N2O emissions originating from agricultural land use include direct emissions from the surface of crop fields as well as indirect emissions caused by nitrogen flows from agricultural fields into adjacent systems (Mosier et al, 1998). Indirect emissions resulting from N leaching into aquatic systems are considered a potentially important N2O source. However, its magnitude is still under debate (Nevison, 2000; Groffman et al, 2002; Weymann et al, 2008), with an uncertainty associated with current estimates of almost two orders of magnitude, which is larger than the uncertainty for other N2O sources (IPCC, 2006). The aquatic pathway of reactive N that originates from leaching (and runoff) from agricultural fields and that ends up in the oceans includes downstream flow through a chain of connected systems, i.e. aquifers, riparian areas, rivers and estuaries (Figure 8.1).
A major fraction of agricultural surplus N is leached as nitrate (NO3~) to the groundwater. N2O produced in soil by nitrification and denitrification can also be leached (Heincke and Kaupenjohann, 1999; Russow et al, 2002). In denitrifying aquifers, NO3~ leached from agricultural soils is partially or completely reduced (Hiscock et al, 1991; Korom, 1992; Böhlke, 2002; Weymann et al, 2008) (Figure 8.1). N2O produced under these conditions can be transported to the atmosphere via upward diffusion (Deurer et al, 2008; von der Heide et al, 2008; Weymann et al, 2009) or groundwater discharge to wells, springs and streams (Mühlherr and Hiscock, 1998; Heincke and Kaupenjohann, 1999). Groundwater containing NO3- and eventually N2O reaches streams by direct discharge or via tile drainage (Hack and Kaupenjohann, 2002; Reay et al, 2003, 2004a, 2004b, 2005) and further flows
indirect N2O emissions to downstream systems, i.e. lakes (Boontanon et al, 2000; Xiong et al, 2006), rivers and estuaries (Garnier et al, 2006), and finally to the open sea (Bange, 2006a). Once discharged to surface water bodies, dissolved N2O may partially or completely degas to the atmosphere (Reay et al, 2004b).
During transport in streams, rivers and estuaries, NO3~ can be denitrified (Laursen and Seitzinger, 2004) or assimilated by the biota (Mulholland et al, 2008). Within the N cycle in the open water bodies, mineral N species can be produced or retained, and N2O can be produced by nitrification of ammonium (NH4+) as well as produced and reduced by denitrification. Point sources such as sewage treatment plants may significantly increase the load of reactive N. Estimates of fluxes at the various stages of the downstream flow chain are based on direct or indirect measurements, on process-based models or empirical emission factors (EFs).
Estimating indirect agricultural N2O emission is complicated by the fact that it is often difficult to differentiate between fluxes originating from agricultural and other N sources. For example, a riparian buffer between agricultural land and a stream receives N via subsurface groundwater flow, atmospheric deposition from industrial, agricultural and natural sources, biological N2 fixation and eventually N from various sources in the stream during flooding events (Figure 8.1). N2O emitted at the soil surface is then a mixture of groundwater-derived N2O of mostly agricultural origin, and N2O
produced in soil that originates from industrial, agricultural and natural sources. Robust estimates of indirect agricultural emissions need to take this complex interaction of pathways into account.
Another major pathway of N loss from agricultural systems is the volatilization of ammonia (NH3). On a global scale NH3 emissions from agricultural systems are in the range of 27-38Tg NH3-N yr-1 (Beusen et al, 2008). Of applied fertilizer or N excreted by animals, 10 to 30 per cent may be volatilized as NH3 (Bouwman et al, 2002), which itself is deposited somewhere in the surrounding region, relatively close to its source. In many regions of Europe, N deposition is dominated by reduced N, i.e. NH3 or NH4+, and deposition rates to natural and semi-natural systems can vary from 5kg N ha-1 yr-1 for unpolluted areas to over 80kg N ha-1 yr-1 (Fowler et al, 2004) in regions of intensive animal farming such as northern Germany or The Netherlands. Nitrogen input into natural and semi-natural systems via atmospheric deposition will increase N availability in the plant soil system and is regarded as one of the main drivers for increased soil N2O emissions from temperate forests (Butterbach-Bahl et al, 1998; Pilegaard et al, 2006).
In this chapter, each system is described with respect to the control of N2O fluxes and reported flux data. Moreover, concepts of EFs are compared and discussed and reported EFs and fluxes are summarized for each system. For the aquatic pathway, a comparison of systems is given which finally leads to some comments on mitigation options.
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