Characterizing Organic Carbon Flux from Litter Sources to Mineral-Soil Sinks
Julie Jastrow, Argonne contact (jdjastrow@anl.gov)
The Enriched Background Isotope Study (EBIS) provides data on carbon flux from litter sources to mineral soil sinks in United States eastern hardwood forests; these data are needed for testing process-level hypotheses and judging the efficacy of soil carbon cycling models. EBIS utilizes releases of radiocarbon that resulted in a whole-ecosystem isotopic label near Oak Ridge, Tennessee, to study issues associated with the terrestrial carbon cycle. EBIS observations of the different pathways leading from leaf litter and root detritus to soil organic matter pools with varying residence times are being used to parameterize and refine carbon dynamics models for the characterization of key process-level features of the soil carbon cycle applicable to eastern temperate forests. EBIS is a multi-institution project conducted in collaboration with Oak Ridge, Lawrence Livermore, and Lawrence Berkeley National Laboratories; the University of California, Irvine; and the U.S. Forest Service.
Following a unique, large release of 14CO2 from an industrial source in 1999, a gradient of labeling occurred; sites nearest the source experienced a pulse about three times that produced by atmospheric weapons testing. At four locations consisting of two soil types and two levels of 14C exposure (enriched and near-background), replicated plots were established where forest litter was manipulated through multi-year reciprocal transplants of enriched vs. near-background litter. The differently labeled root and surface litter sources enabled the tracking of carbon sources into soil organic matter pools. Argonne’s contributions to the project are to (1) investigate soil carbon dynamics in unprotected and protected soil organic matter pools and (2) contribute to estimates of root production, decomposition rates, and turnover into soil organic matter pools.
During the 4-year EBIS experiment, we found that the carbon dynamics associated with root turnover occurred mostly at the macroaggregate scale rather than the microaggregate scale, confirming a longer residence time for carbon in microaggregate-protected particulate organic matter forms. In contrast, 14C from litter sources increased at both aggregate scales suggesting a different mode of carbon incorporation, such as transport and sorption of soluble or colloidal carbon to exchange sites or assimilation by microbes associated with both particulate- and mineral-associated carbon forms. New atmospheric pulses of 14CO2 in 2004 and 2005 led to enrichment of additional plant material. These pulses ended the initial experiment, but the radiocarbon inputs provided a new opportunity to collect and deploy enriched leaf litter, organic layer humus, and fine root litter for soil carbon cycle studies along the climatic and edaphic gradients of eastern hardwood sites in the U.S. AmeriFlux network. The results of these studies, initiated in 2007, will be used to parameterize and refine existing carbon dynamics models, which in turn will be used to address the long-term fate of ecosystem carbon inputs and the potential for ecosystem carbon sequestration. |
For the new experiment — distributed across AmeriFlux sites in New Hampshire, Massachusetts, Michigan, and Missouri — Argonne has prepared and deployed the experimental treatments for quantifying the decomposition and turnover of root litter and the movement of root carbon into soil carbon pools over time. Enriched roots collected from Oak Ridge were washed, sorted, homogenized, and analyzed for radiocarbon content. Soils collected from each of the AmeriFlux sites were sieved and processed to remove existing roots and homogenize the soil. These roots and soils were then used to produce 14C-enriched root litter bags for deployment and sequential harvesting at each of the AmeriFlux sites. We are also collaborating with other project researchers to carry out the soil fractionations needed to quantify the transport of carbon from surface litter, the humus layer, and roots into functionally meaningful soil organic matter pools.
Links
EBIS web page: http://ebis.ornl.gov/
AmeriFlux sites with EBIS experiments:
Bartlett Experimental Forest, NH: http://public.ornl.gov/ameriflux/Site_Info/siteInfo.cfm?KEYID=us.bartlett.01
Harvard Forest EMS Tower, MA: http://public.ornl.gov/ameriflux/Site_Info/siteInfo.cfm?KEYID=us.harvard_forest.01
University of Michigan Biological Station, MI:
http://public.ornl.gov/ameriflux/Site_Info/siteInfo.cfm?KEYID=us.umbs.01
Missouri Ozark Site, MO: http://public.ornl.gov/ameriflux/Site_Info/siteInfo.cfm?KEYID=us.moozark.01