Plant Litter-derived Dissolved Organic Carbon Characteristics And Biodegradability In Subtropical Forests

In forest ecosystems,plant litter decomposition is key to soil organic matter formation and nutrient cycling.Although litter decomposition has received considerable attention,litter-leached dissolved organic carbon（DOC）properties and biodegradability during decomposition are still unclear.In subtropical regions,leaching is a crucial component of plant litter decomposition.Here,leaf litter of common broadleaf trees,coniferous trees and fern species in subtropical forests was selected in this study.First,we used a laboratory leaching experiment to investigate leaf litter-leached dissolved organic carbon（DOC）,dissolved total nitrogen（DTN）,and dissolved total phosphorus（DTP）productions and the specific ultraviolet absorbance at 254 nm(SUVA₂₅₄).Second,we used a standard laboratory incubation experiment to examine the effects of mixing deciduous and evergreen tree leaf litter-leached DOC on biodegradation.Third,using a standard laboratory incubation experiment,we assess the interspecific pattern of litter-derived DOC biodegradation between overstory trees and understory ferns,and then examine the effect of mixing overstory tree and understory fern leaf litter-leached DOC on biodegradation.These observations will provide the theoretical basis for accurate recognition and assessment of hydrologically driven litter decomposition and biogeochemical cycles,and also provides a scientific basis for the design of appropriate management practices in subtropical forests.The main results are as follows:（1）Compared with coniferous tree species,ferns and broadleaf tree species have higherspecific leaf area,water holding capacity,and nutrient concentration of leaf litter,but had lower leaf litter tissue density.Therefore,ferns and broadleaf tree species produced higher amounts of DOC,DTN and DTP during leaching than coniferous tree species.In addition,coniferous tree species had lower SUVA₂₅₄ values in the leaf litter leachates than ferns and broadleaf tree species,indicating the relatively lower aromatic C compounds in the DOC.（2）Among the tree species,leaf litter-leached DOC biodegradation was higher for deciduous trees than for evergreen trees.Moreover,litter-leached DOC biodegradation was negatively correlated with the initial DOC/DTN ratio,although DOC exhibited no significant relationship with either DOC/DTP ratio or SUVA₂₅₄.Non-additive effects on biodegradation were common when deciduous and evergreen tree leaf litter-leached DOC was mixed.The relative mixing effects on DOC biodegradation positively correlated with the interspecific differences in DOC/DTN ratio and SUVA₂₅₄,but exhibited no significant relationship with the interspecific difference in DOC/DTP ratio.Non-additive effects on DOC biodegradation shifted from antagonistic effects to additive effects or synergistic effects when the interspecific differences in DOC/DTN ratio and SUVA₂₅₄ increased.（3）Both broadleaf trees and ferns had higher leaf litter-leached DOC biodegradation thanconiferous trees,albeit there was no significant difference between broadleaf trees and ferns.Non-additive effects,especially synergistic effects,were dominant during DOC biodegradation when tree and fern leaf litter-leached DOC was mixed.Moreover,the relative mixing effects on DOC biodegradation showed a positive relationship with the interspecific differences in DOC/DTP ratio and SUVA₂₅₄,although there was no significant relationship between the relative effects and the interspecific difference in DOC/DTN ratio.In addition,the magnitude of the relative mixing effects on DOC biodegradation showed an increasing trend with the interspecies differences in DOC/DTP ratio and SUVA₂₅₄.In summary,leaf litter-leached DOC quantity and characteristics was mainly dependent on the initial litter physical and chemical properties.Among the species,both broadleaf trees and ferns had greater leaf litter-leached DOC biodegradation than coniferous trees,despite no significant difference between broadleaf trees and ferns.During DOC biodegradation,the DOC/DTN ratio was the main factor determining leaf litter-leached DOC biodegradation,and the interspecies differences in DOC/DTN/DTP stoichiometric ratios and DOC chemical composition controlling the relative mixing effects on DOC biodegradation.These findings indicate that ferns play a key role in the C cycle of subtropical plantations,and highlight that mixed forests will increase DOC biodegradation and thus soil organic C formation and accumulation through soil microbial C pump in subtropical plantations.Therefore,our observations suggest that understory ferns should be retained in the subtropical plantations and mixed forests should be preferentially considered during afforestation in subtropical regions.