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Nitrous oxide Sources - Agricultural soils


Agricultural soils represent a very large, and growing, global source of nitrous oxide. Current estimates for annual emissions from this source range from 2 to about 4 million tonnes of nitrous oxide-N globally. With a rapid increase in population growth, and the consequent need for more food production, both the area of agricultural soils and the intensity of their use is likely to continue to rise rapidly in coming decades.

Direct Sources

ploughed farmlandA major direct source of nitrous oxide from agricultural soils is that of synthetic fertilizer use. Widespread increase in the use of such nitrogen based fertilizers has been driven by the need for greater crop yields, and by more intensive farming practices.

Where large applications of fertilizer are combined with soil conditions favorable to denitrification, large amounts of nitrous oxide can be produced and emitted to the atmosphere.

Similarly, the widespread and often poorly controlled use of animal waste as fertilizer can lead to substantial emissions of nitrous oxide from agricultural soils. Some additional nitrous oxide is thought to arise in agricultural soils through the process of nitrogen fixation, though the true importance of this source remains poorly defined.

Human Impact

Man's need for more food, as a result of an expanding global population, has inevitably led to an increase in the use of both synthetic fertilizer and the wider application of animal waste on agricultural soils. However, the application of such nitrogen based fertilizers in many areas has been excessive, with large proportions of the added fertilizer providing no benefit to crop yield, but inducing elevated nitrous oxide emissions.

Potential for control

The better targeting of fertilizer applications, both in space and time, can significantly reduce nitrous oxide emissions from agricultural soils. Land-management strategies which accurately take account of the optimum amounts of fertilizer addition necessary for maximum crop yield and minimum waste are crucial both environmentally and economically. Similarly, the exact form of nitrogen based fertilizer and the best time of year at which to use them is key information on which to base fertilization campaigns

Indirect Sources

drainage outfall pipeIndirect agricultural sources of nitrous oxide remain poorly defined in most cases. There are several ways in which such indirect emissions occur. The most important of these is nitrous oxide emission arising from nitrogen leaching and run-off from agricultural soils.

After fertilizer application or heavy rain, large amounts of nitrogen may leach from the soil into drainage ditches, streams, rivers and eventually estuaries. Some of the nitrous oxide produced in agricultural soils is lost in exactly this way, being emitted to the atmosphere as soon as the the drainage water is exposed to the air.

Still more nitrous oxide is produced from such drainage waters when the leached nitrogen fertilizer they contain undergoes the processes of nitrification or denitrification in aquatic and estuarine sediments. Other important indirect nitrous oxide sources from agricultural soils include the volatilization and subsequent deposition of ammonia from fertilizer application, and the consumption of crops followed by sewage treatment.

Human Impact

As with direct nitrous oxide emission from agricultural soils, man takes full responsibility for indirect emission. Not only do large quantities of leached nitrogen based fertilizer have a significant impact on indirect nitrous oxide emissions, they have also led to dangerously high nitrate concentrations in drinking water and to eutrophication in rivers and estuaries around the world. Increased food consumption and consequent increases in municipal sewage treatment have also inevitably led to increased indirect nitrous oxide emissions from this source.

Potential for control

Again, it is through properly informed land-management practice and fertilization campaigns that nitrous oxide emissions can primarily be reduced. Much of the impetus for control of nitrogen based fertilizers has come from concern over high nitrate levels in drinking water supplies and the threat of eutrophication in estuaries and coastal waters. Individual governments have enacted changes in policy to bring about reductions in such nitrogen leaching, with the creation of 'Nitrate Sensitive Zones' (NSZs) requiring particular attention in the UK.


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