A review of physicochemical properties of dissolved organic carbon and its impact over mountain glaciers

Investigating the characteristics and transformation of water-soluble carbonaceous matter in the cryosphere regions is important for understanding biogeochemical process in the earth system.Water-soluble carbonaceous matter is a heterogeneous mixture of organic compounds that is soluble in aquatic environments.Despite its importance,we still lack systematic understanding for dissolved organic carbon(DOC)in several aspects including exact chemical composition and physical interactions with microorganisms,glacier meltwater.This review presents the chemical composition and physical properties of glacier DOC deposited through anthropogenic emission,terrestrial,and biogenic sources.We present the molecular composition of DOC and its effect over snow albedo and associated radiative forcings.Results indicate that DOC in snow/ice is made up of aromatic protein-like species,fulvic acid-like materials,and humic acid-like materials.Light-absorbing impurities in surface snow and glacier ice cause considerable albedo reduction and the associated radiative forcing is definitely positive.Water-soluble carbonaceous matter dominated the carbon transport in the high-altitude glacial area.Owing to prevailing global warming and projected increase in carbon emission,the glacial DOC is expected to release,which will have strong underlying impacts on cryosphere ecosystem.The results of this work have profound implications for better understanding the carbon cycle in high altitude cryosphere regions.A new compilation of globally distributed work is required,including large-scale measurements of glacial DOC over high-altitude cryosphere regions,to overcome and address the scientific challenges to constrain climate impacts of light-absorbing impurities related processes in Earth system and climate models.

Dissolved organic carbon fractionation in wet deposition and its potential impact on radiative forcing in the central Tibetan Plateau

As an important component of carbonaceous matters,dissolved organic carbon(DOC)can absorb and scatter the solar radiation at ultraviolet and blue wavelengths.The wet deposition process has great impact on the con-centration and light absorption ability of precipitation DOC,affecting the climatic effect caused by DOC in the atmosphere.In this study,light absorption and fluorescence characteristics of precipitation DOC was investigated in the central Tibetan Plateau(TP).The results showed that the mean DOC concentration and mass absorption cross-section measured at 365 nm(MAC_(365)) in Tanggula(TGL)station were 0.59±0.42 mg/L and 0.37±0.19 m^(2)/g,respectively,while both values showed much higher volatilities than those of aerosols.DOC concentrations had significant negative correlation with the precipitation amount,while MAC_(365) values increase with the precipitation amount in TGL station.Therefore,DOC with high light-absorbing ability was preferred to be retained in the atmosphere during wet deposition.In this study,precipitation DOC contained three fluorescent components(one humic-like component and two tyrosine-like components)mainly from local biomass burning sources.DOC concentration showed a negative relationship with MAC_(365) value in TGL station.The wet deposition of DOC with low light-absorbing ability can reduce the strong negative radiative forcing caused by secondary organic aerosol due to high proportion of DOC in secondary organic carbon.Similar phenomenon was also found in Nam Co,Lulang and Everest stations of previous study,which may have a potential impact on radiative forcing in the atmosphere of TP.

Dissolved organic carbon and nitrogen in precipitation, throughfall and stemflow from Schima superba and Cunninghamia lanceolata plantations in subtropical China

