Greenhouse gas balances of managed peatlands in the Nordic countries – present knowledge and gaps

Abstract

Abstract. This article provides an overview of the effects of land-use on the fluxes of carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) and from peatlands in the Nordic countries based on the field data from about 100 studies. In addition, this review aims to identify the gaps in the present knowledge on the greenhouse gas (GHG) balances associated with the land-use of these northern ecosystems. Northern peatlands have accumulated, as peat, a vast amount of carbon from the atmosphere since the last glaciation. However, the past land-use and present climate have evidently changed their GHG balance. Unmanaged boreal peatlands may act as net sources or sinks for CO 2 and CH 4 depending on the weather conditions. Drainage for agriculture has turned peatlands to significant sources of GHGs (mainly N 2 O and CO 2 ). Annual mean GHG balances including net CH 4 , N 2 O and CO 2 emissions are 2260, 2280 and 3140 g CO 2 eq. m −2 (calculated using 100 year time horizon) for areas drained for grass swards, cereals or those left fallow, respectively. Even after cessetion of the cultivation practices, N 2 O and CO 2 emissions remain high. The mean net GHG emissions in abandoned and afforested agricultural peatlands have been 1580 and 500 g CO 2 eq. m −2 , respectively. Peat extraction sites are net sources of GHGs with an average emission rate of 770 g CO 2 eq. m −2 . Cultivation of a perennial grass (e.g., reed canary grass) on an abandoned peat extraction site has been shown to convert such a site into a net sink of GHGs (−330 g CO 2 eq. m −2 ). In contrast, despite restoration, such sites are known to emit GHGs (mean source of 480 g CO 2 eq. m −2 , mostly from high CH 4 emissions). Peatland forests, originally drained for forestry, may act as Correspondence to: M. Maljanen (marja.maljanen@uef.fi) net sinks (mean −780 g CO 2 eq. m −2 ). However, the studies where all three GHGs have been measured at an ecosystem level in the forested peatlands are lacking. The data for restored peatland forests (clear cut and rewetted) indicate that such sites are on average a net sink (190 g CO 2 eq. m −2 ). The mean emissions from drained peatlands presented here do not include emissions from ditches which form a part of the drainage network and can contribute significantly to the total GHG budget. Peat soils submerged under water reservoirs have acted as sources of CO 2 , CH 4 and N 2 O (mean annual emission 240 g CO 2 eq. m −2 ). However, we cannot yet predict accurately the overall greenhouse gas fluxes of organic soils based on the site characteristics and land-use practices alone because the data on many land-use options and our understanding of the biogeochemical cycling associated with the gas fluxes are limited.

Annual emissions of CH<sub>4</sub> and N<sub>2</sub>O, and ecosystem respiration, from eight organic soils in Western Denmark managed by agriculture

“…Maljanen et al (2010), in a review of GHG balances for organic soils in the Nordic countries, reported emissions from perennial grasslands to be 0.32 ± 0.64 g CH 4 m −2 (n = 11), which is comparable to the 0.47 g CH 4 m −2 of this study. Similarly, for N 2 O the average emission observed at grassland sites on peat in previous studies in Nordic countries was 1.50 ± 1.60 g N 2 O m −2 (n = 12), a range that includes the average emission of 0.5 g N 2 O m −2 of the present study (Table 8).…”

“…Similarly, for N 2 O the average emission observed at grassland sites on peat in previous studies in Nordic countries was 1.50 ± 1.60 g N 2 O m −2 (n = 12), a range that includes the average emission of 0.5 g N 2 O m −2 of the present study (Table 8). For barley an average flux of CH 4 of −0.03 ± 0.18 g m −2 (n = 5) was reported (Maljanen et al, 2010), the range and lower level relative to permanent grasslands was confirmed by the present study. Finally, N 2 O emission factors for barley grown on peat soil were reported to average 1.7 ± 0.5 g m −2 (n = 5), with one study of a potato field in the same range; these values are also comparable to the 2.5 g N 2 O m −2 determined in the present study.…”

“…Drainage will limit the production of CH 4 , but also increase the potential for CH 4 oxidation during passage through the unsaturated zone to the atmosphere. As a result CH 4 fluxes from drained organic soils are consistently low or slightly negative (Langeveld et al, 1997;Drösler et al, 2008;Maljanen et al, 2010). Methane oxidation potentials appear to be highest near the oxic/anoxic interface.…”

“…Maljanen et al (2010), reviewing GHG monitoring studies from the Nordic countries, reported that N 2 O emissions from organic soils in agricultural use were on average four times higher than those from mineral soils, indicating that N 2 O derived from soil organic matter decomposition dominate overall fluxes. According to Maljanen et al (2010) annual N 2 O emissions from managed organic soils range from 0.2 to 5.5 g m −2 , with an average of 1.6 g m −2 , but no studies from Denmark were available.…”

“…In Europe organic soils cover approximately 5 % of the total land area (European Soil Bureau Network, 2005), and this proportion is similar in Denmark, where organic soils are widely used by agriculture as pastures or for crop production (Maljanen et al, 2010). With an organic matter content of 20 % or more in the top soil (FAO, 1998) the C and N turnover and gaseous exchanges of Histosols are significant for the greenhouse gas balance of Danish agriculture, even if their total area is relatively small (Gyldenkaerne et al, 2005).…”