Response of humic acids and soil organic matter to vegetation replacement in subtropical high mountain forests

Reforestation can alter the chemical composition of soil organic matter (SOM) and humification; however, information on how specific plant types impact SOM lability and humification is not well documented. In this study, we used solid-state C-13 nuclear magnetic resonance spectroscopy, photometric analysis, and chemical fractionation to examine carbon (C) components and lability of SOM in a Japanese cedar (Cryptomeria japonica) forest and bamboo (Phyllostachys edulis) plantation that reforested a cutover primary broadleaf forest. The increment logK value of soil humic acids, the inverse index of SOM humification, was lowest in the bamboo plantation, suggesting a higher SOM humification stage in the bamboo plantation. The soil labile C/Total C ratio was highest in the bamboo plantation, and this can be attributed to low aromaticity and alkyl-C/O-alkyl-C ratio (A/O-A) in the bamboo litter. Intensive cultivation of the bamboo plantation accelerated litter breakdown in the strongly acidic soil, resulting in the depletion of SOM. Cedar coniferous leaves, with their high recalcitrant substances and slow decomposition, only slightly lowered SOM humification due to the substantial broadleaf understory. Our results suggest that the type of plants involved in reforestation and understory reestablishment is critical to how SOM humification and lability change during the reforestation through the control of litter C components. Further research into the interaction between microclimate change and forest type in forest conversion will be useful for increasing understanding on the impact of forest conversion on SOM lability and humification in subtropical high mountain forest ecosystems.