Aim. The aim behind this work is: (i) to review the work on Scots pine needle litter in order to construct a model for the decomposition process, from litterfall until a stable fraction is left, (ii) suggest a simple regulating mechanism for its sequestration of carbon. Focus will be on foliar litter of Scots pine and the genus Pinus. Discussion. The chemical composition of newly shed pine litter is in part determined by climate, e.g. mean annual temperature (MAT). Thus concentrations of nitrogen (N) are higher -and those of manganese (Mn) lower -with higher MAT. This may also influence the decomposition process. Mass loss of newly shed pine needle litter is positively influenced by climate (e.g. MAT), as well as by N and phosphorus (P) concentrations. In the late stage (above c. 30% accumulated mass loss) the influence of climate fades and those of lignin (Acid Unhydrolyzable Residue -AUR), N, and Mn are regulating the decomposition process. As the degradation of AUR dominates the decomposition process important parameters are those that influence the degradation of AUR, thus N and Mn. In the humus-near organic matter limit values have been related to litter Mn concentration over a wide climate gradient. Thus, the higher the Mn concentration, the further the process goes and the smaller the stable fraction.Conclusions. It appears that factors regulating the size of the stable litter fraction may be used as a tool on a larger geographical scale to predict carbon sequestration rates in pine forests.

Boreal forest soils play an important role in the global carbon cycle by functioning as a large terrestrial carbon sink or source, and the alteration of fire regime through global change phenomena may influence this role. We studied a system of forested lake islands in the boreal zone of Sweden for which fire frequency increases with increasing island size. Large islands supported higher plant productivity and litter decomposition rates than did smaller ones, and, with increasing time since fire, litter decomposition rates were suppressed sooner than was ecosystem productivity. This contributes to greater carbon storage with increasing time since fire; for every century without a major fire, an additional 0.5 kilograms per square meter of carbon becomes stored in the humus.

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Biological nitrogen (N) fixation is the primary source of N within natural ecosystems, yet the origin of boreal forest N has remained elusive. The boreal forests of Eurasia and North America lack any significant, widespread symbiotic N-fixing plants. With the exception of scattered stands of alder in early primary successional forests, N-fixation in boreal forests is considered to be extremely limited. Nitrogen-fixation in northern European boreal forests has been estimated at only 0.5 kg N ha(-1) yr(-1); however, organic N is accumulated in these ecosystems at a rate of 3 kg N ha(-1) yr(-1) (ref. 8). Our limited understanding of the origin of boreal N is unacceptable given the extent of the boreal forest region, but predictable given our imperfect knowledge of N-fixation. Herein we report on a N-fixing symbiosis between a cyanobacterium (Nostoc sp.) and the ubiquitous feather moss, Pleurozium schreberi (Bird) Mitt. that alone fixes between 1.5 and 2.0 kg N ha(-1) yr(-1) in mid- to late-successional forests of northern Scandinavia and Finland. Previous efforts have probably underestimated N-fixation potential in boreal forests.