Chamber Systems for Measuring Nitrous Oxide Emission from Soils in the Field

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Abstract
Recent concern over N2O losses from soils has highlighted the need for new techniques for measuring N2O exchange in the field. Two systems are described, in which nondispersive infrared gas analysis is used to measure the change in N2O concentration of air passing through a cylindrical chamber driven 0.1 m into the ground. When interfering gases are removed from the air stream, the infrared analyzer has a resolution of ± 12 ppb N2O.In one system, air circulates in a closed loop between the chamber air space and the gas analyzer; in the other, open system, outside air is drawn continuously through the chamber space and its N2O enrichment or depletion measured.Two operational problems (common to many emission chambers) were encountered: the development of a low pressure in the chamber when air is withdrawn, which induces a mass flow of N2O from the soil in addition to the diffusive flow; and a readjustment of the N2O concentration in the soil air whenever the N2O concentration in the chamber air changes from ambient. These can lead to incorrect estimates of the flux. The former problem was overcome by chamber design. The latter appeared to be insurmountable in the closed system, but could be reduced to acceptable proportions in the open system by choice of air flow rate.The open system permits relatively rapid equilibration, automatic and continuous operation, a discrimination less than 2 ng N m−2 sec−1, and the maintenance of environmental conditions within the chamber close to those in the field.