NITROUS-OXIDE EMISSIONS FROM AGRICULTURAL FIELDS - ASSESSMENT, MEASUREMENT AND MITIGATION

In this paper we discuss three topics concerning N2O emissions from ag ricultural systems. First, we present an appraisal of N2O emissions fr om agricultural soils (Assessment). Secondly, we discuss some recent e fforts to improve N2O flux estimates in agricultural fields (Measureme nt), and finally, we relate recent studies which use nitrification inh ibitors to decrease N2O emissions from N-fertilized fields (Mitigation ). To assess the global emission of N2O from agricultural soils, the t otal flux should represent N2O from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N f rom crop residues, N2O from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertili zer and animal manures and the area of fields cropped with legumes hav e sufficient global data to estimate their input for N2O production. T he assessment of direct and indirect N2O emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lan ds by 2% and the area of land cropped to legumes by 4 kg N2O-N ha(-1). No regard to method of N application, type of N, crop, climate or soi l was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assess ments should include these variables and should be used to drive proce ss models for field, area, region and global scales. Several N2O flux measurement techniques have been used in recent field studies which ut ilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N2O fluxes. These studies reveal that it is not the measurement technique that is providing much of th e uncertainty in N2O flux values found in the literature but rather th e diverse combinations of physical and biological factors which contro l gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An ar ray of careful field studies which compare a series of crops, fertiliz er sources, and management techniques in controlled parallel experimen ts throughout the calendar year are needed to improve flux estimates a nd decrease uncertainty in prediction capability. There are a variety of management techniques which should conserve N and decrease the amou nt of N application needed to grow crops and to limit N2O emissions. U sing nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N2O emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N2O emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.