Characteristics Of Dissolved Organic Carbon In A Subtropical Forest Landscape And Its Sequestration Mechanisms

Dissolved organic carbon(DOC)is the most dynamic component in the carbon pool of terrestrial forest landscapes.The continuous flow of DOC within and outside forest ecosystems is a key pathway for carbon movement,and transport of biogenic elements,heavy metals and organic pollutants.Thus,DOC is an essential part of carbon budget of forest ecosystems.Most studies have examined the composition,dynamic and controlling factors for dissolved organic matter(DOM)at forest,but little is known about the transport and transform of DOC when it passes through soil,it is not beneficial to the study of forest carbon dynamic.Therefore,we measured the concentration and quality of DOM in field to ascertain the vertical distribution and controlling factors of DOC.We selected three headwater catchments at Badagongshan National Nature Reserve,collected surficial water(including precipitation(PP),throughfall(TF),stemflow(SF),litter leachate(LL),runoff(RO))and soil pore water at depth 10 cm,30 cm,and 60 cm(PW10,PW30,and PW60)in the growing season.Then we measured DOM concentration and chemistry composition.The results indicated that the mean concentration of DOC in PP was 3.39±0.90 mg L-1.While the concentration of TF,SF,LL and RO increased to 12.51±3.21 mg L-1,21.11±6.60 mg L-1,21.29±8.13 mg L-1 and 15.26 ± 4.73 mg L-1,respectively.The average DOC concentrations in soil pore water at depth 10 cm,30 cm,and 60 cm were 4.75 ± 2.14 mg L-1,3.73 ± 0.52 mg L-1 and 3.30±1.36 mg L-1,respectively,which were lower than surficial water(?=30.643,P<0.01).Hydrophobic organic acid(HPOA)concentration in the surficial water was significant higher than that in soil pore water(F=8.117,P=0.001).But transphilic organic acid(TPIA)and low molecular weight hydrophilic acids(HPI)in the surficial water were lower than in the soil pore water at 10 cm and 30 cm depth(TPIA:F=1.193,P=0.365;HPI:F=3.316,P=0.03).These results indicated that DOC concentrations and composition showed a pronounced trend across different sources.These results indicated that the properties of DOC in surficial water were mainly controlled by vegetation and site conditions while DOC in soil pore water was mainly controlled by soil properties.To verify the transport and transform of DOC in soul,we added 13C-labelled litter-derived DOC into different depth soil columns in a 180 d incubation experiment and quantify the fate of 13C-labelled litter-derived DOC in soil(including retention,mineralization and leaching)and the changes of litter-derived DOC composition when they percolated through soils.And the responses of different depths of SOC mineralization and soil microbial community structure to the addition of DOC.Results showed that about 76%of litter-derived DOC was retained in soil(60%in topsoil and 16%in midsoil).There were respectively 18%,4%,3%of litter-derived DOC mineralized into CO2 in topsoil,midsoil and subsoil.Finally,only 0.04%of litter-derived DOC leached from soil(0-60 cm).These results indicated that soil retention of DOC is likely the dominant mechanism,with minor microbial degradation and minimal leaching.Parallel factor analysis(PARAFAC)indicated that the litter-derived DOC,water soluable organic carbon(WSOC)and the leached dissolved organic carbon(LDOC)could be fractioned into humic-like and protein-like materials.The component composition of WSOC and LDOC were mostly comparable between the control and treatment.This indicated that the quality of SDOC and LDOC is more similar to soil carbon rather than the added DOC,which supported the dynamic exchange model.DOC addition stimulated surface and deep SOC mineralization in the early incubation and this increase was greater in the deep soils,suggesting that a lack of fresh C supply may prevent the mineralization of deep SOC.DOC addition also enhanced the biomass of gram-negative bacteria,gram-positive bacteria,fungi and actinomycetes,and the maximum abundance occurred on day 30.PCA analysis indicated that DOC addition only diminished the magenitude of the changes in microbial community structure,but not the direction of the changes.Moreover,17:0cy and 15:0 PLFAs had greater amount of 13C incorporation than other PLFAs.These results indicated that microbes are limited by energy and soil microbial community composition can be modified by C inputs.In this study,we measured the concentration and composition of DOC in a humid subtropical forest.These results not only demonstrated the characteristics and dynamic of DOC in the subtropical forest ecosystem,but also provided a theoretical basis for formulating sound water quality management strategies for forest watershed.Through laboratory soil column culture experiment,we illustrated that fate and dynamic process of litter-derived DOC in soil.Our findings support the hypothesis of the dynamic exchange model of DOC in soil profile movement.In future studies,the fluxes of DOC in subtropical forest ecosystem should be further monitored to accurately assess its contribution to the carbon cycle.