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Dissolved organic matter (DOM) is a crucial driver of various biogeochemical processes in aquatic systems. Thus, many lakes and streams have been investigated in the past several decades. However, fewer studies have sought to understand the changes in DOM characteristics in the waters of the Three Gorges Reservoir (TGR) areas, which are the largest artificial reservoir areas in the world. Thus, a field investigation of dissolved organic carbon (DOC) concentrations and of chromophoric dissolved organic matter (CDOM) properties was conducted from 2013 to 2015 to track the spatial-temporal variability of DOM properties in the TGR areas. The results showed that the alternations of wet and dry periods due to hydrological management have a substantial effect on the quantity and quality of aquatic DOM in TGR areas. Increases in DOC concentrations in the wet period show an apparent “dilution effect” that decreases CDOM compounds with relatively lower aromaticity (i.e., SUVA 254 ) and molecular weight (i.e., S R ). In contrast to the obvious temporal variations of DOM, significant spatial variability was not observed in this study. Additionally, DOM showed more terrigenous characteristics in the dry period but weak terrigenous characteristics in the wet period. Furthermore, the positive correlation between SUVA 254 and CDOM suggests that the aromatic component controls the CDOM dynamics in TGR areas. The first attempt to investigate the DOM dynamics in TGR areas since the Three Gorges Dam was conducted in 2012, and the unique patterns of spatial-temporal variations in DOM that are highlighted in this study might provide a new insight for understanding the role of DOM in the fates of contaminants and may help in the further management of flow loads and water quality in the TGR area. [ABSTRACT FROM AUTHOR]

Because of heterogeneous properties, dissolved organic matter (DOM) is known to control the environmental fate of a variety of organic pollutants and trace metals in aquatic systems. Here we report absorptive and fluorescence properties of DOM, in concurrence with concentrations of dissolved mercury (Hg), along the Xiaoqing River-Laizhou Bay estuary system located in the Bohai Sea of China. A mixing model consisting of the two end-members terrestrial and aquatic DOM demonstrated that terrestrial signatures decreased significantly from the river into the estuary. Quasi-conservative mixing behavior of DOM sources suggests that the variations in the average DOM composition were governed by physical processes (e.g., dilution) rather than by new production and/or degradation processes. In contrast to some previous studies of river-estuary systems, the Xiaoqing River-Laizhou Bay estuary system displayed a non-significant correlation between DOM and Hg quantities. Based on this and the variation of Hg concentration along the salinity gradient, we concluded that Hg showed a non-conservative mixing behavior of suggested end-member sources. Thus, rather than mixing, Hg concentration variations seemed to be controlled by biogeochemical processes. [ABSTRACT FROM AUTHOR]

The exchange of mercury (Hg) across the air-water interface is an important part of Hg biogeochemical cycle. Mercury fluxes across the air-water interface in paddy fields were measured by a Dynamic Flux Chamber (DFC) coupled with a Lumex® multifunctional mercury analyzer RA-915 at two sites (Chengjiang (CJ) and Caoshang (CS)) in Beibei, Chongqing, China in 2008. The results showed that mercury emission followed a power-law relationship with solar radiation and air temperature, and it increased exponentially with water temperature at both sites. Mercury emission was mainly influenced by the solubility of gaseous elemental Hg, photo-thermal effect, electron activity (Eh) and air Hg concentrations. Solar radiation made the greatest direct contribution to mercury emission during the daytime (0.80), with an 83.60% contribution, whereas at nighttime the water temperature (0.72) contributed to 71.65% of emissions. The temperature gradient between water and air might also influenced mercury emission across the air/water interface at nighttime. These findings suggest that paddy fields could act as a significant source of atmospheric mercury, and it can contribute significantly to the atmospheric mercury in a local region. [ABSTRACT FROM AUTHOR]

Forest ecosystem has long been suggested as a vital component in the global mercury (Hg) biogeochemical cycling. However, there remains large uncertainties in understanding total Hg (THg) and methylmercury (MeHg) variations and their controlling factors during the whole hydrological processes in forest ecosystems. Here, we quantified Hg mass flow along hydrological processes of wet deposition, throughfall, stemflow, litter leachate, soil leachate, surface runoff, and stream, and litterfall Hg deposition, and air-forest floor elemental Hg (Hg0) exchange flux to set up a Hg mass balance in a subtropical forest of China. Results showed that THg concentration in stream was lower than that in wet deposition, while an opposite characteristic for MeHg concentration, and both THg and MeHg fluxes of stream were lower than those of wet deposition. Variations of THg and MeHg in throughfall and litter leachate had strong direct and indirect effects on controlling variations of THg and MeHg in surface runoff, soil leachate and stream, respectively. Especially, the net Hg methylation was suggested in the forest canopy and forest floor layers, and significant particulate bound Hg (PBM) filtration was observed in soil layers. The Hg mass balance showed that the litterfall Hg deposition was the main Hg input for forest floor Hg, and the elemental Hg vapor (Hg0) re-emission from forest floor was the dominant Hg output. Overall, we estimated the net THg input flux of 13.8 μg m−2 yr−1 and net MeHg input flux of 0.6 μg m−2 yr−1 within the forest ecosystem. Our results highlighted the important roles of forest canopy and forest floor to shape Hg in output flow, and the forest floor is a distinct sink of MeHg. Image 1 • Forest canopy and forest floor shape mercury concentrations in runoff. • Both forest canopy and forest floor induce net Hg methylation. • Mineral soil is a distinct methylmercury sink. • Subtropical forest in south China plays a sink role of atmospheric mercury. [ABSTRACT FROM AUTHOR]

