No laughing matter

chlorofluorocarbons (cfcs) are not the only threat to the ozone layer. Nitrous oxide (n₂o), also known as laughing gas, has been quietly eating into the layer that screens out harmful ultraviolet radiations. The gas has a tremendous global warming potential, with a heat absorption capacity 250-290 times that of carbon dioxide (co₂). With world attention focused on cfcs and co₂, the damage caused by n₂o has gone unnoticed (see table: Warming over ).

Scientists have estimated that n₂o lasts in the atmosphere for about 160 years. That makes its life span the second longest among greenhouse gases -- after co₂, which lasts for 250 years. The natural sources of n₂o are oceans (22-28 per cent) and soil (46-55 per cent). Agricultural activity using chemical fertilisers contributes about 15-28 per cent of the n₂o produced, and burning of fossil fuel accounts for another 2-4 per cent. Increased use of fertilisers and production of leguminous crops, which fix atmospheric nitrogen, accent on livestock rearing, and burning of fossil fuel have led to doubling of n₂o concentrations in recent years. Atmospheric n₂o now stands at 310 parts per billion by volume (ppbv), and is increasing at an alarming rate of 2.5 to 4.5 million tonnes per year. The level of n₂o is 0.75 ppbv higher in the Northern than in the Southern Hemisphere, indicating a greater strength of sources in the former.

The gas is produced when nitrogen compounds are converted by the biological processes of nitrification and denitrification. Nitrification involves reaction of oxygen with nitrogen compounds and production of nitrates. Denitrification is the conversion of nitrites and nitrates into free nitrogen, with generation of n₂o in the process.

The agents that assist these processes are chemosynthetic bacteria (such as Nitrosomonas , Nitrosolobus , Nitrosporia , Nitrosococcus , Nitrobacter and Nitrospina) found in the soil and marine environments. Some fungi also release n₂o as a by-product during anaerobic respiration (where, in the absence of air, oxygen is derived from compounds). Non-biological processes that release n₂o include anaerobic oxidation of organic matter by nitrous acid in soil. Soil acidity is a crucial factor in production of n₂o -- the bacterial enzyme that reduces the gas to nitrogen operates best between p H 7.0 and p H 8.0. At a low p H, a lot of n₂o is produced during denitrification.

Various other soil factors control generation of n₂o, such as moisture, temperature, and presence of organic carbon and nitrogen compounds. Moisture limits the oxygen available below the soil surface. When moisture is added to low or moderate level soils, such as after irrigation or rainfall, denitrifying bacteria produce n₂o rapidly. Though flooded soils favour nitrification, the amount of oxygen available is important: anaerobic conditions lead to production of nitrogen, rather than n₂o as an end-product.

The proportion of organic carbon and nitrates and nitrites, as well as soil temperature affect the rate of denitrification. A temperature of 25c has been found to be ideal for denitrification. Deviations from this decreases bacterial activity. For nitrification, a range of 30-35c is optimum. In marine environments, temperatures range from 0c to 15c and have little influence on n₂o production. Despite the numerous sources of n₂o, the only known potential sink is photolysis -- the breaking up of n₂o into nitrogen and oxygen in the stratosphere when exposed to strong light.

There is an urgent need to find options to control emission of n₂o from agricultural activities. Split application of nitrogenous fertilisers seems to be the simplest method to contain n₂o emission. Other methods include the use of nitrification inhibitors such as wax-coated calcium carbide that also reduces formation of methane (another greenhouse gas) from wetland rice cultivation. But such techniques are uncommonly used in Asia. Soil management during fallow periods -- allowing growth of weeds or another crop -- can also help in cycling nitrogen.

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