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Long-Term Monitoring of Soil Gas Fluxes with Closed Chambers Using Automated and Manual Systems

Environmental Division, SAC, West Mains Road, Edinburgh, EH9 3JG, UK.

^* Corresponding author (a.scott{at}ed.sac.ac.uk).

ABSTRACT

We describe two gas sample collection techniques, each of which is used in conjunction with custom made automated or manually operated dosed chambers. The automated system allows automatic collection of gas samples for simultaneous analysis of multiple trace gas efflux from soils, permitting long-term monitoring. Since the manual system is cheaper to produce, it can be replicated more than the automated and used to estimate spatial variability of soil fluxes. The automated chamber covers a soil area of 0.5 m² and has a motor driven lid that remains operational throughout a range of weather conditions. Both systems use gas-tight containers of robust metal construction, which give good sample retention, thereby allowing longterm storage and convenience of transport from remote locations. The containers in the automated system are filled by pumping gas from the closed chamber via a multiway rotary valve. Stored samples from both systems are analysed simultaneously for N₂O and CO₂ using automated injection into laboratory-based gas chromatographs. The use of both collection systems is illustrated by results from a field experiment on sewage sludge disposal to land where N₂O fluxes were high. The automated gas sampling system permitted quantification of the marked temporal variability of concurrent N₂O and CO₂ fluxes and allowed improved estimation of cumulative fluxes. The automated measurement approach yielded higher estimates of cumulative flux because integration of manual point-in-time observations missed a number of transient high-flux events.

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