Estimated emissions from various pathways

Groundwater

Due to the difficulty in measuring diffusive N2O emission from the aquifer surface and convective discharge via streams, wells and springs, N2O emission from aquifers has mostly been estimated from the N2O and NO3~ concentrations at groundwater monitoring wells. Most reported estimates of CEF2 (Table 8.1) are within the range given in the review of Hiscock et al (2003). The lowest value of 0.0003 has been calculated for the central High Plains aquifer in the US, where the impacts of agricultural nitrogen inputs may be yet to be fully realized due to a thick unsaturated zone. Highest values of

Table 8.1 Indirect N2O fluxes from riparian areas, aquifers and land drainage and associated conceptual emission factors

Location and description

CEF.

CEF2

CEF3

Reference

Riparian areas

Eolian sand over glacial till; forested (Alnusglutinosa), The Netherlands 55

Eolian sand over glacial till; grassland

(Glyceria maxima), The Netherlands 6 to 11

Sandy soils in Pleistocene deposits,

(Alnus glutinosa), The Netherlands 2.6 to 13.6

Aquifers

Review of seven aquifers

Cretaceous chalk and weathered bedrock aquifers, UK

Three Pleistocene sand and gravel aquifers in northern Germany Pleistocene sand aquifer in northern Germany Near-surface groundwater of a pleistocene sand aquifer in northern Germany Tayhoo Valley, China, cropped with upland rice Review of three sites

Mix of arable and grassland, Aberdeenshire, UK

<1 to 21771

149 to 1928 (602)2

3357 to 97,250 15 to 571 (161)2 11 to 35,6571 <2500

0.001

0.00003 to 0.042

0.0019

0.001 to 0.0415

0.03 to 0.163 Hefting et al, 2003 0.03 to 0.0 53 Hefting et al, 2003 0.05 to 2.63 Hefting et al, 2006

Hiscock et al, 2003

0.001 to 0.005140.000001 to 0.37 (0.0023)

0.005 to 0.1 0.0001 (0.00003 to 0.005) 0.00003 to 0.04 0.002

Hiscock et al, 2003

Weymann et al, 2008 Deureretal, 2008; Weymann et al, 2008

Well et al, 2005c Xiong etal, 2006 Hiscock etal, 2003 Reay etal, 2003, 2004a, 2004b, 2005

Note: CEF,, CEF2 and CEF3 were estimated using Equations 8.1, 8.3 and 8.4. Individual data sets are only listed if EFs were given or can be calculated from presented data in the reference.

1 Range of aquifer medians.2 Total range (median in brackets).3 Calculation of CEF3 as in Well et al (2005a) based on data given by Hefting et al (2003, 2006).4 CEF3 was calculated for each

o sample in the basic data set of Weymann et al (2008).5 Potential EFs measured at groundwater monitoring wells.

up to 0.1 were observed near the surface of sandy aquifers in northern Germany under agricultural management. A value in between these extremes (0.0019) was found in unconfined chalk groundwater in Cambridgeshire and Norfolk, eastern England.

An estimate of CEF1 (Equation 8.1) at the aquifer scale was obtained for the same aquifers (Hiscock et al, 2003). FN2O of 0.04kg N ha-1 yr-1 was determined from groundwater concentrations and discharge by bore hole extractions and springs. Diffusive flux to the unsaturated zone was estimated to be negligible. FNin was derived from average N fertilizer applications to crop fields (130kg N ha-1 yr-1) and the IPCC default fraction of leached N (0.3), giving a CEF1 of 0.001.

CEF1 can also be determined more directly: summing excess N2 from deni-trification and residual NO3--N of individual groundwater samples reflects initial NO3--N discharge to the groundwater, i.e. FNin (Equation 8.1) (Green et al, 2008; Weymann et al, 2008). If this information is available in addition to N2O, CEF1 can be calculated using Equation 8.1. Excess N2 can also be used to determine CEF3, i.e. N2O accumulation in relation to denitrification (Equation 8.4). However, it must be noted that CEF1 and CEF3 as determined for individual groundwater samples from monitoring wells are potential EFs, since the concentration of dissolved N species may further change during transport within the aquifer. Actual EFs could be obtained from excess N2 and N2O in ground-water abstraction wells or monitoring wells close to groundwater discharge zone, but this was not reported in the studies mentioned above.

