| Climate warming plays an important role in forest ecosystem carbon and nitrogen balance and soil biogeochemical cycling by affecting dissolved organic matter（DOM）.DOM leached by rainfall from litter plays an important role in the carbon and nitrogen balance of forest ecosystems and soil biogeochemical cycles.In this study,we used the zero-pressure method to collect DOM fluxes（including soluble organic carbon（DOM））in the litter layer of subtropical evergreen broad-leaved natural forests by using the zero-pressure method in the field in situ warming test plots（two treatments:control（ambient temperature）and warming（+4℃））at the National Field Scientific Observatory of Forest Ecosystems in Sanming,Fujian.The effects of warming on the DOM flux（including soluble organic carbon（DOC）flux and soluble organic nitrogen（DON）flux）and chemical composition of subtropical evergreen broad-leaved natural forests were investigated.The aromaticity,hydrophobic composition,molecular weight,degree of humification,composition and origin of DOM in the litter layer were investigated by combining UV-visible spectroscopy,fluorescence spectroscopy and excitation-emission matrix（EEM）parallel factor analysis（PARAFAC）.Meanwhile,we collected litter material from the forest surface for indoor analysis and measured litter chemical mass,extracellular enzyme activity and microbial biomass carbon（MBC）to investigate the effects of warming on litter chemical mass,MBC and extracellular enzyme activity and to elucidate the mechanism of warming on the DOM of the litter layer.This study can provide basic data for analyzing the contribution of DOM to soil carbon pools in the surface litter layer of subtropical forests,and provide a theoretical basis and data support for studying the response of soil carbon pools to warming.Through one year（a total of29 rainfall-drenched DOM samples were received）of dynamic monitoring,it was found that.（1）Warming had no significant effect on the concentration and flux of DOM in the litter layer of subtropical broadleaf evergreen natural forest,and DOC and DON did not respond to warming in the same way.The average annual concentration of DOC in the litter layer of subtropical broadleaf evergreen natural forest was 19.40±4.67 mg·L-1,which was reduced by 25%by warming;the DOC flux was 132.36±3.60 kg·ha-1 yr-1,which was reduced by 12.68%by warming;the average annual concentrations of DON,DON flux The average annual DON concentration and DON flux were 2.72±0.51 mg·L-1and 17.06±0.36 kg·ha-1·yr-1,respectively,and both did not change significantly after warming.In addition,warming did not change the seasonal characteristics of DOM concentrations and fluxes,and the seasonal responses of DON and DOC to warming were similar.The concentrations and fluxes of DOC and DON in the litter layer were significantly higher in March than in other months,and warming significantly reduced DOC concentration and DOC flux by 61.78%and 172.20%in March,with no significant changes in the rest of the months.（2）Warming had no significant effect on the UV and fluorescence spectral indices of DOM in the litter layer.The non-response of aromatization index（SUV254）,hydrophobic fraction（SUV260）and molecular weight size（SUV280）to warming indicated that warming did not change the complexity of DOM chemical structure.The higher fluorescence index（FI,values 1.88-2.33）and the lower authigenic index（BIX,values 0.39-0.66）together indicate that litter DOM is predominantly of microbial origin,while the lower humification index（HIX,values 0.78-0.98）and the ratio of the higher fluorescence peak A（associated with fulvic acid components）to peak C（belonging to humic acid-like substances）（IA/IC,values of 1.05-1.79）indicate a low degree of humification of the litter layer DOM.The dynamic changes of fluorescence indicators indicated that the litter layer DOM had strong microbiological properties and high microbial availability.（3）Warming significantly affected the carbohydrate concentration of the litter layer DOM,and the concentration of hexose,which characterizes microbial sources,and pentose,which characterizes plant sources,did not respond to warming.The concentration of hexose in March and April decreased by 190.25%and 357.62%,respectively,but the concentration of pentose increased significantly by 67.90%and1143.17%,respectively,after warming;except for September when hexose/pentose was less than 0.5,all other months were greater than 2,indicating that carbohydrates in the DOM of the litter layer underwent more complex microbial transformation.（4）The main components of the litter layer DOM are humic acid-like and aromatic-like proteins such as tryptophan.component 1（C1）and component 2（C2）identified by PARAFAC model belong to low molecular weight UVA-like humic substances and high molecular weight UVC-like humic substances,respectively,and component 3（C3）belongs to aromatic-like proteins such as tryptophan.After warming,the proportion of C2 decreased,the proportion of C3 increased,and the proportion of C1did not change significantly,indicating that microbial degradation increased after warming,and humic substances were gradually transformed by microbial decomposition（consistent with hexose/pentose,HIX results,FI,and BIX results）.（5）The indoor analysis revealed that the DOC concentration of litter leaching in the undecomposed layer（L layer）and semi-decomposed layer（F layer）increased by 6.04%and 8.38%,respectively,after warming,but did not reach a significant difference level.The total nitrogen（N）content of litter litter in the L layer increased significantly by248.90%after warming,and had no significant effect on the total carbon（C）and phosphorus（P）contents.The MBC content of L-layer apoplast increased by 33.28%after warming,which did not reach a significant level,and the activities of four hydrolases,β-glucosidase（βG）,cellulose hydrolase（CBH）,β-N-acetylaminoglucosidase（NAG）,and acid phosphatase（AP）,did not change significantly after warming.Unlike the response of the L layer to warming,the P content of the F layer was significantly reduced by 44.13%after warming,but there was no significant effect on the C and N contents.The MBC content of F layer increased significantly by 220.25%after warming,but the enzyme activity of CBH in F layer decreased significantly by 230.15%,whileβG,NAG,AP,and NAG enzyme activities did not change significantly.In summary,the results of this study showed that warming did not significantly affect the DOM concentration,flux and chemical composition of the litter layer of subtropical broadleaf evergreen forest,indicating that warming did not cause significant changes in the DOM flux and chemical composition characteristics of the litter layer of broadleaf evergreen forest,which may be related to the fact that the DOC concentration and chemical quality characteristics as well as microbial activity in the litter layer were not significantly correlated after warming.The lack of response of DOM flux and chemical composition in the litter layer of subtropical broadleaf evergreen forests to warming suggests that future climate warming will not significantly affect the DOM in the litter layer of this type of ecosystem.This is a reference value for further revealing the response of forest litter carbon pools to climate change and its mechanisms. |
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