Forests play a significant role in the biogeochemical cycling of mercury (Hg). Litterfall represent a dominant pathway for Hg to reach the ground surface under a forest canopy. In order to understand Hg accumulation in the litterfall, the dynamics of Hg, carbon (C), nitrogen (N), microbial C and N, as well as bacterial community in the decaying litterfall of two typical subtropical forest stands in southwest China were investigated for one year. THg levels in the litterfall after one-year increased to 124.64% and 135.90%, and MeHg levels in the litterfall increased up to 295.65% and 209.38% of the initial values in the mixed broadleaf-conifer forest and evergreen broadleaf forest respectively. Differently, the concentrations of THg and MeHg in the organic layer of the underlying soil were quite stable. The concentrations of THg in the decaying litterfall corresponded negatively with C/N ratios. Bacterial community analysis found that the bacteria previously being confirmed as Hg methylators did not occur in the genera of the decomposing litterfall in the two forest stands, which might imply that the increase of MeHg during decomposing did not mainly due to the contribution of confirmed Hg methylators, and other sources of MeHg might exert certain roles. [ABSTRACT FROM AUTHOR]

Soil-derived dissolved organic matter (DOM) has a major influence in biogeochemical processes related to contaminant dynamics and greenhouse gas emissions, due to its reactivity and its bridging role between the soil and aquatic systems. Within the Three Gorges Reservoir (TGR, China) area, an extensive water-fluctuation zone periodically submerges the surrounding soils. Here we report a characterization study of soil-derived DOM across the TGR areas, using elemental and optical analysis, infrared spectroscopy (FTIR), pyrolysis-GC–MS (Py-GC–MS) and thermally assisted hydrolysis and methylation (THM-GC–MS). The results showed that the soil DOM from the TGR area is a mixture of “allochthonous” (i.e., plant-derived/terrigenous) and “autochthonous” (i.e., microbial) origins. The terrigenous DOM is composed primarily of phenolic and aliphatic structures from lignin and aliphatic biopolymers (i.e. cutin, suberin), respectively. Multivariate statistics differentiated between two fractions of the microbial DOM, i.e. chitin-derived, perhaps from fungi and arthropods in soil, and protein-derived, partially sourced from algal or aquatic organisms. Molecular proxies of source and degradation state were in good agreement with optical parameters such as SUVA 254 , the fluorescence index (FI) and the humification index (HIX). The combined use of elemental analysis, fluorescence spectroscopy, and Py-GC–MS provides rigorous and detailed DOM characterization, whereas THM-GC–MS is useful for more precise but qualitative identification of the different phenolic (cinnamyl, p -hydroxyphenyl, guaiacyl, syringyl and tannin-derived) and aliphatic materials. With the multi-methodological approach used in this study, FTIR was the least informative, in part, because of the interference of inorganic matter in the soil DOM samples. The soil DOM from the TGR's water fluctuation zone exhibited considerable compositional diversity, mainly related to the balance between DOM source (microbial- or plant-derived), local vegetation and anthropogenic activities (e.g., agriculture). Finally, the relationship between DOM composition and its potential reactivity with substances of environmental concerns in the TGR area are also discussed. [ABSTRACT FROM AUTHOR]

Previous research has found total mercury (THg) and methylmercury (MeHg) levels increase with litterfall decay, thus suggesting litterfall decomposition plays an essential role in the biogeochemical transformation of mercury (Hg). However, it remains unclear how Hg accumulates in the decaying litter, how bacterial taxa networks vary and what roles various microorganisms play during litterfall decomposition, especially nitrogen (N)-fixing, MeHg-degrading and Hg-methylating microbes. Here, we demonstrated as degradation proceeded, a gradually-complex network evolved for litterfall bacteria for the subtropical mixed broadleaf-conifer (MBC) forest, whereas a relatively static network existed for the evergreen broadleaf (EB) forest. N-fixing and MeHg-degrading bacteria dominated throughout litterfall decomposition process, with relative abundances of N-fixing genera and nifH copies maximum and relative abundances of MeHg-degrading bacteria and merAB copies minimum in summer. Hence, N-fixing bacteria likely mediate THg increase in the decomposing litterfall, while MeHg enhancement may be regulated by aerobic MeHg-degrading microbes which can transform MeHg to inorganic divalent Hg (Hg2+) or further to elemental Hg (Hg0). Together, this work elucidates variations of N-fixing and MeHg-degrading microbes in decaying litterfall and their relationships with Hg accumulation, providing novel insights into understanding the biogeochemical cycle of Hg in the forest ecosystem. [Display omitted] • A gradually-complex network evolved for the decaying litterfall of MBC forest. • A relatively static network existed for the decomposing EB forest litterfall. • N-fixing bacteria may mediate the increase of THg levels in the decomposing litterfall. • MeHg enhancement is likely regulated by aerobic MeHg-degrading microbes. • Underlying soil is the source of litterfall THg, while MeHg is transported from litterfall to soil. [ABSTRACT FROM AUTHOR]