Abstract：Recent evidence suggests that recalcitrant compounds are not selectively preserved in soils and that labile components are the precursors of humic substances, but the relationship between the slow accumulation of humic substances and the rapid release of labile components during litter decomposition is not well understood. In an in situ litterbag experiment, we quantified the litter mass loss, release of acidsoluble labile components, and accumulation of alkali-soluble humic substances during four years of decomposition of fir (Abies faxoniana) needle and twig litters in an alpine coniferous forest. We found that fresh litter had high contents of humic substances (121 and 151 mg/g for needle and twig litters, respectively), with needle litter showing increases in humic substances contents at the early decomposition stage and decrease later, whereas twig litter showed the reverse pattern. However, the net accumulation of humic substances in both needle and twig litters decreased during decomposition, and the decrease was related to the rapid release of labile components. We conclude that the rapid release of labile components limits the accumulation of humic substances in plant litter during decomposition in this alpine coniferous forest.
Key words: humic substances; labile components; litter decomposition; soil organic matter.