adipic acid are the largest sources of nitrous oxide emissions. The adipic acid emissions specifically arise from the degradation of the nitrolic acid intermediate derived from nitration of cyclohexanone. Biological processes Natural processes that generate nitrous oxide may be classified as nitrification and denitrification. Specifically, they include: aerobic autotrophic nitrification, the stepwise oxidation of ammonia () to nitrite () and to nitrate () anaerobic heterotrophic denitrification, the stepwise reduction of to , nitric oxide (NO), and ultimately , where facultative anaerobe bacteria use as an electron acceptor in the respiration of organic material in the condition of insufficient oxygen () nitrifier denitrification, which is carried out by autotrophic -oxidising bacteria and the pathway whereby ammonia () is oxidised to nitrite (), followed by the reduction of to nitric oxide (NO), and molecular nitrogen () heterotrophic nitrification aerobic denitrification by the same heterotrophic nitrifiers fungal denitrification non-biological chemodenitrification These processes are affected by soil chemical and physical properties such as the availability of mineral nitrogen and organic matter, acidity and soil type, as well as climate-related factors such as soil temperature and water content. The emission of the gas to the atmosphere is limited greatly by its consumption inside the cells, by a process catalysed by the enzyme nitrous oxide reductase. Environmental impact Greenhouse effect Nitrous oxide has significant global warming potential as a greenhouse gas. On a per-molecule basis, considered over a 100-year period, nitrous oxide has 265 times the atmospheric heat-trapping ability of carbon dioxide (). However, because of its low concentration (less than 1/1,000 of that of ), its contribution to the greenhouse effect is less than one third that of carbon dioxide, and also less than water vapour and methane. On the other hand, since 38% or more of the entering the atmosphere is the result of human activity, control of nitrous oxide is considered part of efforts to curb greenhouse gas emissions. A 2008 study by Nobel Laureate Paul Crutzen suggests that the amount of nitrous oxide release attributable to agricultural nitrate fertilisers has been seriously underestimated, most of which presumably, would come under soil and oceanic release in the Environmental Protection Agency data. Nitrous oxide is released into the atmosphere through agriculture, when farmers add nitrogen-based fertilizers onto the fields, and through the breakdown of animal manure. Approximately 79 percent of all nitrous oxide released in the United States came from nitrogen fertilization. Reduction of