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The carbon cycle in lakes has been receiving great concerns and has become a hot issue. This study aimed at investigating the characteristics of Dissolved Organic Carbon (DOC) and its influence on methane emission based on continuous field sampling in a year cycle from algal-dominated zones of Lake Taihu. Results showed that the DOC with mean value of 4. 15 mg/L varied spatially due to external input and internal algal blooms. Overall, peak DOC occurred in Northwest Zone and Meiliang Bay, and temporal variations of DOC in the two zones were associated with basin precipitation, especially for Northwest Zone (R²=0. 67,P<0. 01) . It should be noted that the temporal variation of DOC in Central Zone were highly positively correlated with algal biomass mostly due to the less external input. The CH4 diffusion emission flux with mean value of 0. 083 mmol·m-2·d-1 varied greatly across zones. Algal blooms significantly stimulated the CH4 emissions via increasing DOC concentration. Overall, the algal-dominated lake was a hot spot of CH4 emission due to DOC enrichment. Considering the effects of DOC on lake CH4 emission was regulated by multiple factors related to internal metabolic activities and external loading, further studies are needed to reveal the potential control mechanism. [ABSTRACT FROM AUTHOR]

Dissolved carbon in groundwater plays an important role in carbon cycling and ecological function maintenance, and its concentration level affects the migration and transformation of pollutants in groundwater. To understand the spatiotemporal variation characteristics of dissolved carbon and its driving factors in shallow groundwater around plateau lakes, variations in the concentrations of dissolved organic carbon （DOC）, inorganic carbon （DIC）, and total carbon （DTC） and their driving factors in shallow groundwater （ n = 404） around eight plateau lakes were analyzed. The results indicated that the average values of ρ （DOC）, ρ （DIC）, and ρ （DTC） in shallow groundwater around plateau lakes were 8.23, 49.01, and 57.84 mg·L -1 , respectively, with the ρ （DOC） in 79.0% of shallow groundwater samples exceeding 5 mg·L -1 . There were no significant differences in the DOC, DIC, and DTC concentrations between rainy and dry seasons, whereas the change in dissolved carbon concentrations in shallow groundwater were strongly affected by the intensity of agricultural intensification and the depth of groundwater table； the DOC, DIC, and DTC concentrations in shallow groundwater from facility agricultural regions （SFAR）, cropland fallow agricultural regions （CFAR）, and intensive agricultural regions with deeper groundwater tables （DIAR） were significantly reduced by 25.8% - 56.6%, 14.0% - 32.9%, and 16.6% - 36.7%, respectively, compared with those in intensive agricultural regions with shallower groundwater tables （SIAR）. Additionally, the dissolved carbon concentrations in shallow groundwater from DIAR were significantly lower than those of SFAR and CFAR. RDA revealed that physicochemical factors in water and soil significantly explained the changes in the dissolved carbon concentrations. Moreover, the dissolved carbon concentrations in shallow groundwater around Yilong Lake were significantly higher than those of other lakes, whereas that of Chenghai Lake was significantly lower than that of other lakes. Our study highlights that agricultural intensification intensity and groundwater table depth jointly drove the variations in dissolved carbon concentrations in shallow groundwater around plateau lakes. The study results are expected to provide a scientific basis for understanding the carbon cycle in plateau lake areas with underground runoff flowing into lakes and evaluating the attenuation of pollutants by dissolved carbon in shallow groundwater.

