Nitric oxide emissions from agricultural soils in temperate and tropical climates: sources, controls and mitigation options

Global annual NO emissions from soil are of the order of 10 Tg NO-N. This is about half the amount fossil fuel combustion processes contribute to the annual global NO_x budget. Reducing the emissions of soil derived NO_x requires an understanding of the source of the flux and the processes that determine its magnitude. A thorough investigation of possible mitigation strategies and the consequences of their implementation is also necessary. The ratio of NO and N₂O emissions from soils can be used as an indicator of the dominant NO production pathway operating. Fertilizer application (rate, type and time of application), soil temperature, soil water content and soil management practices all affect the emission rate and are reviewed. Mitigation options include reduction in N fertilizer use through an increase in fertilizer use efficiency, preferential use of NH⁴NO₃ instead of urea, improved timing of fertilizer application, the use of nitrification and urease inhibitors, improving the fertilizer uptake efficiency of crops in tropical agriculture and changes in land management. Several of the viable mitigation strategies, mainly those increasing fertilizer use efficiency, have the capacity to reduce global annual NO emissions by 4% (0.4 Tg NO-N y^-1). For other strategies including use of inhibitors, changing cultivation or land use, the possible reductions are too uncertain to justify quantification on the basis of present knowledge.