HR: 1340h
AN: B13C-0240    [Abstracts]
TI: The Age and Amount of Carbon Released From Incubations of Permafrost Soil From Northeastern Siberia
AU: * Dutta, K
EM: kdutta@ufl.edu
AF: Koushik Dutta, Department of Botany, University of Florida, Gainesville, FL 32611-8526 United States
AU: Zimov, S A
EM: tneh@mail.sakha.ru
AF: Sergei A. Zimov, Northeast Science Station, Republic of Sakha, Cherskii, PO Box 18 Russian Federation
AU: Schuur, E A
EM: tschuur@ufl.edu
AF: Koushik Dutta, Department of Botany, University of Florida, Gainesville, FL 32611-8526 United States
AB: Permafrost soils are important reservoirs of carbon (C) in arctic and boreal ecosystems. Rising global temperatures are expected to enhance decomposition of permafrost organic matter, and in turn, respired CO2 may cause a positive feedback to warming. Yedoma soils from northeastern Siberia represent a large and poorly understood reservoir of permafrost soil organic matter, and are estimated to contain up to 450 Gt of C frozen since the Pleistocene. These mineral soils from the ice-rich areas of northeastern Siberia accumulated as wind blown loess buried organic matter fragments that were subsequently frozen in permafrost. We conducted laboratory incubations of Yedoma soils in order to understand the lability and the temperature dependence of this frozen organic matter. We collected frozen Yedoma soils from four different locations in northeastern Siberia and incubated them at $5\deg$C, $10\deg$C and $15\deg$C to monitor CO2 flux from microbial decomposition. The four locations included two tundra sites close to the Arctic Ocean and two locations located in boreal forest along the banks of the Kolyma River near Cherskii, Siberia. At all sites, permafrost soils were collected from deep within the soil profile, at depths $>$10m. Additionally at one of the boreal forest sites, soils were also collected from the modern mineral soil surface, which is currently unfrozen in summer and at 2m depth, which is below the modern active layer. We found more than a four-fold range in the rate of CO2 flux among sites during the initial period of the incubation. This range in CO2 flux increased to more than a ten-fold range later in the incubation as the rates at some sites declined to very low levels. Incubation temperature had a positive effect on flux rates. The modern surface soil from the boreal forest had among the lowest fluxes, even though it receives current inputs of labile C from actively growing plant roots, dissolved organic C, and root exudates. The low fluxes from the modern soil relative to the permafrost soils demonstrates the lability of organic matter stored frozen in these Yedoma soils. In addition to fluxes, we measured C isotopes to determine the age and sources of respired CO2. Radiocarbon measurements showed that respired CO2 had 14C-ages ranging from 21,000 to 25,000BP across all sites, with the exception of the surface soil where modern `bomb' carbon was observed in respiration. Measurements of stable C isotopes ($\delta$13C) ranged from -23 to -29.7$\permil$ for all soils, and there was a significant relationship with 14C-ages, where younger respired C had more depleted $\delta$13C values. Our results indicate the potential for ancient C to fuel microbial respiration and C release once permafrost Yedoma soils are thawed.
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting