Nitrous oxide emission from agricultural soils

A review of the salient features of N2O emissions from agricultural soils was done to assess our current understanding and associated problems. Nitrous oxide is an important globe warming gas and a destructive agent of ozone in the stratosphere. A major concern is the increasing contribution of chemical fertilizers to atmospheric N2O buildup. There is only a limited understanding of the contributions from manures, biological N2 fixation and crop residues. A recent estimate suggests that agriculture's share of N2O emissions is 80% although such estimates are highly uncertain because of imprecise data and the physical and biological complexities of the production process. As a product of the nitrification and denitrification process in soils, a major problem is our understanding of the proportion of N2O produced, i.e. the product ratios, although there is a good general understanding of the processes involved. Measurements of N2O emissions from the soil surface fail to take into account N2O flux from the bottom of the root zone into the subsoil and aquifers although they are generally considered to be significant. There is a need to apply newly available methodology and for combining this methodology and modelling together to predict N2O emissions on the landscape (or field) scale taking climate, soil and cropping variables into account. There is enough information available now to exercise some control of N2O emissions from cultivated soils. It is suggested that this be done focusing on factors that directly affect the soil microbes involved with the nitrification (NH4+, O2) and denitrification (NO3−, C, O2) processes. Cropping practices and some soil characteristic amendments are suggested herein for this purpose. Key words: Denitrification, nitrification, emission control, gas ratios