The total terrestrial GHG balance is estimated by equation 5.13. We use the annual CH4 emission derived from method 2 and the annual N2O emissions derived from method 3. The average annual CH4 emission is 165 kg CH4 ha-1 year-1 and the average annual N2O is 24 kg N2O ha-1 year-1. The annual terrestrial CO2 emission, net ecosystem exchange (NEE), is taken from Veenendaal et al. (2007), who calculated a value of 134 g C m-2 year-1 for October 2004 to October 2005 which equals to 4910 kg CO2 ha-1 year-1. This leads to an average total terrestrial GHG emission of 16 (±15%) Mg ha-1 year-1in CO2-equivalents with 30%, 25% and 45% from CO2, CH4 and N2O, respectively (Fig 5.6.). This means that the agricultural peatland is an important source for all three greenhouse gases.
Fig 5.6. Annual terrestrial emissions of CO2, CH4 and N2O and the sum of the emissions of these
three gases in CO2-equivalents ha-1 year-1 with their uncertainty measured at a managed peat area in
the Netherlands.
To put this into the right perspective, it is important to realize that there are substantial additional sources due to the full range of agricultural activities on this peatland. If CO2 emissions due to biomass removal (i.e. grass harvest) and CH4 farm emissions (i.e. enteric fermentation by dairy cows) are included, the total emission will increase by 10.5x103 kg CO2 ha-1 year-1 (Veenendaal et al., 2007) and 5911 kg CO2 ha-1year-1 (257 kg CH4 ha-1 year- 1) (Schrier-Uijl et al., 2010a), respectively. The total balance will thus increase by even more than 250%, which emphasizes the importance of further research in which all GHG components will be taken into account.
We also compared the total field GHG emissions at this site (Oukoop) to the GHG emissions in a former agricultural site (Horstermeer) which was converted to a nature reserve 15 years ago by stopping intensive farming and raising the water table (Hendriks et al., 2007). The terrestrial CO2, CH4 and N2O emissions at the Horstermeer site are -11403 kg CO2 ha-1 year-1, 417 kg CH4 ha-1 year-1 and negligible (Hendriks et al., 2007). This leads to a total GHG uptake of -2 Mg ha-1 year-1 in CO2-equivalents. Consequently, a transformation of an intensively agricultural site to a nature reserve will probably lead to a decrease in total GHG emission since the Oukoop site is a strong source of 16 Mg ha-1 year- 1 in CO
2-equivalents. The CO2 and N2O emissions are thus much larger at the intensively managed Oukoop site than at the restored Horstermeer site. In addition, if we also include the farm based emissions to the CH4 emissions, the total CH4 emissions at Oukoop will be also larger than the CH4 emissions at the Horstermeer site (e.g., Schier-Uijl et al., 2010a).
5.5.
Conclusions
The annual CH4 and N2O terrestrial balances were derived from a dairy farm on peat grassland in the Netherlands over 2006, 2007 and 2008 using EC flux measurements. The average terrestrial CH4 balance was 165 kg CH4 ha-1 year-1 and the average terrestrial N2O balance 24 kg N2O ha-1 year-1. The CH4 estimate was more accurate than the estimate by static chambers and the N2O estimate was a step forward since the background fluxes could not be detected by static chamber measurements using a photo acoustic instrument at the same site.
Furthermore, we investigated the direct emission factor EF1 from agricultural soils which is used in the IPCC guidelines. We calculated the possible EF1 range for six fertilization events ranging from EFlmin to EFlmax since the real background emission was not known. The minimum EF1 was determined by subtracting the background emission and the maximum EF1 without subtracting the background emission. The average EFlmin was 1.2% and the average EFlmax 2.8%. Both values are larger than the IPCC default EF1 value of 1%. Finally, the total terrestrial GHG balance was estimated at 16 Mg ha-1 year-1 in CO2- equivalents averaged over 2006, 2007 and 2008 with contributions of 30%, 25% and 45% by CO2, CH4 and N2O. This agricultural peatland was thus a serious source for all three greenhouse gases. The total emission would be even larger since the farm emissions and the biomass removal were not taken into account.
5.6.
Acknowledgements
This research was part of the Dutch National Research Program BSIK ME1. We are grateful to two anonymous referees for their remarks and suggestions. Thanks are due to
these measurements. We are also grateful to A. Vermeulen and E. de Beus for their assistance in data analyses. In addition, we would like to take the opportunity of thanking C. Flechard from Inra in France for the productive discussion and advices. Finally, we owe a special debt of gratitude to the farmer T. Van Eyk for using his farm site.
5.7.
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