The genus Pinus encompasses c 120 species and has a global distribution. Today we know more about the decomposition of pine needle litter than litter from any other genus. This paper presents a developed conceptual three-phase model for decomposition, based on pine needle litter, starting with newly shed litter and following the process until a humus-near stable residue. The paper focuses on the mass-loss dynamics and factors regulating the process in the early phase, the late one, and the humus-near phase. For the late phase, the hampering influence of N and the rate-enhancing effect of Mn on the decomposition are given extra attention. Empirical factors related to the limit value/stable residue are discussed as well as the decomposition patterns and functions for calculating limit values. The climate-related litter concentrations of N and Mn are discussed as well as their possible influence on the size of the stable residue, which may accumulate and sequester carbon, for example, in humus layers. The sequestration of carbon in humus layers is discussed as well as the effect of tree species on the process. Although the paper focuses on litter of pine species, there are comparisons to studies on other litter genera and similarities and differences are discussed. 1. Introduction The process “plant litter decomposition” is quantitatively as large as the photosynthesis. The process is necessary for the recirculation of nutrients and a continued buildup of plant biomass as well as for the maintenance of food webs through the energy released by the degradation of organic compounds. The main process which we, a bit simplified, call “plant litter decomposition” is extremely complex and can be subdivided into a multitude of subprocesses. These include not only release of nutrients but also a stepwise degradation of the main chemical compounds present in the shed litter. Also, synthesis processes are included in the concept, resulting in new compounds, which in turn may recombine, often creating a recalcitrance of the remaining litter mass. In spite of its importance, this system of subprocesses is mainly unknown. The concept “plant litter” is wide both regarding components and their chemical composition and foliar litter appears to be the litter fraction that has been most studied. Still, with foliar litter of different species having very different chemical composition (e.g., DELILA III data base; http://www.eko.uj.edu.pl/deco/) we may expect different decomposition patterns among species or at least among genera. Also, bark, twig, branch, root, and foliar
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