Effects Of Dry-wet Cycles On Litter-derived Dissolved Organic Carbon Production And Spectral Characteristics In Common Tree Species Subtropical Plantations

In forest ecosystems,due to periodic rainfall and drought events,plant litter often undergoes repeated drying-wetting processes.Due to global warming and associated changes in precipitation patterns,the extreme weather events frequency is expected to increase.Accordingly,plant litter will experience more frequent drying-wetting cycles,which will affect forest ecosystem structure and function.Although the pulse effect of atmospheric precipitation on plant litter decomposition has received widespread attention,it is still unclear how the drying-wetting frequency events,especially the increase in drying-wetting frequency and the extension of drought time,will affect the release pattern of dissolved organic carbon（DOC）during plant litter decomposition.Thus,this paper collects leaf litter of common tree species from subtropical plantations,and used laboratory drying-wetting simulation experiments to assess the effects of drying-wetting cycles,drying-wetting frequency,and drought period extension during drying-wetting cycles on plant litter-derived DOC production and spectral characteristics.These results will provide a theoretical basis for understanding and predicting plant litter decomposition process and its ecological consequences in forest ecosystems.The main results are as follows:（1）For all tree species,litter mass loss under drying-wetting cycle treatment was comparable to that under wet treatment,but was significantly higher than under drought treatment.For deciduous and evergreen broad-leaved trees,litter-derived cumulative DOC production was higher in drying-wetting cycle treatment than in the drought treatment,but was lower than in the wet treatment.For evergreen coniferous trees,litter-derived cumulative DOC production in the drying-wetting cycle treatment was lower than that in both drought and wet treatments.During the entire culture process,the SUVA₂₅₄,SUVA₂₈₀,and SUVA₃₅₀values of litter-derived DOC in the wet and drying-wetting cycle treatments were higher than in the drought treatment,showing the higher aromatic degree of DOC.These results indicate that the drying-wetting cycles is an important factor regulating the production and quality of litter-derived DOC in subtropical plantations.（2）With the increase of drying-wetting frequency,litter-derived DOC production significantly decreased,while litter-derived particulate organic carbon（POC）production and POC/DOC ratio showed an increasing trend.Moreover,deciduous broad-leaved tree litter-derived POC and DOC productions are higher than those of evergreen broad-leaved trees.With the increase of drying-wetting frequency,SUVA₂₅₄ and SUVA₂₈₀ values of litter-derived DOC show an initial increase and a subsequent decline,whereas S_275-295 and S_350-400values showed an opposite changing trend.Compared with evergreen broad-leaved trees,deciduous broad-leaved trees had higher SUVA₂₅₄,SUVA₂₈₀,S_275-295,and S_350-400 values,meaning the higher aromatic degree of litter-derived DOC.In addition,litter mass loss,cumulative POC production,and cumulative DOC production significantly positively correlated with the litter specific leaf area and saturated water capacity.These results imply that increasing frequency of drying-wetting cycles will change the amount and characteristics of litter-derived DOC,but the changing trends vary with tree species.（3）Extended drought periods significantly changed litter-derived DOC production.With increasing drought periods,leaf litter-derived DOC production exhibited an initial increase and subsequently decreased.In all treatments,litter mass loss and cumulative DOC production were higher for deciduous broad-leaved trees than for evergreen broad-leaved trees and evergreen coniferous trees.When drought periods increased,SUVA₂₅₄,SUVA₂₆₀,and SUVA₂₈₀ values of litter-derived DOC showed an increasing trend,while S_275-295,S_350-400,and S_Rvalues showed a decreasing trend.In addition,there were significant differences in litter mass loss among growth form during the relatively shorter drought periods,but this interspecific pattern disappeared when the drought period extended to 120 days.In summary,plant litter-derived DOC production and characteristics were affected by the drying-wetting cycles,drying-wetting frequency,and prolonged drought periods in subtropical plantations,and the changing trends changed with tree species.These findings are essential for accurately evaluating and predicting hydrological-driven plant litter decomposition processes and their ecological consequences in subtropical plantations.