Carbon storage in the organic layers of boreal forest soils under various moisture conditions: A model study for Northern Sweden sites

A typical feature of the boreal forest landscape is a gradient from dry to wet sites, with associated increases in the depth of the soil organic layers. In this study, the coupled ecosystem-soil biogeochemistry model GUESS-ROMUL is used to explore how the specific features of soil organic matter decomposition and vegetation dynamics account for an observed difference between the soils formed under contrasting moisture conditions. Two sites, one mesic and one mesic-to-wet, representative of the natural forest in Northern Sweden, are simulated. In addition to the assumptions underlying the GUESS-ROMUL model, it is assumed that the fire frequency was higher at the mesic site. The model shows that with a natural fire regime, the soil organic layers at the mesic-to-wet site store 6.0 kg C m−2 compared to 3.1 kg C m−2 at the mesic site. Forty-seven percent of the difference between the sites in this respect is explained by suppressed decomposition under higher moisture conditions, 37% by the decreased litter input into the soil (more frequently disturbed ecosystems have lower productivity) and 16% by direct consumption of the forest floor in fires. It is predicted that due to anthropogenic fire suppression the organic soil layers of mesic sites will, in the future, sequester carbon at an average rate of 0.0103 kg C m−2 year−1 and have an equilibrium storage capacity of 5.4 kg C m−2. For the mesic-to-wet site, the model predicts an extremely slow sequestration rate of 0.0022 kg C m−2 year−1. The effect of increased precipitation on the carbon storage at the landscape level is also investigated.