Despite growing attention to the role of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in forest nutrient cycling, their monthly concentration dynamics in forest ecosystems, especially in subtropical forests only were little known. The goal of this study is to measure the concentrations and monthly dynamics of DOC and DON in precipitation, throughfall and stemflow for two planta- tions of Schima superba (SS) and Chinese fir (Cunninghamia lanceolata, CF) in Jianou, Fujian, China. Samples of precipitation, throughfall and stemflow were collected on a rain event base from January 2002 to December 2002. Upon collection, all water samples were analyzed for DOC, NO3 -N, NH4 -N and total dissolved N (TDN). DON was calculated by subtracting NO3 -N and NH4 -N from TDN. The results - + - + showed that the precipitation had a mean DOC concentration of 1.7 mg·L-1 and DON concentration of 0.13 mg·L-1. The mean DOC and DON concentrations in throughfall were 11.2 and 0.24 mg·L-1 in the SS and 10.3 and 0.19 mg·L-1 in the CF respectively. Stemflow DOC and DON concentrations in the CF (19.1 and 0.66 mg·L-1 respectively) were significantly higher than those in the SS (17.6 and 0.48 mg·L-1 respectively). No clear monthly variation in precipitation DOC concentration was found in our study, while DON concentration in precipita- tion tended to be higher in summer or autumn. The monthly variations of DON concentrations were very similar in throughfall and stemflow at both forests, showing an increase at the beginning of the rainy season in March. In contrast, monthly changes of the DOC concentrations in throughfall of the SS and CF were different to those in stemflow. Throughfall DOC concentrations were higher from February to April, while relatively higher DOC concentrations in stemflow were found during September-November period.

Distribution characteristics of dissolved organic carbon in annular wetland soil-water solutions through soil profiles in the Sanjiang Plain,Northeast China

Overwhelming evidence reveals that concentrations of dissolved organic carbon （DOC） have increased in streams which brings negative environmental impacts. DOC in stream flow is mainly originated from soil-water solutions of watershed. Wetlands prove to be the most sensitive areas as an important DOC reserve between terrestrial and fluvial biogeosystems. This reported study was focused on the distribution characteristics and the controlling factors of DOC in soil-water solutions of annular wetland, i.e., a dishing wetland and a forest wetland together, in the Sanjiang Plain, Northeast China. The results indicate that DOC concentrations in soilwater solutions decreased and then increased with increasing soil depth in the annular wetland. In the upper soil layers of 0-10 cm and 10-20 cm, DOC concentrations in soil-water solutions linearly increased from edge to center of the annular wetland （R^2 = 0.3122 and R^2 = 0.443）. The distribution variations were intimately linked to DOC production and utilization and DOC transport processes in annular wetland soil-water solutions. The concentrations of total organic carbon （TOC）, total carbon （TC） and Fe（II）, DOC mobility and continuous vertical and lateral flow affectext the distribution variations of DOC in soil-water solutions. The correlation coefficients between DOC concentrations and TOC, TC and Fe（II） were 0.974, 0.813 and 0.753 respectively. These distribution characteristics suggested a systematic response of the distribution variations of DOC in annular wetland soil-water solutions to the geometry of closed depressions on a scale of small catchments. However, the DOC in soil pore water of the annular wetland may be the potential source of DOC to stream flow on watershed scale. These observations also implied the fragmentation of wetland landscape could bring the spatial-temporal variations of DOC distribution and exports, which would bring negative environmental impacts in watersheds of the Sanjiang Plain.

Concentrations and seasonal dynamics of dissolved organic carbon in forest floors of two plantations （Castanopsis kawakamii and Cunninghamia lanceolata） in subtropical China

The concentrations and seasonal dynamics of DOC in forest floors of monoculture plantations of Castanopsis kawakamii and Chinese fir （Cunninghamia lanceolata） were assessed in Sanming, Fujian, China （26°11′30″N, 117°26′00″E）. Forest floor samples were taken in January, April, July and October in 2002 and divided into undecomposed material （horizon Oi）, partially decomposed organic material （horizon Oe）, and fully decomposed organic material （horizon Oa）. Upon collection. DOC concentrations of samples were analyzed by a High Temperature TOC. The results showed that the annual average DOC concentration of Chinese fir （1341.7 mg·kg^-1） in the forest floor was higher than that of Castanopsis kawakamii （ 1178.9 mg·kg^-1）. Difference in DOC concentrations was observed among three horizons of the forest floor. DOC concentration of forest floor in the two forests was the highest in horizon Oe. Seasonal trends of DOC concentrations in different horizons of forest floors were similar and the maximal value occurred in autumn （or winter）. The concentration and temporal change of DOC in studied forests were probably related to the variation in moisture, temperature, biological activity and quantity of organic matter in the forest floor.

