HR: 0800h
AN: B11A-0993    [Abstracts]
TI: Measurement of the Isotopic Signature of Soil Carbon Dioxide: Methods Development and Initial Field Results
AU: * Kayler, Z
EM: zachary.kayler@oregonstate.edu
AF: Department of Forest Science, 321 Richardson, Oregon State University, Corvallis, OR 97331 United States
AU: Rugh, W
EM: rughb@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Burt Hall, Oregon State University, Corvallis, OR 97331 United States
AU: Mix, A C
EM: amix@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Burt Hall, Oregon State University, Corvallis, OR 97331 United States
AU: Bond, B J
EM: barbara.bond@oregonstate.edu
AF: Department of Forest Science, 321 Richardson, Oregon State University, Corvallis, OR 97331 United States
AU: Sulzman, E W
EM: elizabeth.sulzman@oregonstate.edu
AF: Department of Crop and Soil Science, 3017 ALS Bldg, Oregon State University, Corvallis, OR 97331 United States
AB: Soil respiration is a significant component of ecosystem respiration and its isotopic composition is likely to lend insight into ecosystem processes. We have designed probes to determine the isotopic signature of soil-respired CO₂ using a two end-member mixing model approach (i.e., Keeling plot). Each probe consists of three 35 ml PVC chambers cased in fiberglass mesh and connected to the soil surface via stainless steel tubing with a septa-lined swagelok fitting. Chambers are vertically connected such that they sample gases at depth intervals centered on 5, 15, and 30 cm. Gases are sampled via a hand vacuum pump equipped with a two-way valve, which allows vials pre-filled with N₂ gas in the laboratory to be evacuated and re-filled with only a single septa puncture in the field. Data indicate samples can be stored reliably for up to three days if punctured septa are coated in silicone sealant. To test whether this field sampling method was robust, we constructed a carbon-free sand column out of PVC pipe into which we plumbed a tank of known CO₂ concentration and isotopic composition. We have tested the effects of wetting and flow rate on our ability to reproduce tank values. A linear model (geometric mean regression) yielded a more negative isotopic value than the actual gas, but a simple polynomial curve fit the tank value. After laboratory testing, the probes were established in a steep drainage in the H.J. Andrews LTER site in the Cascade Mountains of western Oregon (as part of the Andrews Airshed project). We established a transect of five 10 m² plots with four soil probes and a companion respiration collar and measured soil CO₂ efflux and soil δ¹³CO₂ values biweekly from June-Sept. Results indicate there is a clear difference in isotopic and respiration flux patterns between the north- and south-facing slopes, with the north facing slope exhibiting higher fluxes and more ¹³C enriched respiration. The temporal pattern of respiration correlates well with decreasing soil moisture over the summer. In addition, flux and isotopic samples collected every 4 hours over a 24 hour period suggested strong diel patterns in both measures, with more enriched δ¹³C respired from soils in early morning and more δ¹³C depleted values during the day, suggesting that photosynthetic uptake and CO₂ recycling by the aboveground vegetation influence soil-respired CO₂ values.
DE: 1040 Radiogenic isotope geochemistry
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 3322 Land/atmosphere interactions (1218, 1631, 1843)
SC: Biogeosciences [B]
MN: Fall Meeting 2005