Abstract
Abstract. Afforestation has been proposed as a strategy to mitigate the often high greenhouse gas (GHG) emissions from agricultural soils with high organic matter content. However, the carbon dioxide (CO2) and nitrous oxide (N2O) fluxes after afforestation can be considerable, depending predominantly on site drainage and nutrient availability. Studies on the full GHG budget of afforested organic soils are scarce and hampered by the uncertainties associated with methodology. In this study we determined the GHG budget of a spruce-dominated forest on a drained organic soil with an agricultural history. Two different approaches for determining the net ecosystem CO2 exchange (NEE) were applied, for the year 2008, one direct (eddy covariance) and the other indirect (analyzing the different components of the GHG budget), so that uncertainties in each method could be evaluated. The annual tree production in 2008 was 8.3 ± 3.9 t C ha−1 yr−1 due to the high levels of soil nutrients, the favorable climatic conditions and the fact that the forest was probably in its phase of maximum C assimilation or shortly past it. The N2O fluxes were determined by the closed-chamber technique and amounted to 0.9 ± 0.8 t Ceq ha−1 yr−1. According to the direct measurements from the eddy covariance technique, the site acts as a minor GHG sink of −1.2 ± 0.8 t Ceq ha−1 yr−1. This contrasts with the NEE estimate derived from the indirect approach which suggests that the site is a net GHG emitter of 0.6 ± 4.5 t Ceq ha−1 yr−1. Irrespective of the approach applied, the soil CO2 effluxes counter large amounts of the C sequestration by trees. Due to accumulated uncertainties involved in the indirect approach, the direct approach is considered the more reliable tool. As the rate of C sequestration will likely decrease with forest age, the site will probably become a GHG source once again as the trees do not compensate for the soil C and N losses. Also forests in younger age stages have been shown to have lower C assimilation rates; thus, the overall GHG sink potential of this afforested nutrient-rich organic soil is probably limited to the short period of maximum C assimilation.
Highlights
After drainage, organic soils continuously lose CO2 to the atmosphere, irrespective of their use as agricultural or forest land
For Sweden, it has been estimated that organic soils used for agriculture release 6–10 % of the total national emission of greenhouse gases (GHG) (Kasimir Klemedtsson et al, 1997; Berglund and Berglund, 2010), despite the fact that only 5–10 % of agricultural land is on such soils
Other studies on the net ecosystem CO2 exchange (NEE) of forests on organic soils have been undertaken by Dunn et al (2007) and Flanagan and Syed (2011), but the comparability is limited because site conditions differ considerably from our site with respect to vegetation, management history and the much higher forest age of 160 yr
Summary
Organic soils continuously lose CO2 to the atmosphere, irrespective of their use as agricultural or forest land. Using micrometeorological measurements to determine the CO2 flux and chamber measurements to quantify the N2O and CH4 fluxes, Lohila et al (2011) found that a pine forest on a drained organic soil was an intermediate C sink, in which N2O and CH4 only had a minor impact on the total GHG budget. This site had not been in agricultural use previously and had a high soil C : N ratio (34–41), which is why low N2O emissions could be expected (Klemedtsson et al, 2005). The site is which the chambers for gas flux measurements were placed
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