Methods for Measuring Atmospheric Gas Transport in Agricultural and Forest Systems : Agricultural Ecosystem Effects on Trace Gases and Global Climate Change
The paper reviews methods for measuring the exchanges of trace gases between soil-plant systems and the atmosphere, with emphasis on the greenhouse gases N2O, CH4, and CO2. The methods vary in scale from chambers covering < 1 Quadratmeter to CBL budgets for regions of 100 to 1000 Quadratkilometer. They should be seen as complementary rather than alternatives, each having a special role. Chambers permit replication and experiments with many land treatements, but can suffer from their interference to the microclimate and the large spatial variability of soil gas fluxes. Micrometeorological techniques are nondisturbing and integrate over larger areas. Mass balance methods can be used with small plots, up to 1 ha, for example. They are best suited to investigations where the background concentration of the gas is small and fluxes are large compared with normal emissions. Gradient and eddy correlation techniques integrate over areas of several hectares and can have high precision. Their application is limited presently by the availability of sensors that combine sensitivity, fast response, continuous operation, ruggedness, and low cost, and for eddy correlation, the need to apply large corrections to the apparent eddy flux. Two new micrometeorological approaches are filling gaps in our measurement capabilities: Lagrangian methods that offer a relatively simple means for inferring source-sink distributions of trace gases in plant canopies, and CBL budget methods that permit estimates of trace gas fluxes on regional scales, again with relatively simple measurements.