The satellite reversion of dissolved organic carbon (DOC) based on the analysis of the mixing behavior of DOC and colored dissolved organic matter: the East China Sea as an example

The retrieval of dissolved organic carbon （DOC） distribution by remote sensing is mainly based on the em- pirical relationship of DOC concentration and colored dissolved organic matter （CDOM） concentration in many literatures. To investigate the nature of this relationship, the distributions and mixing behaviors of DOC and CDOM are reviewed in the world＇s major estuaries and bays. It is found that, generally, the C- DOM concentration is well correlated with the salinity in most estuaries, while DOC usually shows a non- conservative behavior which leads to a weak correlation between the DOC concentration and the CDOM concentration. To establish a good satellite reversion of the DOC concentration, the East China Sea（ECS） was taken as an example, and the mixing behavior of DOC and CDOM as well as the influence of biogeo- chemical processes were analyzed except for the physical mixing process with the data from late autumn （November, 2010） and winter （December, 2009） cruises. In the two ECS cruises, the CDOM concentration was found to be tightly correlated with the salinity, influenced little by the photochemical or biological pro- cesses. The data from the winter cruise show that DOC followed a conservative mixing along the salinity gradient, while in the late autumn cruise it was significantly affected by the biological activities, resulting in a poor correlation between the DOC and the CDOM. Accordingly, an improved DOC algorithm （CSDM） was proposed： when the biological influence was significant （Chl a greater than 0.8 μg/dm3）, DOC was retrieved by the conservative and biological model, and if the conservative mixing was dominant （Chl a less than 0.8 μg/dm3）, the direct DOC concentration and CDOM concentration relationship was used. Based on the pro- posed algorithm, a reasonable DOC distribution for the ECS from satellite was obtained in this study, and the proposed method can be applied to the other large river-dominant marginal sea.

Effects of Fertilization Types and Cultivation Years on Dissolved Organic Carbon(DOC) in Paddy Soil under Water-logging Condition

In this study, the spatial and temporal distribution characteristics of dis- solved organic carbon （DOC） in water-logged paddy soil in Yanbian were investigat- ed under conditions of different fertilization types （single application of chemical fer- tilizer, mixed application and chemical and organic fertilizers and single application of organic fertilizer） and cultivation years （80 years and 120 years）. The results showed that the spatial and temporal distribution of DOC in water-logged paddy soil changed significantly with time going by. The single application of chemical fertilizer or mixed application of chemical and organic fertilizers contributed to the release of DOC in top paddy soil under water-logging condition; the single application of organ- ic fertilizer promoted the accumulation of DOC in bottom paddy soil, resulting in great heterogeneity of DOC in the vertical space, but the single application of chem- ical fertilizer weakened the vertical spatial heterogeneity of soil DOC; the DOC con- tent in the 80-year-old water-logged paddy soil was higher and more stable than that in the 120-year-old soil.

Temporal and Spatial Variability of Dissolved Organic Carbon Concentration in the Xijiang River, South China

A whole year analysis of riverine dissolved organic carbon (DOC) concentrations in the Xijiang River (XJR), South China, showed that the mean riverine DOC concentration (1.24 mg L-1) in the XJR was notably lower than the averaged value (5.75 mg L-1) of the global riverine DOC concentration in several major rivers. There is an inconspicuous monthly fluctuation of the DOC signal in the XJR, but on a semi-yearly time scale, however, the riverine DOC concentration had significant difference between hydrological seasons. The DOC level during the flood season (1.18 mg L-1) was less than that during the non-flood season (1.40 mg L-1). Owing to the concomitance of the flushing and dilution effects of the runoff during the high-water period, the variation of riverine DOC concentration with discharge in the XJR differed from that reported in many other major rivers. The DOC export flux above the city of Wuzhou was about 0.62× 106 g C km-2 yr-1. The DOC transported during the "056" Massive Flood period comprised 30.35% of the annual total, while the discharge accounted for 36.32% of the total annual flow. The characteristics in riverine DOC concentration in the XJR were attributed to the combined effect of the geomorphologic, monsoon climatic and hydrological processes as well as land-use within the drainage basin.

