HR: 1340h
AN: H53F-1483    [Abstracts]
TI: Quantification of Microbial Activities in Near-Surface Soils
AU: * Schroth, M H
EM: martin.schroth@env.ethz.ch
AF: Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitaetstr. 16 CHN G50.2, Zurich, CH-8092, Switzerland
AU: Nauer, P
EM: philipp.nauer@env.ethz.ch
AF: Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitaetstr. 16 CHN G50.2, Zurich, CH-8092, Switzerland
AU: Zeyer, J
EM: josef.zeyer@env.ethz.ch
AF: Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitaetstr. 16 CHN G50.2, Zurich, CH-8092, Switzerland
AB: Microbial processes in near-surface soils play an important role in carbon and nutrient cycling, and specifically in the turnover of greenhouse gases such as CO2 and CH4. We modified a recently developed technique, the gas push-pull test (GPPT), to allow for the in-situ quantification of microbial activities in near-surface soils. A GPPT consists of the controlled injection of a gas mixture containing reactive gases (e.g., CH4, O2, CO2) and nonreactive tracer gases (e.g., Ar, Ne) into the soil, followed by the extraction of the gas mixture/soil-air blend from the same location. Rates of microbial activities are computed from the gases" breakthrough curves obtained during the GPPT's extraction phase. For a GPPT to be applied successfully, it is important that sufficient mass of the injected gases can be recovered during the test, even after prolonged incubation in soil. But this may be difficult to achieve during GPPTs performed in near- surface soils, where gas loss to the atmosphere can be substantial. Our modification consisted of performing GPPTs within a steel cylinder (8.4-cm radius), which was previously driven into the soil to a depth of 50 cm. During the GPPTs, the cylinder was temporarily closed with a removable lid to minimize gas loss to the atmosphere. We performed a series of numerical simulations as well as laboratory experiments to test the usefulness of this modification. Numerical simulations confirmed that without use of the cylinder, typical near- surface GPPTs (e.g., injection/extraction depth 20 cm below soil surface) are subject to extensive gas loss to the atmosphere (mass recovery < 20% for most gases), whereas mass recovery of injected gases increased dramatically when the cylinder was employed (mass recovery > 90% for most gases). Results from laboratory experiments confirmed this observation. We will also present results of a first field application, in which a near- surface GPPT was successfully conducted in a sandy soil to quantify in-situ rates of CH4 oxidation.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 1875 Vadose zone
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Fall Meeting