Amount and biodegradation of dissolved organic matter leached from tree branches and roots in subtropical plantations of China

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Dissolved organic matter (DOM) plays a crucial role in biogeochemical cycles and ecosystem functions in forests. However, our understanding of wood litter-derived DOM amounts and biodegradation in forest ecosystems remains limited. In this study, we collected branches and roots with varying diameters (2.0–5.0 mm and 5.0–12.0 mm in diameter) from four common tree species (Liquidambar formosana, Schima superba, Pinus massoniana, and Pinus elliottii) in subtropical plantations of southern China. We measured the amounts of dissolved organic carbon (DOC), dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP), and DOM aromaticity in the litter leachates from a short-term leaching experiment. Here, the specific ultraviolet absorbances at 254 nm and 350 nm were used to indicate DOM aromaticity. We subsequently performed a 28-day incubation experiment to investigate wood litter-derived DOM biodegradation. Our results showed that, irrespective of tree species, branches often leached lower amounts of DOC with lower aromatic content than roots at each diameter class. For both branches and roots, DOC quantity generally increased with decreasing diameter class, but aromatic content showed an opposite trend. Moreover, DOM biodegradation increased with increasing diameter class, and branches had greater DOM biodegradation than did roots. In addition, DOM biodegradation exhibited no significant relationship with DOC:DTN and DOC:DTP ratios, but correlated negatively with DOM aromaticity. These observations highlight the crucial roles of organ type and diameter in regulating wood litter-derived DOM amount and biodegradation, and suggest that DOM chemistry rather than stoichiometry drives the variation of wood-derived DOM biodegradation in subtropical plantations of China.