Leaf Litter-derived Dissolved Organic Matter Characteristics In Typical Subtropical Plantations Of Jiangxi Province

Dissolved organic matter(DOM),as a relatively active fraction of organic C in forest soils,provides the main substrates and nutrients for microbial growth and metabolism,and thus plays an important role in forest carbon and nutrient cycles,the transport and transformation of pollutants,and the maintenance of regional water quality.Although litter-derived DOM has been regarded as the main source of soil DOM in forests,the effects of litter type and composition on DOM production,chemical structure,and biodegradability and the regulatory mechanisms are still unclear.Therefore,we selected the leaf litter of four common afforestation tree species(coniferous trees:Pinus massoniana and Pinus elliottii;broadleaf trees:Liquidambar formosana and Schima superba)from subtropical plantations in the vegetation restoration experiment station in the degraded red soil region in the Taihe County,Jiangxi Province.Subsequently,we set up the treatments of single litter,litter richness,and litter mixing ratio,and used the laboratory simulated leaching experiment coupled with UV-vis spectrum analysis technique to explore the effects of tree species,species richness,and mixing ratio on DOM production and chemical properties.Then,we determined the biodegradability of litter-derived DOM using a 56-day laboratory incubation experiment.These results will help to reveal the hydrological-driven carbon and nutrient cycles in subtropical plantations,and provide scientific basis for improving the function of soil and water conservation of subtropical plantations from the viewpoint of species arrangement.The results were as follows:(1)Litter mass loss and DOM characteristics significantly varied with litter type during leaching.Compared with coniferous trees(P.massoniana and P.elliottii),broadleaf trees(L.formosana and S.superba)had higher litter mass loss and greater production of dissolved organic carbon(DOC),dissolved total nitrogen(DTN),and dissolved total phosphorus(DTP)during leaching.The optical spectral analysis showed that broadleaf trees had higher SUVA₂₅₄,SUVA₂₈₀,SUVA₃₅₀and SUVA₃₇₀ values,but lower S_R value in the litter leachates than coniferous trees,indicating that the aromaticity and molecular weight of leaf litter-derived DOM were higher for broadleaf trees than for coniferous trees.By the end of 56-day incubation,leaf litter-derived DOM biodegradability was higher for broadleaf trees than for coniferous trees.The negative correlation between DOM biodegradability and DOC:DTP ratio or DTN:DTP ratio showed that nutrient availability was the main factor driving the interspecific variations of DOM biodegradability.(2)The effect of species richness on litter mass loss and DOM characteristics was significantly modulated by litter type and associated shifts in substrate quality during leaching.In general,litter mass loss and litter-derived DOC,DTN,and DTP productions increased with elevating polyphenol content and nutrient concentrations in leaf litter mixtures.The optical spectral analysis exhibited that,among the litter mixtures,the treatment L.formosana+S.superba+P.massoniana+P.elliottii had the greatest SUVA₂₅₄,SUVA₂₈₀,SUVA₃₅₀,and SUVA₃₇₀ values(e.g.,the highest aromaticity of DOM),while the treatment P.massoniana+P.elliottii had the lowest aromatic content of DOM.After 56 days of incubation,litter derived-DOM biodegradability was observed to be greatest in the treatments L.formosana+S.superba+P.massoniana and L.formosana+S.superba+P.massoniana+P.elliottii,but lowest in the treatments S.superba+P.elliottii,S.superba+P.massoniana+P.elliottii,and P.massoniana+P.elliottii.In total,the biodegradability of DOM derived from litter mixtures with contrasting species richness was largely dependent on the intrinsic nature of mixed litter and associated DOM chemical composition,especially phosphorus availability in the leaf litter leachates.(3)Litter mixing proportion and associated shifts in litter chemical properties significantly affected litter mass loss and DOM properties during leaching.Generally,litter mass loss and the amounts of DOC,DTN,and DTP increased with elevating proportion of broadleaf tree leaf litter.The optical spectral properties showed that litter mixed ratio had no effect on the aromatic degree of litter-derived DOM in the litter mixtures containing L.formosana leaf litter.However,with increasing mixing proportions of S.superba leaf litter in the litter mixtures,SUVA₂₅₄,SUVA₂₈₀,SUVA₃₅₀,and SUVA₃₇₀ values increased,but S_Rvalues decreased,indicating increased DOM aromaticity and molecular weight in the litter leachates.Compared with coniferous trees,coniferous and broadleaf tree leaf litter mixtures had greater DOM biodegradability.However,the effect of litter mixing ratios on DOM biodegradability was largely dependent on the component species in the litter mixtures.Litter-derived DOM biodegradability declined with increased proportion of broadleaf tree litter in the litter mixtures containing P.massoniana litter,but increased in the litter mixtures containing P.elliottii litter.In summary,tree leaf litter type,richness,and mixing ratio exerted significant influences on litter-derived DOM production,chemical composition,and biodegradability in the subtropical plantations of southern China.Moreover,litter richness-and mixing proportion-induced alterations of substrate quality was the predominant factor driving the changes in DOM production and biodegradability.In addition,mixed broadleaf and coniferous tree litter significantly enhanced litter-derived DOM production and biodegradability.These observations will not only help to understand and assess ecosystem services and functions,but also provide scientific guidelines for optimizing stand structure and selecting afforestation tree species in subtropical plantations.