Dissolved organic carbon concentration and its seasonal variation in the Huguangyan Maar Lake of Southern China

The dissolved organic carbon(DOC)in the surface water of Huguangyan Maar Lake was continuously monitored based on once per week sampling frequency of 4 locations from June 2018 to May 2019.The DOC concentrations and its seasonal variation were discussed by correlating with the rainfall,water temperature,and p H of lake water.The results showed that the weekly DOC varied from 8.15 to 32.65 mg/L with an annual average concentration of 17.96 mg/L.There was a significant difference in the average DOC concentrations between the rainy and dry seasons as the monthly average DOC concentration was 21.72 mg/L for the wet season compared to the dry season concentration of 14.21 mg/L.The rainfall shows a significant effect on the DOC concentration of lake water,as DOC concentration was much high during the wet season.There were no significant spatial variations in the average monthly concentration among the four sampling locations except occasional variation during the wet season.The reason for the wet season DOC differences among four sampling locations is likely due to the uneven runoff and underground water inputs and the relatively slow circulation of lake water.Finally,the seasonal fluctuation of DOC concentration in this closed lake water suggests that dissolved soil organic matter inputs through the rainfall related to surface runoff and subsurface infiltration from the surrounding watershed land is important to the primary production and organic matter deposition in the lake sediments.

Temporal and spatial characteristics of dissolved organic carbon in the Wujiang River,Southwest China

River systems play an important role in the global carbon cycle. Rivers transport carbon to the ocean and also affect the carbon cycle in the coastal ocean. The flux from land to the ocean is thought to be a very important part of the land carbon budget. To investigate the effect of dam-building on dissolved organic carbon(DOC)in rivers, three reservoirs of different trophic states in the Wujiang basin, Guizhou Province, were sampled twice per month between May 2011 and May 2012. Temporal and spatial distributions of DOC in the reservoirs and their released waters were studied. It was found that different factors controlled DOC in river water, reservoir water, and released water. DOC in the rivers tended to be affected by primary production. For reservoirs, the main controlling factors of DOC concentration varied by trophic state. For the mesotrophic Hongjiadu Reservoir, the effect of primary production on DOC concentration was obvious. For the eutrophic Dongfengdu Reservoir and the hypereutrophic Wujiangdu Reservoir, primary production was not significant and DOC came instead from soil and plant litter.

Effect of Different Irrigation Methods on Dissolved Organic Carbon and Microbial Biomass Carbon in the Greenhouse Soil

The objective of this study was to investigate the contents and distribution of dissolved organic carbon （DOC） and microbial biomass carbon （MBC） at 0-100 cm soil depth under three irrigation treatments, viz., subsurface, drip and furrow irrigation in the greenhouse soil. The soil samples were collected at different depths （0-100 cm）, and the contents of soil total organic carbon （TOC）, DOC and MBC were analysed. The experiment was conducted for 10 yr, during which period the application of fertilizers and crop management practices were kept identical. The results showed that the contents of TOC, DOC and MBC were significantly affected by different irrigation regimes, decreased with the increase of soil depth. TOC at 0-10 and 80-100 cm soil depths followed the order of furrow irrigation 〉 subsurface irrigation 〉 drip irrigation, whereas at the depth of 10-80 cm followed the order of subsurface irrigation 〉 furrow irrigation 〉 drip irrigation. DOC and MBC contents at 0-100 cm soil depths followed the order of furrow irrigation 〉 drip irrigation 〉 subsurface irrigation, and drip irrigation 〉 furrow irrigation 〉 subsurface irrigation, respectively. The ratios of DOC and MBC to TOC accounted for 4.98-12.87% and 1.48-2.82%, respectively, which were the highest in the drip irrigation treatment, followed were in the furrow irrigation treatment, and the lowest in subsurface irrigation treatment. There were significant positive correlations among the contents of DOC, MBC and TOC in all irrigation treatments. The furrow irrigation facilitated the accumulation of TOC and DOC, while drip irrigation increased the MBC. The content of TOC and the ratios of DOC to TOC were the lowest in subsurface irrigation treatment.

