| Dissolved organic carbon(DOC)and total dissolved nitrogen(TDN)contain the important and highly active chemical components for the biogeochemical cycles of C and N in terrestrial ecosystems.Rainfall is the main driving force in the process of forest hydrology and the pathway of water and nutrients input,partitioned into throughfall and stemflow after interception by the forest canopy,and its nutrient content changes.The lateral output of nutrients through surface runoff is important in connecting terrestrial and aquatic ecosystems and has important contribution to the assessment of nutrient migration in the hydrological process of forest ecosystem.But the hydrological fluxes of DOC and TDN have not been fully investigated in different forest types in subtropical secondary forest.Three forests in subtropical secondary forest were selected:coniferous forest(Pinus massoniana-Lithocarpus glaber),deciduous broadleaved forest(Choerospondias axillaris),and evergreen broadleaved forest(L.glaber-Cyclobalanopsis glauca).Rainfall,throughfall,stemflow,and surface runoff amount were monitored by using the rain gauges and field data acquisition device for two years.Water samples were collected to measure and analyze concentrations of DOC,TDN,NH4+-N and NO3--N.Input and output fluxes as well as leaching and migration coefficients of DOC and TDN were calculated to compare the nutrient cycling differences in the three secondary forests.The main findings are as follows:(1)Concentrations of DOC and TDN were enriched in throughfall and stemflow,and DOC concentrations in autumn and TDN concentrations in autumn and winter were significantly higher(p<0.05).In throughfall,DOC and TDN concentrations were not significantly among the different forest types.In stemflow,DOC concentration in the evergreen broadleaved forest was markedly lower(p<0.001)than that in another two forests.Average annual DOC concentrations among the three forests were 21.19±10.94 mg L-1,20.92± 16.04 mg L-1,and 8.29±4.66 mg L-1,respectively.TDN concentrations in stemflow decreased significantly among the three stands,and average annual TDN concentrations were 6.04 ± 4.06 mg L-1,3.79± 1.88 mg L-1,and 3.03 ± 1.73 mg L-1,respectively.(2)The amount of DOC leached was primarily influenced by rainfall and was significantly related to fluxes in throughfall,with the highest value in the deciduous broadleaved forest(57.78 kg C ha-1).However,the amount of TDN leached was affected by rainfall and the absorption of vegetation and was not significantly related to fluxes in throughfall,except for the deciduous broadleaved forest.The highest value of TDN leached was in the coniferous forest(11.90 kg N ha-1).(3)Concentrations of DOC and TDN in autumn in surface runoff were significantly different(p?0.05),but there was no significant difference among the three forest types.Both average annual concentrations and annual output fluxes of DOC in surface runoff were the highest in the deciduous broadleaved forest,which performing open nutrient cycle.However,average annual TDN concentrations in surface runoff tended to increase but annual TDN output fluxes decreased among the different forest types,which indicating close nutrient cycle.Concentrations of DOC and TDN in surface runoff were closely related to surface runoff,decomposition rate of litterfall,and soil organic matter content.The annual output fluxes of DOC and TDN in surface runoff were less than 30%of the fluxes in throughfall,indicating nutrient accumulation in subtropical secondary forests. |
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