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Nitrous oxide

Nitrous oxide (N₂O) is a very stable greenhouse gas with a life-time of over 100 years, and therefore contributes significantly to the total radiative forcing. Because of its stability it migrates to the stratosphere were N₂O is involved in reactions leading to stratospheric ozone reduction. Atmospheric N₂O has no direct impacts on natural habitats in the UK, or any other country. However, as a greenhouse gas it contributes to climate change.

The soil is the dominant source of atmospheric N₂O, contributing about 57% (9 Tg y^-1) of the total annual global emission (IPCC, 1997). In soil the microbial processes nitrification (the oxidation of ammonium to nitrate) and denitrification (the anaerobic reduction of nitrate to gaseous forms of nitrogen) are the principal sources of N₂O (Bouwman, 1990, Granli and Bockman, 1994). The rate of production and emission of N₂O depends primarily on the availability of a mineral N source, the substrate for nitrification or denitrification, on soil temperature, soil water content, and (when denitrification is the main process) the availability of labile organic compounds. These variables are universal and apply to cool temperate and also warm tropical ecosystems.

The global budget for N₂O is the most uncertain of all the greenhouse gases, due to its inherent spatial and temporal variability and poor quantification of some of the soil sources. These include tropical agricultural and natural soils, and indirect processes that lead to nitrogen production in soil other than fertiliser or manure application (i.e. ploughing, leaching of nitrate to rivers and subsequent N₂O production at the river margins, in rivers and estuaries). However, it is generally agreed, that intensification of agriculture and with it the increased usage of nitrogen and livestock production are mainly responsible for the global increase in N₂O emissions in the last few decades (Kroeze et al, 1999).

Nitrous oxide emissions from UK soils

In the UK annual N₂O emissions are currently estimated at 142 kt N₂O (NetCen 2002). NetCen’s brake-down of these sources is as follows: Agricultural soils are the largest source of N₂O, contributing to 64% (91 kt N₂O) of the total annual emission. A small, but rising source of N₂O are vehicles fitted with catalytic converters. All transport has been estimated to contribute to 19% of the total annual emission. The former largest emission sources of N₂O, the production of adipic and nitric acid (Salway, 1998), have now been reduced from 70 kt N₂O y^-1 to 20 kt N₂O y^-1, and only contribute to 14% of the total annual emission.

CEH has estimated N₂O emissions from the different soil ecosystems in the Great Britain as visualized by the map and table. The map shows the spatial distribution of soil N₂O emission calculated from the sum of nitrogen fertiliser applied, the animal manure produced and the atmospheric N deposited (Skiba et al, 2001, Sozanska et al, 2002). Nitrous oxide emissions from N fertilised agricultural soils were calculated from typical N fertiliser inputs and subsequent N₂O emission rates. For forest, heath and moorland ecosystems, estimates were based on measurements carried out in Scotland and Northern England (Skiba et al, 1996, 1998a).

In the UK grasslands are the largest soil source of N₂O and together with manure from housed and grazed animals returned to grassland CEH estimated annual emissions at 74 kt N₂O. Per unit area grassland emissions are larger than from fertilised arable land, because a) grasslands receive larger rates of mineral fertiliser and manure, b) grasslands occur in high rainfall regions, c) grazed grasslands have compacted soils. Such conditions are all favourable for N₂O production. The location of intensively managed grasslands is the western wetter part of Great Britain is therefore reflected by the higher N₂O emission rates (see map). In Scotland the grasslands in Galloway and Renfrewshire are responsible for the 'hot' spots of N₂O emissions (Sozanska et al, 2002).

The contribution of arable land to the total N₂O emission is of similar importance as N deposition derived N₂O emission rate (1% of N deposited is emitted as N₂O, Skiba et al, 1998b). In Scotland, the fertile soils in Fife and East Lothian, capable of producing a wide variety of arable crops contributed significantly to the local N₂O emission.

Forests, heath and moorlands contribute an estimated 3 kt N₂O. The impact of N deposition is likely to contribute to a further 3 kt N₂O from these ecosystems.

The importance of riparian wetland areas, rivers and estuaries to the total UK N₂O emission is still very uncertain. Based on UK information on river flow rates, river nitrate concentrations, rates of nitrate leaching in the catchments and denitrification rates and N₂O emission factors (0.3 to 3% of the DIN input) (Kroeze and Seitzinger, 1998), CEE estimated that UK rivers may contribute 0.9 to 9 kt N₂O and estuaries to 0.3 to 3 kt N₂O. Riparian zones were estimated to contribute < 0.2 kt N₂O.

The table below shows the break down of N₂O emissions for Great Britain. Northern Ireland has not been included yet. For Scotland the annual emissions were estimated at 19 kt N₂O, 18 kt from soils and < 1 kt from transport.

Table: CEH’s estimate of the main biological sources of nitrous oxide in Great Britain (Fowler et al, 2000, Skiba et al, 2001, Sozanska et al, 2002) and NetCen’s estimate of the non-biological sources (NetCen, 2002).

References:

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