Foliar Litter Decomposition: A Conceptual Model with Focus on Pine (Pinus) Litter—A Genus with Global Distribution

Figure 11

Conceptual model for rate-regulating factors and chemical changes during decomposition, modified from [28, 34]. The decomposition of water-soluble substances and unshielded cellulose/hemicellulose is stimulated by high levels of the major nutrients such as N, P, and S (early stage—phase 1). When the main part of all unshielded holocellulose is decomposed, lignin-encrusted holocellulose and lignin remain. The early phase has been suggested to last for even up to c. 40% accumulated mass loss for some pine litters [34]. For other foliar litters, for example, spruce and oak, the early phase has been found to be very short or possibly nonexisting. In the late stage—phase 2, the degradation of lignin dominates the litter decomposition rate. Nitrogen hampers the degradation of lignin and higher N concentrations suppress the decomposition, whereas Mn appears to have a stimulating effect on the degradation of lignin and thus on litter. Finally, in the humus-near stage (phase 3), the litter decomposition rate is (close to) zero and the accumulated mass loss reaches its limit value. The model is modified according to a suggestion of Klotzbücher et al. [81]. They found that there is a loss of lignin from the start of the incubation. Still it appears that for, for example, Scots pine and other pine litters the early phase is not dominated by lignin degradation.