Accumulation of microplastics in soil could interfere with the germination and growth of plants. However, the chemical risks raised by leachate of microplastics remain unknown. Here, we prepared microplastic leachate at different extraction temperatures (25 and 50℃) using microplastic fibers derived from polyamide (PA) and polyethylene (PE) and conducted the seed germination test of microplastic leachates to investigate the toxic effects of microplastic leachates on lettuce ( Lactuca sativa L.). Furthermore, characteristics of the microplastic leachate, such as DOC and DON concentrations and parameters of UV-vis, were measured. The results revealed that the concentration of DOC and DON in the leachate of PA was significantly higher than that of PE. DOC and DON concentration in the leachate of PA increased with extraction temperature. Additionally, the aromaticity, hydrophobic component content, and molecular weight of leachates were significantly affected by the polymers of microplastic, whereas the extraction temperature had no effect. Compared to those in CK, the microplastic leachates reduced the indicators such as germination vigor, germination index, and vigor index of lettuce seeds; however, it had no impact on agronomic traits, such as plant height, root length, fresh weight, and dry weight. Meanwhile, some seeds had abnormal developments of radicles and cotyledons under the microplastic leachate treatments. This demonstrates that the substances leached from microplastics could interfere with the germination process of lettuce seeds. Therefore, the chemical risks exerted by the microplastics to the soil and plant system require further attention.

A field experiment was conducted in a lightly Cd-contaminated rice field in Ningxiang City, Hunan Province, to study the effects of straw removal measures on the soil Cd bioavailability and rice Cd accumulation. The results showed that:① two consecutive seasons of straw removal measures (T1-T4 treatments) effectively increased soil pH by 0.04-0.58 units, reduced soil organic matter by 0.68%-25.87%, and reduced the Cd content of rhizosphere soil by 3.76%-12.78%. ② The proportions of Cd in the acid-extractable fraction and oxidizable fraction decreased, and the proportion of Cd in the residual fraction increased. Furthermore, straw removal measures significantly reduced the bioavailability of Cd in rhizosphere soil, and the Cd contents in TCLP, DTPA, and CaCl 2 extracts all significantly decreased compared with those in CK. ③ The straw removal measure could significantly reduce the content of DOC and Cd in soil pore water; and the contents of Cd in soil pore water decreased by 4.54%-40.00% and 2.75%-67.34% under the straw removal measure (T1-T4) for two consecutive seasons, respectively, indicating that DOC was one of the key factors affecting the content of Cd in soil pore water. ④ Two consecutive straw removal measures (T1-T4) reduced the accumulation of Cd in different rice tissues, among which, under the treatment of all straw and root removal (T4), the Cd contents of brown rice in late rice planting in 2020 and early rice planting in 2021 decreased by 18.52% and 39.69%, respectively. Therefore, full or partial removal of straw in Cd-contaminated rice fields is a powerful measure to reduce the risk of exceeding Cd levels in brown rice.

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Lake Qinghai is the largest lake in China and is of great significance to maintain the ecological security of the Qinghai-Tibet Plateau. Few studies have been carried out to investigate the optical composition and source of chromophoric dissolved organic matter (CDOM) in large lakes on the Qinghai-Tibet Plateau. It is of great significance to study the source and optical dynamics of CDOM in Lake Qinghai watershed for water quality protection and filling in the gaps in the knowledge of CDOM variability in a remote area. Two sampling campaigns in the Lake Qinghai watershed were carried out, and excitation-emission matrices coupled with parallel factor analysis (EEMs-PARAFAC) were used to unravel the optical composition and the sources of CDOM. Our results indicated that the mean dissolved organic carbon (DOC) concentration, a 250 : a 365 , and the spectral slope of CDOM absorption S 275-295 in the lake were significantly higher than that in the inflow river ( P <0.0001, t -test), whereas the mean absorption coefficient of CDOM a 350 , humification index (HIX), fluorescence peak integration ratio I C : I T, and specific ultraviolet absorbance at 254 nm SUVA 254 of CDOM were shown to be lower in the lake than in the inflow river ( P <0.0001), indicating that compared with the lake itself, CDOM in the inflow was humic-rich and highly aromatic. Four fluorescent components were obtained using PARAFAC, including a terrestrial human-like component C1, a microbial human-like component C2, a tyrosine-like C3, and a tryptophan-like C4. The mean DOC concentration, S 275-295 , and a 250 : a 365 in the headwater streams of the Lake Qinghai watershed were lower than those in the downstream estuary, indicating that the CDOM abundance increased, and the molecular weight decreased, from the headwaters to the downstream river mouths. The mean of SUVA 254 , C1, and the first axis of principal component analysis were positively related to terrestrial input (i.e., the PC1 values were significantly higher in rivers than in lakes ( P <0.001)), indicating that the aromaticity of CDOM in rivers was higher than that in lakes. Particularly, the contribution of terrestrial humic-like C1 was higher in the Quanji River, Shaliu River, and Khargai River compared with that in other tributaries due to an intensified cultivated land use at the downstream estuary of these rivers.

