A new sampling technique to monitor concentrations of CH4, N2O and CO2 in air at well-defined depths in soils with varied water potential

A new sampling technique for measuring the concentrations of trace gases (C H4, CO2 and N2O) in the soil atmosphere from well-defined depths is describ ed. Probes are constructed from silicone tubing closed with silicone septa on both ends, thereby dividing an inner air space from the outer soil atmos phere without a direct contact. The gas exchanges between the inner and out er atmosphere only by diffusion through the walls of the silicone tube. Tes ts revealed that the gases N2O, CO2 and CH4 in the enclosed space reached 9 5% equilibrium with the surrounding atmosphere at 20 degreesC within 7 h or faster. The probe measurements are reproducible: the standard deviation of samples taken from 26 probes stored in the laboratory atmosphere equalled that of a standard gas. The probes can easily be constructed and installed at specified depths in the soil. The method has the following advantages compared with other methods that us e spaces with holes in them for gas exchange: (i) the silicone probe enable s trace gases to be sampled in wet soils, including ones that are waterlogg ed or temporarily saturated; (ii) the sampling itself does not create low p ressure and hence does not create mass flow in the soil matrix from undefin ed depths; and (iii) the probe can be made to take samples of gas of any re quired size. The silicone probes did not show ageing effects during 18 mont hs of use in the field in a mineral soil under grass. The probes yielded co mparable results: three probes inserted at 5 cm depth in a uniformly treate d 100-m(2) plot provided nearly identical average trace gas concentrations within the measurement period.