Minkkinen, Kari

Ojanen, Paavo

Penttilä, Timo

Helsingin yliopiston digitaalinen arkisto

14th International Peat Congress  1  Extended abstract No. 239   GREENHOUSE GAS BALANCE OF FORESTRY-DRAINED BOREAL PEATLANDS: SINKS OR SOURCES?  Paavo Ojanen, University of Helsinki, Department of Forest Sciences, P.O. Box 27, FI-00014 University of Helsinki, Finland, +358 9 191 581 42, paavo.ojanen@helsinki.fi Kari Minkkinen, University of Helsinki; Timo Penttilä, Finnish Forest Research Institute   SUMMARY  We estimated the current greenhouse gas balance (in CO2 equivalents, CDE) for 68 forestry-drained sites in Finland, ranging from hemiboreal to north boreal vegetation zones and from fertile to poor sites. We found net emissions from soil at fertile but not at poor sites. The role of CH4 and N2O emissions was minor. The large CO2 sinks of the growing tree stands caused the ecosystem balances to be clearly positive (sink) even at fertile sites.  KEY WORDS: forestry-drainage, boreal peatland, greenhouse gas balance   INTRODUCTION  Pristine boreal peatlands are sinks of atmospheric carbon dioxide (CO2) as carbon is accumulated in the peat (Turunen et al., 2002). In the same time, methane (CH4) is released (Waddington and Roulet, 2000). Nitrous oxide (N2O) emissions are generally low, occurring mainly at fertile sites (Regina et al., 1996; Drewer et al., 2010; Lohila et al., 2010). As a consequence, pristine boreal peatlands have a long-term climate cooling effect (Frolking and Roulet, 2007).  Drainage of boreal peatlands for forestry may have both climate warming and climate cooling impacts on the greenhouse gas (GHG) balance: When ground water table is lowered, CH4 emissions decrease or even cease (Ojanen et al., 2010). If the peat layer starts to degrade, as is the case after drainage for agriculture (Maljanen et al., 2007), peatland turns into a source of CO2. In forestry-drained peatlands N2O emissions are generally low, but substantial emissions may occur at fertile sites (Ojanen et al., 2010). If drainage is successful, tree stand biomass starts to increase, which results in a considerable CO2 sink (Tomppo, 1999; Minkkinen et al., 2001).  There is widely data on tree growth and CO2, CH4 and N2O fluxes in forestry-drained boreal peatlands (Ojanen et al. 2010; Statistics Finland, 2011; Swedish Environmental Protection Agency, 2011). On the other hand, very little data exists on net CO2 exchange (Hargreaves et al., 2003; Lohila et al., 2011) and soil C stock changes after drainage (Minkkinen and Laine, 1998; Minkkinen et al., 1999; Laiho et al., 2008). Also, studies estimating the current balance of all the three GHGs at the same sites, thus empirically testing if the sites are GHG sinks or sources, are lacking.  14th International Peat Congress  2  In this study we estimated the CO2, CH4, and N2O balances for 68 peatland sites in Finland, by measuring the soil–atmosphere fluxes of GHGs and estimating net primary production and litter production for each site.   MATERIAL AND METHODS  The 68 study sites were located in all parts of Finland except for the northernmost part where drainage for forestry is scarce (Ojanen et al., 2010). Sites on different drained peatland site types (Vasander and Laine, 2008) were equally included to represent the continuum from the most fertile Herb-rich type via Vaccinium myrtillus type and Vaccinium vitis-idaea type to the poor Dwarf shrub type.  GHG balance was defined as the sum of the ecosystem–atmosphere net fluxes of CO2, N2O, and CH4, as converted to CO2 equivalents (CDE). For CH4 and N2O, net fluxes were those annual soil–atmosphere fluxes estimated by Ojanen et al. (2010). The CDEs for N2O and CH4 were calculated by multiplying their net fluxes by their global warming potentials (GWP100): 25 for CH2 and 298 for N2O (IPCC 2007). For CO2, the balance consisted of soil–atmosphere net flux and the CO2 sink of the growing tree stand (Ojanen et al., manuscript).  The soil–atmosphere net flux of CO2 was defined as litter production (L) – aerobic decomposition (D). L was estimated following the procedures presented by Laiho et al. (2003). D was estimated as the sum of measured heterotrophic soil respiration (Ojanen et al. 2010) and modelled decomposition of the litter layer. Tree stand CO2 sink was estimated as the difference in living tree biomass between two consecutive estimations. Positive values indicate sinks and negative values sources of GHG.   