As an important part of the riverine carbon cycle, dissolved inorganic carbon (DIC) has attracted continuous attention. The stable carbon isotope ( δ 13 C DIC ) of DIC can reflect its sources and transformations. However, the effects of land use on DIC and δ 13 C DIC are rarely investigated. To study the influencing factors of DIC and δ 13 C DIC , river water samples were collected and analyzed from the Chishui River basin, a typical karst river basin in southwestern China. The upper reaches of the Chishui River basin were predominantly underlain by carbonate sedimentary rocks, whereas the lower reaches contained mostly siliciclastic sedimentary rocks, and the forest coverage in the lower reaches was relatively high. The elemental compositions of the river water revealed that the hydrochemistry of rivers in the upper reaches of the basin was mainly controlled by carbonate weathering, whereas the hydrochemistry of some tributaries in the lower reaches was mainly affected by silicate weathering. During the wet season, the average values of c (DIC) and the δ 13 C DIC value were (1940±493) μmol·L -1 and (-9±1) ‰, respectively, whereas the c (DIC) was relatively high, and the δ 13 C DIC value was more positive in the dry season, with the average values of (2334±626) μmol·L -1 and (-7.3±1) ‰, respectively. The DIC of most samples was mainly controlled by carbonate weathering under an open system and was derived from carbonate minerals and atmospheric and soil CO 2 . The seasonal variations in the c (DIC) and δ 13 C DIC values suggested that c (DIC) was regulated by climate, hydrology, and biology. The increasing contribution of biological carbon to DIC in summer was the main cause of the more negative δ 13 C DIC value in the wet season, whereas the dilution effect of higher discharge was the main cause of the low c (DIC) in the wet season. The samples from three tributaries in the lower reaches with the highest proportion of silicate and forest distribution had the highest dissolved organic carbon concentration[ c (DOC)], the lowest c (DIC), and the most negative δ 13 C DIC value in the wet season. The proportion of carbonate distribution had positive correlations with c (DIC) in the wet and dry seasons, indicating that lithology was the main controlling factor of c (DIC). The rivers draining the carbonate areas had a lower c (DIC) and a more negative δ 13 C DIC value in the wet season than those in the dry season, whereas for the rivers draining non-carbonate areas with high forest coverage, the c (DIC) was higher and the δ 13 C DIC value was significantly more negative in the wet season than those in the dry season. This implies that c (DIC) and δ 13 C DIC are significantly affected by land use when they are less affected by lithology.

Although the pollution of freshwater systems by microplastics and the resulting ecological effects have attracted widespread attention from scholars at home and abroad, the fragmentation of different types of microplastics in the natural environment has not yet received enough attention. To analyze the fragmentation processes and products of different types of microplastics, a 40 d natural light fragmentation experiment was carried out using four microplastics commonly found in water:polystyrene (PS), low-density polyethylene (LDPE), polypropylene (PP), and polyhydroxybutyrate (PHB). The pH, ORP, EC, and DO of the four types of plastic-aging liquids changed significantly during the aging process. During the weathering process, cracks and pores formed on the surface of LDPE and were later identified through scanning electron microscopy. Fourier transform infrared spectrometry indicated that the carbonyl index of LDPE increased the most in the experiment (an increase of 31.48%), suggesting that PP experienced significant aging. Dissolved organic carbon (DOC), a product of microplastics fragmentation, increased as weathering time progressed, and the concentration of DOC increased significantly after weathering for about 40 d. Compared with the baseline value, the concentration of DOC from PHB, PP, and LDPE in the leaching solution increased significantly, by 61.29%, 69.49%, and 89.15%, respectively. These results suggest that evident aging of microplastics in natural aquatic environments releases significant amounts of dissolved organic matter, and the ecological effects of this should be the subject of future research.

