A fertile peatland forest does not constitute a major greenhouse gas sink

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 (N₂ O) 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 N₂ O fluxes were determined by the closed-chamber technique and amounted to 0.9 ± 0.8 t C_eq 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 C_eq 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 C_eq 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.

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