Review and suggestions for estimating particulate organic carbon and dissolved organic carbon inventories in the ocean using remote sensing data

Dissolved organic carbon（DOC） and particulate organic carbon（POC） are basic variables for the ocean carbon cycle.Knowledge of the distribution and inventory of these variables is important for a better estimation and understanding of the global carbon cycle.Owing to its considerable advantages in spatial and temporal coverage,remote sensing data provide estimates of DOC and POC inventories,which are able to give a synthetic view for the distribution and transportation of carbon pools.To estimate organic carbon inventories using remote sensing involves integration of the surface concentration and vertical profile models,and the development of these models is critical to the accuracy of estimates.Hence,the distribution and control factors of DOC and POC in the ocean first are briefly summarized,and then studies of DOC and POC inventories and flux estimations are reviewed,most of which are based on field data and few of which consider the vertical distributions of POC or DOC.There is some research on the estimation of POC inventory by remote sensing,mainly in the open ocean,in which three kinds of vertical profile models have been proposed：the uniform,exponential decay,and Gauss models.However,research on remote-sensing estimation of the DOC inventory remains lacking.A synthetic review of approaches used to estimate the organic carbon inventories is offered and the future development of methods is discussed for such estimates using remote sensing data in coastal waters.

Spatial Variation of Dissolved Organic Carbon in Soils of Riparian Wetlands and Responses to Hydro-geomorphologic Changes in Sanjiang Plain, China

Spatial variation of dissolved organic carbon(DOC) in soils of riparian wetlands and responses to hydro-geomorphologic changes in the Sanjiang Plain were analyzed through in situ collecting soil samples in the Naoli River and the Bielahong River. The results showed that the average contents of DOC for soil layer of 0–100 cm were 730.6 mg/kg, 250.9 mg/kg, 423.0 mg/kg and 333.1 mg/kg respectively from riverbed to river terrace along the transverse directions of the Naoli watershed. The content of the soil DOC was the highest in the riverbed, lower in the high floodplain and much lower in the river terrace, and it was the lowest in the low floodplain. The difference in the content and vertical distribution of DOC between the riverbed and the three riparian wetlands was significant, while it was not significant among the low floodplain, the high floodplain and the river terrace. The variability of soil DOC was related to the hydrological connectivity between different landscape position of the riparian wetlands and the adjacent stream. Extremely significant correlations were observed between DOC and total organic carbon(TOC), total iron(TFe), ferrous iron(Fe(II)) whose correlation coefficients were 0.819, –0.544 and –0.709 in riparian wetlands of the Naoli River. With the increase of wetland destruction, soil p H increased and soil DOC content changed. The correlation coefficients between soil DOC and TOC, TFe, Fe(II) also changed into 0.759, –0.686 and –0.575 respectively in the Bielahong River. Under the impact of drainage ditches, the correlations between soil DOC and TFe, Fe(II) were not obvious, while the soil p H was weakly alkaline and was negatively correlated with soil DOC in the previous high floodplain. It indicates that riparian hydro-geomorphology is the main factor that could well explain this spatial variability of soil DOC, and the agricultural environmental hydraulic works like ditching also must be considered.