Dissolved organic matter (DOM) in karst water is one of the most important carbon sink components, whose origins, distributions, and transport processes are of significance to carbon sink studies. Chromophoric dissolved organic matter (CDOM) can be utilized to express the composition and structural properties of DOM. In this paper, water samples were collected monthly from Xueyu Cave in a karst underground river from both inside and outside the cave. The changes in hydrochemistry and CDOM spectral characteristics as well as the influencing factors for CDOM changes were studied, with the aim of providing experimental support for carbon sink fluxes in karst processes. The main results were as follows. ① The dominant type of CDOM in Xueyu Cave karst water consisted of small molecular weight organic matter that was autochthonously derived and easily bio-degraded, and the tryptophan-like and tyrosine-like components accounted for more than 60% of the total CDOM. ② The most effective influencing factor inside the cave was microbes, which degraded the small molecular DOM in groundwater and induced slight decreases in the TOC and DOC concentrations; they also increased the humic index (HIX). ③ However, the dominant factor outside the cave was submerged aquatic plant photosynthesis, which induced significant increases in the TOC and DOC concentrations and autochthonous small molecular weight DOM contents.

Soil organic carbon (SOC) is the most important part of the soil carbon pool and soil labile organic carbon is a key indicator of changes in the soil carbon pool. In order to study changes in SOC and its labile components with different biochar additions to soil, a field experiment was conducted at Chongqing National Purple Soil Fertility and Fertilizer Efficiency of Long-Term Monitoring Station. Different amounts of biochar (0 kg·hm -2 , CK; 4000 kg·hm -2 , 0.5 BC; 8000 kg·hm -2 , BC; 16000 kg·hm -2 , 2BC) combined with fertilizer were applied to purple soil in this hilly area. SOC and soil labile carbon content were determined for purple soil rape-maize rotation systems. Results showed that:① Application of biochar can significantly improve SOC content; within a certain range, the amount of added biochar is correlated with SOC content. The content of soil microbial biomass carbon (SMBC) increased with application of 8000 kg·hm -2 biochar, but decreased with application of 4000 and 16000 kg·hm -2 . The content of dissolved organic carbon (DOC) and readily oxidized carbon (ROC) in soil increased with different amounts of biochar; the highest DOC (198.83 g·kg -1 ) and ROC (4.86 g·kg -1 ) were obtained under low biochar addition treatments. ② The application of biochar significantly decreases the ratio of soil ROC to SOC and microbial entropy. The ratio of soil ROC to SOC and microbial entropy of 0.5 BC treatment are 20.45% and 4.11%, respectively, lower than for CK. Moreover, 0.5 BC and BC treatments can significantly improve the ratio of soil DOC to SOC. In summary, biochar addition in soil leads to low soil microbial activity but higher soil stability. An appropriate biochar treatment can stimulate accumulation of SOC, and 0.5BC treatment can improve soil DOC and ROC.

