The BOREAS Information System
Trace Gas Biogeochemistry (TGB)

TGB-12: Input, Accumulation, and Turnover of Carbon in Boreal Forest Soils: Integrating 14C Isotopic Analyses with Ecosystem Dynamics

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].

TGB-12 Data Sets

  • Gas Flux Winter Flux
  • Isotopes Soil Gas d13CO2
  • Isotopes Soil Gas d14CO2
  • Isotopes Soil Gas 14C(dom)
  • Isotopes Soil Gas soil C
  • Isotopes Soil Gas soil N
  • Tower Soil Temp
  • Tower Other Soils Data

  • Get some TGB-12 data using FTP (BOREAS Investigators only, password required). [FTP Help]

    TGB-12 BOREAS Operations 1994

    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:

    1. Fall and winter flux measurements by Sundquist and Winston have shown that significant net loss of carbon dioxide, especially during the fall and early winter. Preliminary calculations based on our data, combined with those of Wofsy et al. (TF-3), suggest that this non-growing efflux amounts to the equivalent of about 1/3 to 1/2 of the net CO2 uptake during the growing season. The good agreement demonstrated between our chamber measurements and Wofsy's OBS tower flux measurements of CO2 suggests that our winter measurements at other sites (OJP and YJP) should also provide useful estimates of winter C efflux. Concurrent measurements of soil moisture and temperature, soil gas concentrations, as well as winter snowpack properties and CO2 isotopes are the only data available to determine the sources (and causes of variation) of winter CO2 efflux. Preliminary data for the winter of 94-95 show warmer soil temperatures associated with generally higher CO2 flux.
    2. Soil C inventory and accumulation rates vary widely, with lowest values for both occurring in sandy, dry jack pine sites, and highest values in Sphagnum-moss-dominated upland clay soils. Long-term accumulation of C appears tied to variables such as moss C inventory, fire frequency and fire intensity.
    3. Short-term (decadal) rates of C sequestration are determined from moss C inventory changes following stand-killing fire events, and, for Sphagnum mosses, from 14C data. Values for annual average C accumulation in feather mosses show NEE of these mosses is of similar order to those estimated for trees at the OBS site. We are also collecting data for wetland sites.

    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