Calculating carbon changes in peat soils drained for forestry with four different profile-based methods

Boreal peatlands are an important carbon (C) sink. The effect of drainage for forestry on the soil C balance in those peatlands is a controversial debate. The Lakkasuo peatland, central Finland, comprise a minerotrophic and an ombrotrophic part, both partially drained for forestry. A pair-wise comparison was conducted and four different profile-based methods were applied to calculate the soil C balance. The first two methods used differences in ash content (I) between the upper and lower part of the profile and (II) between the drained and natural site of the peatland, respectively. The third method (III) used radiocarbon dated samples to calculate C accumulation rates at the natural site and compared these to the current C-stocks at the drained sites. The fourth method (IV) used radiocarbon dated samples to define a 1000-year layer in the profiles for comparing the C-stocks above this layer. Stable carbon isotope depth profiles, used for a qualitative assessment of the peatland status, identify both undrained site as undisturbed. All four methods indicate a C loss at the minerotrophic drained site but of different magnitude (0.057–0.272 kg C m−2 yr−1). At the ombrotrophic drained site both radiocarbon methods (III and IV) indicate a C gain (0.139–0.179 kg C m−2 yr−1) whereas methods I and II suggest a C loss (0.084–0.270 kg C m−2 yr−1). Method IV is considered the most stringent and robust one. Yet, the comparison of profile-based methods for C balance assessment suggests them to be applicable depending on site-specific conditions of nutrient status and presence of a natural reference site.