RESULTS  There was a clear distinction between the soil CO2 balance of the fertile and the poor sites (Fig. 1). The poor sites were on average a small sink of CO2, the fertile sites a source. The source at the fertile sites increased as the temperature sum increased. The CO2 sink of the growing tree stand increased from poor to fertile sites and from north to south  Both fertile and poor sites were, on average, GHG sinks when the tree stands were included (Fig. 2). The negative soil balance of the fertile sites, when compared to the positive soil balance of the poor sites, was more than compensated for by the larger CO2 sink of the tree stand. The effect of CH4 and N2O on the ecosystem balance was small.  14th International Peat Congress  3    Figure 1. Soil CO2 balance of the study sites. Poor sites are those classified as Vaccinium vitis-idaea type or Dwarf shrub type; fertile sites are those classified as Herb rich type or Vaccinium myrtillus type. Lines depict the running averages. Positive value indicates sink and negative value indicates source.    Figure 2. Average ecosystem greenhouse gas balance (in CO2 equivalents, CDE) of the study sites. The balance is calculated as the sum of the balances of CO2, CH4 and N2O. Poor sites are those classified as Vaccinium vitis-idaea type or Dwarf shrub type; fertile sites are those classified as Herb rich type or Vaccinium myrtillus type. Positive value indicates sink and negative value indicates source.  14th International Peat Congress  4   DISCUSSION  The results of this study confirm the earlier findings (Minkkinen and Laine, 1998; Minkkinen et al., 1999; Lohila et al., 2011) in that peat accumulation at nutrient poor boreal peatlands may continue even after drainage for forestry. Thus, climatically sustainable ditching-based forestry seems to be possible on nutrient-poor peatlands. On the other hand, fertile peatland soils turn into CO2 sources after drainage.  Although the current ecosystem GHG sink of the fertile sites was even higher than that of the poor sites, forestry is unlikely to remain sustainable in the long-term, as the positive balance depends on the growing tree stand biomass. Only if the harvested biomass is later stored, for example in wooden buildings or as bio char in agricultural soils, the current high sink of the growing tree stand could be seen as a means to mitigate the climate change.   REFERENCES  Drewer, J., Lohila, A., Aurela, M., Laurila, T., Minkkinen, K., Penttilä, T., Dinsmore, K.J., McKenzie, R.M., Helfter, C., Flechard, C., Sutton, M.A., Skiba, U.M. (2010). Comparison of greenhouse gas fluxes and nitrogen budgets from an ombrotrophic bog in Scotland and a minerotrophic sedge fen in Finland. European Journal of Soil Science 61, 640–650. Frolking, S., Roulet, N. (2007). Holocene radiative forcing impact of northern peatland carbon accumulation and methane emissions. Global Change Biology 13, 1079–1088. Hargreaves, K., Milne, R., Cannell, M. (2003). Carbon balance of afforested peatland in Scotland. Forestry 76, 299–317. IPCC (2007). Climate Change 2007: The physical science basis. Contribution of working group I to the Fourth assessment report of the IPCC, 996 pp. Cambridge University Press, Cambridge, UK and New York, USA. Laiho, R., Minkkinen, K., Anttila, J., Vávrová, P., Penttilä, T. (2008). Dynamics of litterfall and decomposition in peatland forests: Towards reliable carbon balance estimation? In: Vymazal, J. (ed.). Wastewater treatment, plant dynamics and management in constructed and natural wetlands, p. 53–64. Springer Science + Business Media, Dordrecht. Laiho, R., Vasander, H., Penttilä, T., Laine, J. (2003). Dynamics of plant-mediated organic matter and nutrient cycling following water-level drawdown in boreal peatlands. Global Biogeochemical Cycles 17, 1053, doi: 10.1029/2002GB002015. Lohila, A., Aurela, M., Hatakka, J., Pihlatie, M., Minkkinen, K., Penttilä, T., Laurila, T. (2010). Responses of N2O fluxes to temperature, water table and N deposition in a northern boreal fen. European Journal of Soil Science 61, 651–661. Lohila, A., Minkkinen, K., Aurela, M., Tuovinen, J-P., Penttilä, T., Ojanen, P., Laurila, T. (2011). Greenhouse gas flux measurements in a forestry-drained peatland indicate a large carbon sink. Biogeosciences 8, 3203–3218. Maljanen, M., Hytönen, J., Mäkiranta, P., Alm, J., Minkkinen, K., Laine, J., Martikainen, P.J. (2007). Greenhouse gas emissions from cultivated and abandoned organic croplands in Finland. Boreal Environment Research 12, 133–140. Minkkinen, K., Laine, J. (1998). Long-term effect of forest drainage on the peat carbon stores of pine mires in Finland. Canadian Journal of Forest Research 28, 1267–1275. Minkkinen, K., Laine, J., Hökkä, H. (2001). 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Greenhouse gas balance of forestry-drained boreal peatlands: Sinks or sources?

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