The Measurement of Nitrous Oxide Emissions from Soil by Using Chambers
Small flux chambers are widely used to measure emissions of nitrous oxide, N2O, from soil, the gas being determined by gas chromatography with an electron capture detector. The technique is relatively cheap, and is adaptable to a wide range of site conditions and emission rates: from the order of 1 mu g m-2 h-1 to more than 10 mg m-2 h-1. Increasingly, systems are being automated, to get more information on short-term temporal variability and to collect data over long periods to improve estimates of total annual emissions. Such systems are being used in the field and with soil monoliths installed in a greenhouse. Large chambers 50-60 m2 in area, with gas analysis by long-path infrared spectrometry, offer a way of overcoming small-scale spatial variability, and are useful in conditions where micrometeorological methods may not be applicable, or when long runs of data are needed from the same site. In studies with small closed chambers, we have measured N2O emissions from grassland ranging from negligible values to about 4 mg N2O-N m-2 h-1 (nearly 1 kg N2O-N ha-1 d-1), with total losses in the range 0.14-5.1% of the nitrogen applied as fertilizer, depending on factors such as soil structure, water potential and temperature, and the chemical form of the fertilizer. Reasonable agreement can be obtained between chamber and micrometeorological flux measurements on homogeneous sites.