Greenhouse gas flux measurements in a forestry-drained peatland indicate a large carbon sink

Abstract. Drainage for forestry purposes increases the depth of the oxic peat layer and leads to increased growth of shrubs and trees. Concurrently, the production and uptake of the greenhouse gases carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) change: due to the accelerated decomposition of peat in the presence of oxygen, drained peatlands are generally considered to lose peat carbon (C). We measured CO₂ exchange with the eddy covariance (EC) method above a drained nutrient-poor peatland forest in southern Finland for 16 months in 2004–2005. The site, classified as a dwarf-shrub pine bog, had been ditched about 35 years earlier. CH₄ and N₂O fluxes were measured at 2–5-week intervals with the chamber technique. Drainage had resulted in a relatively little change in the water table level, being on average 40 cm below the ground in 2005. The annual net ecosystem exchange was −870 ± 100 g CO₂ m⁻² yr⁻¹ in the calendar year 2005, indicating net CO₂ uptake from the atmosphere. The site was a small sink of CH₄ (−0.12 g CH₄ m⁻² yr⁻¹) and a small source of N₂O (0.10 g N₂O m⁻² yr⁻¹). Photosynthesis was detected throughout the year when the air temperature exceeded −3 °C. As the annual accumulation of C in the above and below ground tree biomass (175 ± 35 g C m⁻²) was significantly lower than the accumulation observed by the flux measurement (240 ± 30 g C m⁻²), about 65 g C m⁻² yr⁻¹ was likely to have accumulated as organic matter into the peat soil. This is a higher average accumulation rate than previously reported for natural northern peatlands, and the first time C accumulation has been shown by EC measurements to occur in a forestry-drained peatland. Our results suggest that forestry-drainage may significantly increase the CO₂ uptake rate of nutrient-poor peatland ecosystems.