Saturday, 15 July 2006

A Plume Runs through it: Elucidating the Soil Carbon Cycle Using Enriched Radiocarbon Additions.

Christopher Swanston1, Paul Hanson2, Susan E. Trumbore3, Philip Jardine2, Julie Jastrow4, Margaret Torn5, Wilfred Post2, and Charles T. Garten2. (1) Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, PO Box 808, L-397, Livermore, CA 94551, (2) Oak Ridge National Laboratory, Bethel Valley Rd, Oak Ridge, TN 37831, (3) Univ of California, Irvine, 204 Physical Sciences Research Facility, Irvine, CA 92697-3100, (4) Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, (5) Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720

The Enriched Background Isotope Study (EBIS) takes advantage of an extraordinary opportunity: tracing a large atmospheric pulse of enriched radiocarbon through an entire forest ecosystem on the Oak Ridge Reservation (ORR) in eastern Tennessee.  During the summer of 1999, emissions from a local incinerator added a large pulse of 14CO2 to the local atmosphere, which was subsequently fixed by photosynthesizing vegetation and incorporated into leaves, stems, and roots.  Fallen leaves were collected from 14C-enriched and near-background areas of the ORR in the fall of 2000.  These litter sources were deployed in a reciprocal litter transplant experiment.  Highly enriched leaves were placed in background-14C stands for three consecutive years, while background-14C leaves were placed in highly enriched stands.  The combination of local 14C gradients and litter manipulations has allowed researchers to trace the flux of enriched 14C from known sources (leaves, roots) through various soil carbon pools, giving us insights into the sources, movement, and stabilization of soil carbon.   

Radiocarbon flux through these soils is characterized using numerous approaches, including measurement of radiocarbon in bulk organic and mineral soils, dissolved and particulate organic matter, microbial biomass, organo-mineral complexes, and soil gas.  A common theme emerges from the results of each of these approaches: recent inputs to the mineral soil carbon cycle appear to be dominated by root litter, whereas canopy litter contributes to a largely independent surface carbon cycle.  For example, litter input has had negligible impact on bulk soil 14C values, and only marginal impact on the most responsive particulate carbon pools.  Conversely, bulk mineral soils with 14C-enriched roots are rapidly enriched in radiocarbon, as are nearly all of the component carbon pools.  Likewise, microbial biomass and belowground gas measurements do not reflect enriched litter values, but quickly respond to root enrichment.  Dissolved organic carbon responded quickly to enriched litter input, with high values visible even in the lower profile; however, the total amount of carbon transported via soil water was quantitatively small.  The combined observations suggest that soil carbon cycling in eastern deciduous hardwood forest soils might be better characterized as a two-compartment system with surface litter inputs representing the primary carbon source for an organic-layer carbon cycle, and root litter inputs the primary carbon source for a mineral-soil carbon cycle. 


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