Diffusion probe for gas sampling in undisturbed soil

Soil-atmosphere fluxes of trace gases such as methane (CH4) and nitrous oxide (N2O) are determined by complex interactions between biological activity and soil conditions. Soil gas concentration profiles may, in combination with other information about soil conditions, help us to understand emission controls. This paper describes a simple and robust diffusion probe for soil gas sampling as part of flux monitoring programmes. It can be deployed with minimum disturbance of in-situ conditions, and also at sites with a high or fluctuating water table. Separate probes are used for each sampling depth, in this study ranging from 5 to 100 cm. The probe has a 10-ml diffusion cell with a 3-mm diameter opening covered by a 0.5-mm silicone membrane. At sampling the diffusion cell is flushed with 10 ml N-2 containing 50 mu l l(-1) ethylene (C2H4) as a tracer; tracer recovery is used to calculate sample concentrations. Ethylene is immediately removed by flushing with unamended N-2. Equations are presented to correct for dead volumes of connecting tubing and valves. Laboratory tests evaluated recovery of CH4, N2O and carbon dioxide (CO2), removal of C2H4 and equilibration of CH4, N2O and CO2 in air and water. Field tests on peat soils used for grazing showed soil gas concentrations of CH4 and N2O as influenced by topography, site conditions and season. The applicability of the diffusion probe for trace gas monitoring is discussed.