Abstract
We have reviewed literature on the topic 'carbon sequestration in forest soils'. We review a theory/model for the sequestration process based on pine (Pinus) needle litter that is transformed into stable matter during decomposition. The decomposition process is divided into two phases with lignified tissue in the latter developing a recalcitrance with a calculable limit value for decomposition. This is just one of several possible models, this one developed for boreal forests. It appears likely that part of the stabilization process is related to oxygen limitation for micro-organisms, high concentrations of heavy metals developed during decomposition and/or allelopathic effects that suppress the decomposition process and allow an accumulation of organic matter. We divide the concept into primary sequestration and secondary sequestration. Primary sequestration occurs in an organic layer, e.g. a humus layer, and is a process that encompasses the stabilization of decomposing plant litter remains and their accumulation. This layer is vulnerable to e.g. fire and other disturbances. Secondary sequestration occurs in the mineral soil, where the buildup of sequestered carbon is a slower process. This carbon material was leached from the organic layers and has become adsorbed to mineral particles and thus stabilized. We may consider the concept stability as a very relative one. Stability appears to be related to a set of conditions on e.g. environment and micro-organisms. It may also be related to nutrients in the litter and/or environment. The review is based on available data and has its focus on coniferous boreal and northern temperate forests.