Effect of nitrogen fertilization, cropping and irrigation on soil air composition and nitrous oxide emission in a loamy clay

Most of the nitrous oxide (N2O) in the atmosphere, thought to be involved i n global warming, is emitted from soil. Although the main factors controlli ng the production of N2O in soil are well known, we need more quantitative data on the interactions of soil and the environment in the soil that affec t the emission. We therefore studied the effects of irrigation, cropping (f allow, barley with grass undersown) and N fertilization (unfertilized, 103 kg N ha(-1)) on the composition of soil air and direct N2O emission from so il (using the closed chamber method) in a factorial field experiment on a w ell-structured loamy clay soil during 1 June-22 October 1993. The measureme nts were made weekly during the growing season and three times after harves ting. The composition of the soil air did not indicate severe anoxia in any treatment or combination of treatments, but the accumulation of N2O in the soil air indicated that hypoxia was common. At the start of the irrigation the emissions were small, even though there was much ammonium and nitrate in the soil and therefore a potential for emission of N2O produced by both nitrification and denitrification. Larger emissions occurred later. The lar gest emissions were found when 60-90% of the soil pore space was filled wit h water. Irrigation and fertilization with N both roughly doubled the cumul ative N2O emission. Growing a crop decreased it by a factor of 3-7. Most N2 O was lost from the irrigated fertilized soil under fallow (3.5 kg N ha(-1) ), and least from the unirrigated unfertilized soil under barley (0.1 kg N ha(-1)).