Emissions of N 2 O and NO associated with nitrogen fertilization in intensive agriculture, and the potential for mitigation

Abstract. Increases in the atmospheric concentrations of nitrous oxide (N 2 O) contribute to global warming and to ozone depletion in the stratosphere. Nitric oxide (NO) is a cause of acid rain and tropospheric ozone. The use of N fertilizers in agriculture has direct and indirect effects on the emissions of both these gases, which are the result of microbial nitrification and denitrification in the soil, and which are controlled principally by soil water and mineral N contents, temperature and labile organic matter. The global emission of N 2 O from cultivated land is now estimated at 3.5 TgN annually, of which 1.5 Tg has been directly attributed to synthetic N fertilizers, out of a total quantity applied in 1990 of about 77Tg N. This amount was 150% above the 1970 figure. The total fertilizer‐induced emissions of NO are somewhere in the range 0.5‐5 Tg N. Mineral N fertilizers can also be indirect as well as direct sources of N 2 O and NO emissions, via deposition of volatilized NH 3 on natural ecosystems and denitrification of leached nitrate in subsoils, waters and sediments. IPCC currently assume an N 2 O emission factor of 1.25 ± 1.0% of fertilizer N applied. No allowance is made for different fertilizer types, on the basis that soil management and cropping systems, and unpredictable rainfall inputs, are more important variables. However, recent results show substantial reductions in emissions from grassland by matching fertilizer type to environmental conditions, and in arable systems by using controlled release fertilizers and nitrification inhibitors. Also, better timing and placement of N, application of the minimum amount of N to achieve satisfactory yield, and optimization of soil physical conditions, particularly avoidance of excessive wetness and compaction, would be expected to reduce the average emission factor for N 2 O. Some of these adjustments would also reduce NO emissions. However, increasing global fertilizer use is likely to cause an upward trend in total emissions even if these mitigating practices become widely adopted.