the accumulation of carbon in physically and chemically defined pools on decadal (time since fire) and millennial (time since soil formation) scales, and estimates of rates of soil respiration and 14C content of respired CO2. We will explore several of the factors controlling soil carbon accumulation and dynamics, including quality of the detrital sustrata, and availability of CO2 as expressed by soil moisture, gas exchange rate, or drainage class.
Location: NSA and SSA
View an electronic poster from TGB-12, in Acrobat Format (PDF) [help with Acrobat].
We are determining rates of C accumulation and turnover of soils in the range of conditions found in the BOREAS NSA (wetlands to dry jack pine sites). In addition, TGB-12 (Sundquist and Winston) has collected non-growing season CO2 flux data. Selected results to date:
Types of Data Collected, Equipment Used:
We stratified soil sampling using a series of sites on clay and sand soils representing times varying from several years to nearly a century since the last stand-killing fire. At each site, we laid out a transect with 10 soil pits to gather data on intra-site variability of organic matter. (Biomass data along these transects were collected by Brian Stocks' group from Forestry Canada). We sampled the organic layers of each pit to determine the total gC m-2 in moss and decomposed organic layers. In all, a total of about 400 samples were collected from upland sites for moisture, density, carbon and nitrogen analyses. 14C will be measured on a subset of these to determine C turnover and accumulation rates.
To answer the second question, we had to develop a freeze- coring method to sample the upper meter of wetland sediment/soil without compressing the sample (not an easy task in standing water). We sampled the wetland sites being measured by TGB-3 (Bubier), and will work with TGB-3 to tie together net ecosystem exchange, C flux measurements and C accumulation rates (from 14C) at fen, collapse bog and collapse fen sites in the NSA.
TGB-12 measurements of winter fluxes of CO2, CH4, as well as soil gas and temperature profiles have been made at four NSA sites. We have also measured C isotopes in soil gases to determine the turnover time of C decomposing to produce CO2.
Known Problems or Caveats:
Care must be taken in scaling C accumulation data beyond the sites where we have measured them, without reference to maps of relevant soil properties. C accumulation rates we have measured in mosses (the top of the soil profile) are offset by decomposition deeper in the profile -- thus these rates are greater than the net C sequestered in soils. We are working with other TGB and TE groups to work out the balance of C accumulation and decay in soils.
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Last Updated: November 25, 1998