Distribution of dissolved organic carbon in and near the Prydz Bay, Antarctica

During the 16th Chinese National Antarctic Research Expedition (CHINARE) (from November 1999 to April 2000) seawater samples were collected for (DOC) determination in the Prydz Bay and its nearby sea areas. DOC concentrations were determined-by high temperature catalytic oxidation (HTCO) method. The results shows that DOC concentrations, in the upper water column (0 similar to 100 in) range from 14.3 to 181.1 mumol/dm(3), with averaging 52.5 mumol/dm(3) (n = 55). These values are slightly higher than those reported for the Ross Sea, the Pacific Ocean and others. Profiles of DOC concentration in the study areas show a decreasing concentration with increasing depth in the upper 100 m, which is related to biological activities in the water column. DOC concentrations below 100 in are relatively constant with a mean of 40.4 gammamol/dm(3). These DOC are unactive for physical and biological activities and are called refractory DOC. Concentration of the refractory DOC in the study area is consistent with the previous reported values for the Southern Ocean, which is about 41 mumol/dm(3). Based on the difference between the measured DOC concentration and the refractory concentration, the excess DOC concentration in the upper column can be calculated at every station. The excess DOC shows a spatial variability with a higher excess in the north of 64degreesS and little excess in the south of 64degreesS. In conclusion, DOC concentrations in the Prydz Bay and its nearby sea areas,are, consistent with the previous reported values in the Southern Ocean, which show a low DOC concentration with respect to the other oceans. Distribution of surface DOC concentrations in the study areas shows an increase from the southwestern to the northeastern, which is ascribed to the northern spread of continental shelf water from the Prydz Bay in summer. Contents of DOC and their distribution in the Prydz Bay and its nearby sea areas are mainly controlled by physical and biological processes.

Rapid, Semi-Automated Fractionation of Freshwater Dissolved Organic Carbon Using DAX 8 (XAD 8) and XAD 4 Resins in Tandem

Natural dissolved organic carbon (DOC) consists of different bio-molecular classes of compounds that are currently very difficult and time-consuming to isolate as individual compounds. However, it is possible to separate natural DOC into hydrophobic and hydrophilic fractions. Such characterisation approaches are becoming increasingly important because, over the past 20 years natural DOC concentrations have been rising rapidly in many parts of the world, most likely influenced by climate change. Higher DOC concentrations in drinking water catchments present a serious problem for the water industry because DOC can form disinfection by-products DBPs during water treatment (e.g. chlorination). Hence, there is an urgent need to better characterise natural DOC before, during and after water treatment. However, current DOC fractionation procedures are extremely laborious requiring days and continual manual monitoring to separate sufficient quantities of DOC for subsequent analysis. This seriously limits sample throughput and the parameter space which can be studied. In this paper, we propose a much more rapid semi-automated method (12.5 hours/litre/sample) which utilises readily available equipment, i.e., HPLC pump or similar and sequential columns of Amberlite DAX 8 and XAD 4 resins. The method reduces the manual input from continual attention to minutes. This paper describes the development of the method and its application in the fractionation of natural DOC from reservoir and lake samples fed from upland peat-land catchments. Recoveries are found to be comparable to those using the manual technique, with the dominant component being hydrophobic acid accounting for 35% - 40% of the natural DOC with the second largest, being hydrophilic acid at 20% - 27%.

Key Physical Factors Affecting Spatial-temporal Variation of Labile Organic Carbon Fractions by Biochar Driven in Mollisols Region of Northeast China

Biochar is widely used to improve soil physical properties and carbon sequestration. However, few studies focuse on the impact of maize stalk biochar on labile organic carbon(LOC) pool and the relationship between physical properties and LOC fractions. A field positioning experiment was performed in Mollisols region of Northeast China to evaluate the influence of maize stalk biochar on the spatial distribution and temporal changes of physical properties and LOC fractions. Maize stalk biochar treatments included C1(1.5 kg·hm^(-2)), C2(3 kg·hm^(-2)), C3(15 kg·hm^(-2)), C4(30 kg·hm^(-2)), and CK(0). The results showed that maize stalk biochar increased soil water contents(SWC) and soil porosity(SP), but reduced bulk density(BD). Maize stalk biochar reduced dissolved organic carbon(DOC) contents in the 0-20 cm soil layer, ranging from 0.25 g·kg^(-1) to 0.31 g·kg^(-1) in harvest period, while increased in the 20-40 cm soil layer. In addition, the application of biochar had a significant impact on the spatial distribution and temporal change of SWC, BD, SP, DOC, hot-water extractable carbon(HWC), acid hydrolyzed organic carbon(AHC Ⅰ, Ⅱ), and readily oxidized organic carbon(ROC). High amounts of maize stalk biochar up-regulated the contents of soil organic carbon SOC, HWC, AHC Ⅰ, AHC Ⅱ, and ROC. In addition, SWC and SP were the key physical factors to affect LOC fractions. In conclusions, maize stalk biochar could improve physical properties, and then influence LOC fractions, and maize stalk biochar could be used as an organic amendment for restoring degraded soils governed by their rates of addition.

