The various estimates of carbon sequestration by European forests indicate a value of ~0.1 Pg C/year. This is comparable to European grasslands that are also thought to be a sink for atmospheric CO2 of 0.1 Pg C/year (CarboEurope
GHG, 2004). Inversely, Janssens et al. (2003) estimated that arable land in Europe loses 0.3 Pg C/year. Thereby, arable land is the largest annual biospheric source of CO2 to the atmosphere in Europe. However, it must be acknowledged that the uncertainty in estimates for forests, grasslands and arable land is large.
Boeckx and Van Cleemput (2001) evaluated the emission of N2O from agricultural soils in Europe using the IPCC methodology (Mosier et al., 1998). They estimated that the average direct N2O emission from agricultural soils across Europe is 5.6 kg N/ha/year. Lower numbers were calculated by Freibauer and Kaltschmitt (2003), who considered climate, soil and management factors in addition to the amount of nitrogen fertilization. For agricultural, minerotrophic soils for the temperate and sub-boreal climate regions in Europe, they estimated a mean N2O emission of 2.0 kg/ha/year. For forest soils, we estimated an average emission of 0.5-0.6 kg N/ha/year. This implies that forest soils are approximately four- to tenfold smaller as a source for N2O than agricultural soils are. With the data derived by Freibauer and Kaltschmitt (2003), Freibauer (2003) estimated the annual total N2O emissions from agriculture in the EU-15 at 535,000 t N2O-N, which included the N2O emissions from animal husbandry (emissions from animal houses and manure storages). The percentage of soil-borne emissions was estimated at 82% or 440,000 t N2O-N. Assuming estimated N2O emissions by forest soils of 80,000 t N2O-N (this study) would imply a contribution of 15% when neglecting the N2O emissions from agricultural soils outside the EU-15. The agriculturally utilized area in EU-15 is 128 million hectares and that in EU-25 is 166 million hectares. The area used for agricultural in the non-member states of the EU (e.g. Norway, Switzerland, Belarus, Ukraine, Bulgaria, Russia) is relatively large but agricultural practices are less intensive, and hence N2O emissions per unit surface area are probably less in these countries than in EU-15. All in all, this would suggest that the emission strength of forest soils in Europe most likely ranges between 5% and 15% of the emission strength of agricultural soils. As agricultural soils are the major source of N2O in Europe (e.g. Freibauer, 2003), we consider
European forests a significant source of atmospheric N2O in Europe.
For CH4 the situation is quite different. Although the uncertainties are large, there is enough evidence to conclude that exchange rates of CH4 between forest soils and atmosphere are very low compared to the release rates from wetlands. For forests, literature suggests a range in mean net CH4 uptake of 0.2-3.0 kg CH4/ha/year (Brumme et al., 2004; Lindner et al., 2004). Using an average value of 1.5 kg CH4/ha/year and an area of 162 million hectares for European forests leads to a CH4 sink of ~240,000 t . Freibauer (2003) estimated the CH4 emissions from agriculture at 7,980,000 t CH4 for the EU-15 countries of which ~75% is due to enteric fermentation and the remaining 25% due to animal housing and manure storage. Uptake of CH4 by agricultural soils was not taken into account in the study of Freibauer (2003), but is probably in the same order of magnitude as that by forest soils. Comparision of those numbers shows that the contribution of forests to the net CH4 emissions from the biosphere in Europe is small (<3%), specifically considering the fact that the estimates for agriculture presented by Freibauer (2003) are for the EU-15 only. Clearly, mitigation measures for CH4 emissions should focus on animal agriculture and not on forests.
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