How large is the potential error caused by neglecting denitrification when using CEF2? This can be seen by comparing CEF2 and CEF1 within the same data sets. For example, the data set given by Weymann et al (2008) yields CEF1 site medians between 0.0006 and 0.01, which is much lower than the site medians of CEF2 for the same sites (0.001-0.04). This demonstrates the importance of accounting for denitrified N in reduced aquifers.

EFs based on dissolved N in groundwater samples are estimates of the potential lateral N2O flux to springs, streams or wells. But the total ground-water-derived N2O flux also includes vertical diffusive flux at the aquifer surface. When taking this into account, care must be taken to avoid double-counting of total agricultural N2O fluxes, since the diffusive groundwater emissions under agricultural land are included in the total emission at the soil surface. Strictly speaking, estimates of direct N2O emission from agricultural land based on surface flux measurements thus include some of the indirect emissions. But the question is; is the magnitude of these diffusive fluxes significant? Extremely high N2O concentrations of up to 100^M (Table 8.1) (Well et al, 2005c) result in significant N2O fluxes of up to 3g N ha-1 day-1, or approximately 1kg N ha-1 yr-1. But such high values are rare individual observations. Recently, a large number of vertical N2O profiles at 10cm resolution were determined in a sandy aquifer in northern Germany (Deurer et al, 2008; von der Heide et al, 2009b). Resulting diffusive fluxes were 0.00090.3kg N ha-1 yr-1. It could be shown that these values were related to aquifer properties such as dissolved and particulate organic C, O2, pH, and NO3" (von der Heide et al, 2008, 2009a). In the same aquifer, vertical fluxes from the groundwater to the soil surface were studied using an in situ 15N-tracer experiment (Weymann et al, 2009) in which an 8m2 area of the aquifer surface was amended with 15N-labelled NO3" solution. Groundwater-derived fluxes estimated from 15N2O in flux chambers at the soil surface were very small (0.0022 to 0.0207pg N2O-N m"2 hour"1, equivalent to 0.0002-0.0018kg N2O-N ha"1 yr"1). When comparing locations with similar concentration gradients among different studies, direct flux estimates (Weymann et al, 2009) were similar to the indirect estimates obtained from concentration gradients (Deurer et al, 2008). Thus the gradient-derived fluxes were confirmed. Peak 15N2O fluxes during falling groundwater levels suggested that a release of N2O from entrapped gas bubbles or from residual soil water in drained layers might be a fast path for vertical N2O fluxes (Weymann et al, 2009). However, the magnitude of this emission path still needs to be investigated.

Overall, the latest studies on CEF1, CEF2 and diffusive groundwater fluxes confirm that the downward revision of EF5g was adequate.

Riparian area including constructed wetland buffers

A variety of recent studies supplies data on N2O emission from riparian buffer zones (Blicher-Mathiesen and Hoffmann, 1999; Bernal et al, 2003; Hefting et al, 2003, 2006; Dhondt et al, 2004; Machefert and Dise, 2004), demonstrating that these systems are potential locations of substantial indirect N2O emissions. In addition to N2O fluxes, Hefting et al (2003) investigated NO3" loading in grassland and forested buffer zones along first-order streams in The Netherlands, and could thus determine EFs for these systems. EF5g determined using Equation 8.1 (CEF1) ranged between 0.03 and 0.06 in the forested and between 0.02 and 0.03 in the grassland buffer zone (Table 8.1). The influx of nitrate to these zones was 467 and 192g N m"2 yr"1, respectively. N2O emissions were 20 and 2-4kg N ha yr"1, respectively. The authors concluded that the IPCC EF5g underestimates riparian buffer zones of this type. This supports previous suggestions that a separate EF for riparian systems should be defined (Groffman et al, 1998). Using the nitrate removal and N2O fluxes reported for riparian systems (Hefting et al, 2003, 2006), CEF3 can be calculated, giving 0.021-0.042. This range is about one order of magnitude higher than the site medians of CEF3 of the aquifer studies (0.001-0.005). This demonstrates that, in comparison with aquifers, NO3" removal within these riparian buffers can be more harmful to the atmosphere. However, care must be taken before generalization, since the reported fluxes were much higher compared to the other riparian studies cited above.

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