Samples collected from 12 rivers with typical vegetation types in the permafrost regions on the Qinghai-Tibetan Plateau were incubated in the laboratory, and the relationships among the vegetation types, river discharges, the compositions of dissolved organic carbon (DOC), permafrost areas, riverine DOC concentration, biodegradability of dissolved organic carbon (BDOC), and the biodegradation kinetics were examined. The results showed that the DOC concentrations of typical vegetation types in the basin, such as alpine meadow (AM), alpine swamp meadow-alpine meadow (ASM-AM), alpine meadow-alpine steppe (AM-AS), and alpine meadow-alpine steppe-bare soil (AM-AS-BL), were (5.17±0.21), (5.02±0.50), (3.55±0.25), and (2.79±0.41) mg ·L -1 , respectively. The values for the bioavailability of river DOC of different vegetation types were (23.54±2.62)%, (23.66±3.31)%, (18.17±5.26)%, and (11.72±15.56)%, respectively. Correspondingly, the riverine DOC aromaticity increased along with the vegetation cover, while the biodegradation and degradation rates decreased gradually. During the incubation, the reaction of BDOC was in accordance with the first-order kinetics equation. Furthermore, the BDOC in continuous permafrost regions of the rivers was greater than that in the non-continuous permafrost regions. The BDOC in higher discharges were lower than those with lower discharges. Taken together, the results suggested that the vegetation types were the main controlling factors for the BDOC, and BDOC was also related to the discharge and permafrost.

Carbon and nitrogen in soils play an important role in the global carbon and nitrogen cycle. The enhancement of ultraviolet radiation (predominantly UV-B) resulting from the depletion of stratospheric ozone has raised significant concern. The effects of UV-B radiation on soil carbon and nitrogen transformation is connected directly to the physical and chemical properties of the soil. In order to clearly understand the effects of soil moisture on UV-B radiation, this study collected soil samples from two paddy fields with different levels of organic matter in a subtropical region of China. The response of the total organic carbon (TOC), dissolved organic carbon (DOC), ammonia nitrogen (NH 4 + -N), nitrate nitrogen (NO 3 - -N) and cumulative net nitrogen mineralization to UV-B radiation under three different moisture gradients (W1=25%, W2=50%, and W3=100%) were monitored in laboratory for 120 h. After this period, the results were compared with a control treatment (CK) and it was found that:the TOC content had significantly decreased under UV-B radiation ( p <0.05). From low to high moisture content (W1, W2 and W3), the TOC decreased by 9.9%, 4.5% and 6.3%, respectively for soil with low organic matter (L), and by 10.9%, 5.6% and 6.3%, respectively for soil with high organic matter (H), under UV-B radiation. However, UV-B radiation was found to enhance the DOC content in the soil compared with the CK. Furthermore, the DOC for soil moisture contents under 100% (W3) was higher than for other moisture contents (W1, W2). The measured DOC increased by 21.5% (W1), 9.4% (W2), and 26.3% (W3) for soil with L. In addition, the measured DOC increased by 26.7% (W1), 14.2% (W2) and 33.8% (W3) for soil with H under UV-B radiation after 120 h. Compared with control treatment (CK), UV-B radiation decreased the NH 4 + -N content significantly, but there was an increased NO 3 - -N content. The decrease of the NH 4 + -N content was largest for W3 and smallest for W1. The increase in NO 3 - -N content was largest for W2 and smallest for W1 for the two soil samples under UV-B radiation. UV-B radiation demonstrated an obvious effect on the cumulative net nitrogen mineralization ( p <0.05) after 24 h compared with the CK and the effect of different soil moisture treatment was also significant ( p <0.05). Overall, light degradation played a major role in the stabilization of soil organic matter, soil moisture, and UV-B radiation could accelerate the loss of soil organic carbon and has a major impact on the transformation of mineral nitrogen in the soil. Therefore, in agricultural production systems, completely bare surfaces should be avoided. For example, paddy rice-upland crop rotation systems could reduce the use of fallow periods.

