| Summary: | This thesis describes investigations into techniques for measuring trace gas emissions from soils, and related modelling activities. Using nitrous oxide, N<sub>2</sub>O, as an example, two major questions concerning the closed chamber technique have been studied. A major source of potential error has been identified in a commonly used chamber design that includes a vent tube to equalise inside and outside pressures. It was found that depressurisation of the chamber due to the Venturi effect led, depending on wind speed, to up to five-fold over-estimations in flux. Also, previous modelling exercises have assumed that the trace gas source in the soil remains at constant concentration, and consequently predicted non-linear accumulation in the chamber, requiring repeated sampling during chamber closure to make a flux measurement. However, measurements commonly show little departure from linearity, rendering repeated sampling superfluous, and modelling the process on the basis of a more realistic constant production by the source has explained these results. Another model has been developed to predict N<sub>2</sub>O fluxes at a field scale, based on the measurement of three key parameters in the soil: temperature, water-filled pore space and mineral N content. This has given a good fit with measured fluxes from grassland and cereal fields, but not from some other crops. With regard to methane, CH<sub>4</sub> fluxes from a rice field were measured over two full growing seasons by the closed chamber technique. Additional measurements made it possible to differentiate between the two main emission pathways, namely via the plant and by ebullition. Temporal variations in CH<sub>4</sub> flux are discussed in relation to these processes.
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