Use of chamber systems to measure trace gas fluxes

Trace gas exchange between soil-plant systems and the atmosphere is a complex phenomenon driven by a different set of physical, chemical, and biological processes for each chemical species and each environment (Fig. 4-1). For example, the exchange rate of a relatively inert gas like N20 represents simply the difference between its integrated rates of production and consumption by soil biochemical processes. More reactive gases are subject to additional emission and deposition processes, such as foliar exchange (NH3) or photochemical oxidation followed by deposition of the reaction products (NO). Further confounding the measurement and understanding of trace gas exchange across the surface-atmosphere boundary, the relative importances of the source and sink processes shown in Fig. 4-1 also vary with the rate of gas transport by diffusive, advective, and plant-mediated processes, and with the time and space scales over which the exchange is considered. Because of this complexity, great care must be exercised in drawing inferences about regional to global and seasonal to annual trace gas exchange rates from fluxes measured over small areas and short times. Denmead and Raupach (1993) introduced the two most common approaches to field measurement of trace gas exchange-chamber methods and micrometeorology. Because tower-based micrometeorological techniques integrate the flux over a larger area (typically 102_103 m2), they offer a potential advantage where the exchange rate is highly variable on the local scale measured by chamber methods (typically < 1 m2). Chambers offer other advantages, including low cost and ease of use, and they represent the measurement technique of choice in process-level studies and other research requiring replicate measurements coincident in space or time. Our purpose is to provide sufficient insight into the use of chamber systems to enable poten-