A comparison of the net exchange of the three investigated GHGs by European forests can best be done in terms of their GWP. The GWP is an index defined as the cumulative radiative forcing between the present and a chosen future time horizon (by convention 100 years), caused by a unit mass of gas emitted at present (by convention CO2). Using this approach, N2O and CH4 emissions are expressed in terms of CO2 equivalents. In this study it is assumed that 1 kg N2O equals 296 kg CO2 equivalents and 1 kg CH4 equals 23 kg CO2 equivalents (Ramaswamy,
2001). Other values frequently used in the literature are 310 and 21 kg CO2 equivalents for N2O and CH4, respectively (IPCC, 1996).
An overview of the net exchange of CO2, N2O and CH4 by European forests in terms of CO2 equivalents is presented in Table 17.6. Data on the carbon sequestration are based on results by Kauppi et al. (1992), Nabuurs et al. (1997), Liski et al. (2002) and De Vries et al. (2005b) for forests (trees), and by Nabuurs and Schelhaas (2002), Liski et al. (2002) and De Vries et al. (2005b) for forest soils. These data are quite consistent and a reliable estimate can be made of 0.080.12 Pg C/year for trees and 0.015-0.04 Pg C/year for soil. Divided by a forested area of 162 million hectares, this implies an average carbon sequestration of 500-650 kg C/ha/year by stem wood and ~100-250 kg C/ha/year by soil. This represents an average total sink of ~600-900 kg C/ha/year or ~2200-3300kg CO2 equivalents/ha/year.
For N2O the average source estimate for Europe ranges between 0.3 and 0.7 kg N2O-N/ha/year (Schulte-Bisping et al., 2003). Multiplying these values with 44/28 (N2O versus N) and the GWP of 296. gives an average total source of 140-325 kg CO2 equivalents/ha/year. For CH4 the average sink estimate for Europe ranges from 0.2 to 3.0 kg CH4/ha/year (Table 17.3: Schulte-Bisping et al., 2001; Brumme et al., 2004; Lindner et al., 2004). This represents an average total sink of ~5-70 kg CO2 equivalents/ha/year.
Using the average values given above, it is clear that CO2 sequestration by forests is much larger than their N2O emissions and CH4 uptake. The CO2 sequestration by forests is approximately ten times larger than the N2O emissions, in terms of CO2 equivalents (N2O emission is ~10% of the CO2 sequestration with a variation of 4-18%; see Table 17.6), whereas the CH4 sink is almost negligible. The results imply that carbon sequestration in forests is clearly outweighing the N2O emissions, but a more reliable N2O estimate is important to estimate the counter-effect. Especially during the re-establishment of a forest plantation several years of high N2O emissions (>3 kg N/ha/year) can be expected due to the missing plant nitrogen sink, which is not accounted for at present.
Table 17.6. Estimated ranges in long-term annual average carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) emissions and the impact of nitrogen deposition on those emissions including a comparison of their global warming potential (GWP) in CO2 equivalents.
Long-term average emissions (kg/ha/year) GWP (kg CO2 equivalents/ha/year)
Greenhouse Nitrogen deposition Nitrogen deposition gas Total estimates impacts3 Total estimates impacts3
CO2-C -600-900 (-750) -60-90 (-75) -2200-3300 (-2750) -220-330 (-275)
N2O-N +0.3-0.7 (0.5) +0.05-0.11 (0.08) +140-325 (230) +20-50 (35)
CH4 -0.2-3.0 (1.6) +0.003-0.05 (0.026) -5-70 (-40) +0.1-1.1 (0.6)
aThe nitrogen deposition impacts are given for an estimated increase in total nitrogen deposition of 2.8 kg N/ha/year and 1.2 kg NH4-N/ha/year for the period 1960-2000.
The impact of nitrogen deposition on the net exchange of CO2, N2O and CH4 by European forests has also been assessed in terms of GWP, as shown in Table 17.6. The average estimated contribution of nitrogen deposition to the increase in CO2 exchange (sink) is ~26 kg C/ha/year per kilogram of nitrogen of which 11.1 kg C/ha/year is sequestered in stem wood and 14.8 kg C/ha/ year in soil. During 1960-2000, the average additional nitrogen deposition was 2.8 kg N/ha/year, causing an additional overall carbon sequestration of ~75 kg C/ha/year. The uncertainty is likely to be 20% (De Vries et al., 2005b) leading to a range of 60-90 kg C/ha/year, implying a nitrogen deposition impact of ~10% (see Table 17.6). The average contribution of nitrogen deposition to the increase in N2O emissions is estimated at 18-40 g/ha/year per kilogram of nitrogen, which is the range found in DNDC model estimates for Europe (first estimate) and in empirical data for coniferous and deciduous forest (second estimate). Using an average additional nitrogen deposition of 2.8 kg N/ha/year during 1960-2000 has led to an average increase of 0.05-0.11 kg N2O-N/ha/ year. The average contribution of nitrogen deposition to the reduction of the CH4 sink is 0.041 kg CH4/ha/year per kilogram of NH4 for a spruce forest in Germany (Hoglwald). This forest has an estimated CH4 sink of ~3.0 kg NH4-N/ha/year at zero NH4-N input, which is the upper value of the CH4 sink estimate for Europe. At an additional NH4-N input of 1.2 kg during 1960-2000, this becomes 0.05 kg
CH4/ha/year, 1.6% that of forests. Assuming a constant percentage of nitrogen deposition impact, the range in reduction of the CH4 sink becomes 0.003-0.05 kg CH4/ha/year.
In summary, Table 17.6 shows that the impact of nitrogen deposition on the exchange in GHGs in terms of GWP is on average -275 kg CO2 equivalents due to carbon sequestration by forests which is offset by ~35 kg CO2 equivalents due to N2O emissions (13%), whereas the contribution of a reduced CH4 sink is negligible. This means that the increase in carbon sequestration in response to nitrogen deposition clearly outweighs the increased N2O emissions, but again a more reliable N2O estimate in response to nitrogen deposition is important to estimate the counter-effect more precisely.
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