Dissolved Organic Carbon Dynamics and Controls of Planted Slash Pine Forest Soil in Subtropical Region in Southern China

Soil dissolved organic carbon （DOC） is an active fraction of the soil organic carbon （SOC） pool and links terrestrial and aquatic systems. The degradation of DOC can affect carbon cycling, nutrient dynamics and energy supply to microorganism, and consequently change biogeochemical processes. This study investigated the vertical and seasonal variability of soil DOC concentrations and its controls in a 23-year-old planted slash pine （Pinus elliottii） forest at Qianyanzhou Forest Experimental Station （QFES） in Southern China. Soil solutions were collected at bimonthly intervals at depths of 10, 20 and 30 cm by a mechanical-vacuum extractor from November 2007 to March 2009, and at monthly intervals at depths of 10, 30 and 50cm from April 2009 to October 2010. The DOC concentrations were determined with a total organic carbon （TOC） analyzer. Mean （±standard deviation） DOC concentrations at depths of 10cm, 20cm, 30cm and 50cm were 12.4±4.4, 10.6±6.3, 8.7±2.6, and 8.0±5.9 mg L^-1, respectively. Both seasonal and spring means of DOC concentration showed a decreasing trend with increasing depth, while there was no clear trend for the summer, autumn, or winter seasons. DOC concentrations during spring, summer, autumn and winter ranged from 4.8 to 21.5, 4.9 to 26.2, 5.4 to 17.1, 4.9 to 14.6 mg L-1, respectively, their mean DOC concentrations were 10.2, 10.5, 10.8 and 8.3 mg L 1, respectively. No consistent pattern of seasonal variability of DOC concentrations at different depths was observed. No obvious relationship between organic carbon content of forest litter and DOC concentration was found. There was a positive linear relationship between SOC and DOC concentration （R2=0.19, p〈0.01）, which showed that SOC was one of the main controls of DOC. A positive exponential relationship existed between soil temperature at 5 cm and DOC concentrations at 10 cm depth in slash pine, masson pine （Pinus massoniana） and Chinese fir （Cunninghamia lanceolata） planted forests （R2=0.12, p〈0.01）. DOC concentrations showed a negative linear relationship with soil moisture at all depths in slash pine forest （R2=0.15, p〈0.001）, and DOC concentrations at depth of 10 cm demonstrated a negative exponential relationship with soil moisture at 5 cm depth in three planted forests （R2=0.13,/）〈0.001）. Precipitation in sampling months and mean seasonal DOC concentration were not correlated. However, a more detailed analysis of precipitation events at different times before sampling and seasonal DOC concentration showed that the timing of precipitation events prior to sampling had different effects on seasonal DOC concentrations at different depths. Our study highlights the importance of DOC dynamics for the carbon cycle in planted slash pine forest and it provides evidence for evaluating the effects of ecological restoration in subtropical red soil region.

Spurious Thermoluminescence Characteristics of the Ediacaran Doushantuo Formation(ca. 635–551 Ma) and Its Implications for Marine Dissolved Organic Carbon Reservoir