Leaf litters of Pinus elliottii (needle-leaved) and Schima superba (broad-leaved), which are commonly found in subtropical regions of South China, were decomposed in a field for 0, 30, 60, 90, 150, 210, 240 and 365 d to evaluate changes in concentrations, compositions and properties, and amount of dissolved organic matter (DOM) released and dissolved organic carbon (DOC) in soil. Although the concentrations of DOM derived from S. superba were higher than those from P. elliottii, similar variation patterns of DOM characteristics (e.g., concentrations, composition and properties) were observed between the two types of litter during decomposition. The concentrations of DOM showed a downward trend. Upward trends were obtained for the aromatization and molecular mass of DOM, as well as fulvic acid and humic acid levels in DOM, while a downward trend was found for levels of simple aromatic proteins, such as tyrosine, in DOM. During the initial phase of decomposition, DOM was generally composed of hydrophilic neutral and acidic components, which were easily decomposed and transferred, and their effects on surface soil DOC were not significant. At later stages, DOM was mainly composed of humic and fulvic acid and other aromatic, complex structure substances, which could easily adsorb on soil, and decreased the DOC concentration of surface soil.

As an important sources of dissolved organic matter (DOM) in aquatic system, DOM releases from flooding or submerged soils is a key process in water-level fluctuation zones. In this study, four typical sites in water-level fluctuation zones of Three Gorges Reservoir areas were selected to conduct simulated soil flooding experiments, under ambient (open air) and anoxic conditions. By using fluorescence spectrum technique, the dynamic and geochemical characteristics of DOM releases from flooding soils were investigated. Results showed that the trend or model of DOM releases observed by fluorescence spectrum in all soils from four sampling sites was similar to the observation by UV-Vis spectrum. Fluorescence property of DOM releases showed an important contribution to DOM fluorescence in overlying waters. The rapid releases at initial stage and removal mechanism for later dynamic equilibrium were crucial to explain the dynamic "source-sink" equilibrium in whole flooding period. Effect of inorganic mineral adsorption-desorption on humic-like components (A and C peaks) was significant. Also, impact of microbial utilization on protein-like components (B and T peaks) was confirmative. Additionally, all DOM samples had "autochthonousness plus allochthonousness" property. Except Shibaozhai (SB) site in Zhongxian county, during the whole flooding period, all three other sites didn't show any significant difference between ambient and anoxic conditions. They could be explained by the high heterogeneity of soil property including minerals and organic components. Meanwhile, both the UV-Vis and fluorescence spectra were complementary for each other, and they evidentially showed that the "source-sink" (release and removal) mechanism in DOM releases from submerged soils was the core to decide the dynamics of DOM in overlying waters. Importantly, DOM showed higher aromaticity and humification at the initial release stage when flooding occurred, as well as the greater terrestrial source characteristics, which further helped to explain the environmental fates of pollutants in these environmental sensitive areas. It would also be useful for unveiling the role of DOM in environmental system in future.

Water-level fluctuation zone is an important area in Three Gorges Reservoir areas, because dissolved organic matter (DOM) releases from submerged soils is a crucial source of DOM in overlying waters when flooding occurs. But there was little qualitative and quantitative analysis for DOM. In this study, soils of four typical water-level fluctuation zones were selected to conduct simulated flooding experiment. Also, UV-Vis spectroscopy was utilized to investigate the DOM spectral characteristics in two individual flooding sets including ambient (open air) and anoxic conditions. The results showed, for all soils, the DOM release patterns all showed a "rapid release, then decrease, and final dynamic equilibrium" trend. Also, through calculations of DOM releasing rates and fluxes, DOM releases clearly showed a feature of "source-sink dynamic exchange". By comparison between two flooding treatments, although DOC and CDOM didn't show any significant differences, but higher aromaticity (SUVA 280 ) was observed in anoxic experiment. In contrast, spectra slope ratio ( S R ) was insensitive to redox conditions of different flooding. Meanwhile, correlation between DOC and CDOM in anoxic was also higher than that in ambient flooding. Additionally, obvious differences among soils from different sampling sites were observed, Shibao Zhai (SB) of which showed the greatest releasing potential. Considering the various soil properties, soil organic matter amounts in soils is one of the key factors to explain the differences among sampling sites, but other geochemical factors should also be considered in the processes of DOM releases. Summarily, only focusing on the changes of DOC concentrations in submerging process is not enough to comprehensively describe the DOM release characteristics.