Agricultural soils represent a very large, and growing, global source of N2O to the atmosphere. Current estimates for annual emissions from this source range from ~2 to ~4 Tg N2O globally. With a rapid increase
□ Agricultural soils | Biomass burn O Industry O Cattle and feed
Fig. 13.2. Anthropogenic sources of nitrous oxide (N2O).
in population growth, and the consequent need for more food production, both the area of agricultural soils and the intensity of their use are likely to continue to rise rapidly in the coming decades.
A major direct source of N2O from agricultural soils is 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 applications of nitrogen-based fertilizers are combined with soil conditions favourable to denitrification, large amounts of N2O can be produced.
Similarly, the widespread and often poorly controlled use of animal waste as fertilizer can lead to substantial emissions of N2O from agricultural soils. Some additional N2O arises in agricultural soils through the introduction of nitrogen by biological fixation of atmospheric N2 by leguminous crops (and symbiotic associations in rice fields).
The application of nitrogen-based fertilizers and manures in many areas has been excessive, with large proportions of the added fertilizer providing no benefit to crop yield, but inducing increased leaching and elevated N2O emissions. Better targeting of fertilizer applications, both in space and time, can significantly reduce N2O emissions from agricultural soils. Land management strategies that accurately take account of the optimum amounts of fertilizer addition necessary for maximum crop yield and minimum waste are therefore crucial.
Indirect agricultural sources of N2O remain poorly defined in most cases. There are several ways in which such indirect emissions occur. The most important of these is thought to be the N2O arising from nitrogen leaching and runoff 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 N2O produced in agricultural soils is also lost in this way, as it is emitted to the atmosphere as soon as the drainage water is exposed to the air. Still more N2O is produced from such polluted drainage waters when the leached nitrogen undergoes the processes of nitrification or denitrifica-tion in aquatic and estuarine sediments. Other important indirect N2O sources from agricultural soils include the volatilization of NH3 and subsequent deposition of this nitrogen on other soils. Increased food consumption and consequent increases in municipal sewage treatment - also rich in nitrogen - have led to increased indirect N2O emissions.
Again, it is through properly informed land management practice and fertilization campaigns that N2O emissions from agriculture can 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 nitrogen leaching, with areas designated as 'nitrate-vulnerable zones' (NVZs) requiring particular attention in the UK.
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