Monday, November 13, 2006 - 3:15 PM

Tracing Carbon-14 Flux and Stabilization the Enriched Background Isotope Study (EBIS).

Christopher Swanston, USDA Forest Service, 410 MacInnes Drive, Houghton, MI 49931, Margaret Torn, Earth Sciences Division, Lawrence Berkeley National Laboratory, One Cyclotron Road MS 90-1116, Berkeley, CA 94720, and Paul Hanson, Oak Ridge National Laboratory, Oak Ridge National Laboratory, Bethel Valley Road Building 1062, Oak Ridge, TN 37831-6422.

Research using tracer-level radiocarbon can provide valuable information on the dynamics and stabilization of soil carbon, but 14C-tracer studies are typically small in scale due to cost and environmental concerns.  The Enriched Background Isotope Study (EBIS) takes advantage of an opportunity to trace a large pulse of enriched radiocarbon through a forest ecosystem at Oak Ridge Reservation, ORNL, Tennessee.  The opportunity arose following an incinerator release of highly enriched radiocarbon to the atmosphere, which was subsequently fixed by vegetation.  Highly 14C-enriched leaves were placed in ‘near background’ stands, while near-background leaves were placed in highly enriched stands, enabling us to measure rapid enrichment or dilution of soil organic fractions with differing rates of turnover and potentials for C stabilization. These fractions include accessible particulate organics (‘free light fraction’, FLF), protected particulate organics (‘occluded light fraction’, OLF), mineral-associated organic matter (dense fraction, DF), and dissolved organic matter (DOM).  Over four years of soil collection (0-15 cm) and analysis, 14C abundance in the FLF decreased in the enriched stand as incorporation of ambient 14C diluted the initial enrichment.  In the near-background stand, addition of enriched litter had no discernable effect on the FLF, but appeared to enrich the OLF through time.  The OLF also showed gradual enrichment in the 14C-enriched stand.  The DF showed little change.  The OLF appears to incorporate DOM from litter and soil organic matter, and may also be largely derived from occlusion of the FLF.  Most EBIS results indicate that primary short term additions to soil carbon in this system come from root exudates and senescence.  This research shows that litter may also play a role, though more limited and at a different temporal scale.  We are currently working on a dynamic model (ECHOSoil) to simulate the movement of carbon and radiocarbon between the litter and soil fractions.