Long-term effect of forest drainage on the peat carbon stores of pine mires in Finland

Abstract

The drawdown of the water level has been hypothesized to change peatlands from carbon (C) sinks to C sources to the atmosphere as a result of increased oxidation of organic matter. We measured the change in peat thickness (subsidence of peat surface) ca. 60 years after drainage in 273 peatland sites (875 points) representing three nutrient levels of pine fens and five macroclimatic regions from southern to northern Finland, and we calculated the changes in peat C densities and peat C stores using direct measurements and estimates drawn from regression models for C density derived in this study. On average, the peat surface had subsided ca. 22 ± 17 cm (mean ± SD), the C density had increased by 26 ± 15 kg·m-3, and the C stores had increased by 5.9 ± 14.4 kg·m-2 after drainage. Therefore, the hypothesis that drainage of peatland always makes them sources of C to the atmosphere is incorrect. We concluded that the post-drainage subsidence of peat surface was mainly caused by changes in the physical structure of peat after the removal of water while the oxidation of peat was of less importance. The changes in C density and C stores seemed to be dependent on the input of new C into the system through net primary production, especially through the fine roots of trees. Our results suggest that tree stand development plays an important role in the post-drainage C balance of peat soils.