The Ediacaran Doushantuo Formation（ca. 635–551 Ma） deposited immediately after the last Neoproterozoic glaciations and recorded the most prominent negative excursions of carbonate carbon isotopic composition（δ^（13）C（carb））. These excursions have been interpreted as a result of widespread remineralization of a large dissolved organic carbon（DOC） reservoir in the Ediacaran deep oceans. However, there is no direct evidence so far found in rocks for the proposed DOC reservoir, which devalues such an interpretation. Here, we conducted a detailed study on the glow-curves characteristics and signal origins of spurious thermoluminescence（TL） of the Doushantuo Formation at Jiulongwan in Yangtze Gorges area, South China, through sequential tests under CO2, N2 and air. Spurious TL intensities for test samples before and after removing soluble organic matter via accelerated solvent extraction（ASE） are nearly identical. Further, significant positive correlation between the spurious TL intensity and total inorganic carbon（TIC） content（R^2=0.7） indicate that the Doushantuo spurious TL with the characteristic peak at 393.5 °C from the sequential test is chemiluminescence（CL） which is derived from the oxidation of a type of non-volatile organic matter strongly associated with carbonate mineral lattice（termed as “X-OM”）. A most likely explanation is that the X-OM is a type of dissolved organic matter which co-precipitated with carbonate minerals into sediments in the Ediacaran Doushantuo Ocean. Furthermore, a significant exponential negative correlation（R^2=0.55） is observed between the CL data and the isotopic difference between carbonate and coexisting bulk organic matter（i.e., Δ^（13）C（carb-org）, a proxy for remineralization degree of DOC reservoir in proposed DOC hypothesis）, suggesting that the X-OM was derived from the oxidation of the DOC reservoir in the Ediacaran Ocean. We thus propose that the X-OM and its CL detected in our study may have recorded the evolution of the possible DOC reservoir in the Ediacaran Doushantuo Ocean. If this is correct, the stratigraphic variations of the CL intensity in the Doushantuo Formation at Jiulongwan support the pulsed oxidation of the DOC reservoir in the Ediacaran Ocean. Our findings indicate that the CL derived from the oxidation of non-volatile organic matter which is strongly associated with carbonate mineral lattices in rocks may provide a feasible approach for probing the evolution of DOC reservoir in the ancient oceans, thus likely provide direct geological evidence for the development of oceanic DOC reservoir in geological times.

Dissolved organic carbon,a critical factor to increase the bioavailability of phosphorus during biochar-amended aerobic composting

Considerable research efforts have been devoted to increase phosphorus(P)availability during aerobic composting.However,there is little discussion weather the dissolved organic carbon(DOC)controls the transformation among P-fractions.Thus,we investigated the changes in DOC compositions and P-fractions during biochar-amended composting(wet weight basis,5%and 10%).TP content continuously increased since the'concentration effect'during aerobic composting.NaHCO_(3)-Pi,NaOH-Pi and HCl-Pi were main P-fractions,and biochar can improve P-bioavailability by transforming NaOH-Pi and HCl-Pi into NaHCO_(3)-Pi.Structure equation models(SEMs)indicated that biochar enhanced the P-bioavailability through regulating the decomposition of DOC.Our results at least hint that the activation mechanism on P under the influence of DOC during biochar-amended composting.

SEASONAL CHANGES OF DISSOLVED AND PARTICULATE ORGANIC CARBON IN DONGHU LAKE,CHINA

Concentrations of dissolved and particulate organic carbon (DOC and POC) were documented in 1996-1997 at 4 different trophic state stations in Donghu Lake, a typical shallow eutrophic lake along the Changjiang River’s middle reaches. The mean concentrations of DOC were 15.11±3.26, 15.19±4.24, 14.27±3.43, and 13.31±3.30 mg/L in Station I, II, III, and IV, respectively. The DOC concentrations of the studied area were very similar to that in other lakes along the Changjiang River’s middle reaches. The POC mean of the whole lake was 5.01 mg/L due to the large amount of organic detritus of both allochthonous and autochthonous origin. Significant linear relationship was found between POC and chlorophyll a at all 4 stations, which presumably reflect that phytoplankton, its exudates and its metabolic products were the main contributors to the POC pool in the water column. The slope of such linear relationship at Station IV was significantly steeper than that at Station I, II and III. In addition, the DOC/POC ratios (mean value: 4.40) indicated that the organic detritus was the most important component of the particulate organic matter; in other words, next to organic detritus, phytoplankton dominated the particulate organic matter in Donghu Lake.