Search

Rice plants accumulate Hg from the soil and ambient air, however, evaluating the contribution of Hg from these two sources remains challenging. Here, we proposed a practical method to predict the contribution of total gaseous mercury (TGM) to Hg in white rice in Wanshan Hg mine area (WMM). In this study, rice was planted in the same low-Hg soil at different sites of WMM with varying TGM levels. Comparing to the control sites at IG (Institute of Geochemistry, Guiyang), TGM is the dominant source of Hg in rice leaves and white rice at TB (Tianba) and ZJW (Zhangjiawan) sites of WMM. Subsequently, a good correlation between the Hg concentrations in rice leaves and the concentration contributions of TGM to Hg in white rice was obtained. Such a correlation enabled feasible quantification of the contribution of TGM to Hg in white rice collected from the Wanshan Hg mine. The contribution of TGM to Hg in white rice across the WMM area was also estimated, demonstrating that white rice receives 14–83% of Hg from the air. Considering the high contribution of TGM to Hg in white rice, we compared the relative health risks of Hg via inhalation and rice consumption and found that inhalation, rather than rice consumption, was the major pathway for bioaccessible Hg exposure in adults at high-TGM sites. This study provides new knowledge of Hg biogeochemistry in Hg-mining areas. [Display omitted] • Leaf THg is used to predict the contribution of TGM to IHg in white rice at WMM. • TGM contributes to 14%–83% of IHg in rice in the WMM area. • Inhalation is the major pathway for bioaccessible THg intake at the high-TGM sites. [ABSTRACT FROM AUTHOR]

Recent studies show that the carbon sink attributable to the weathering of carbonate rocks may have been greatly underestimated if the biological carbon pump (BCP) effect in transferring dissolved inorganic carbon (DIC) to organic carbon (autochthonous OC) by aquatic photoautotrophs is neglected. The uptake of DIC by aquatic photoautotrophs may reach 0.2 to 0.7 Pg C/a globally, indicating that the carbon sink by the coupled carbonate weathering with aquatic photosynthesis mechanism (CCW) may be an important control in climate change. In order to understand the sensitivity of the CCW carbon sink to changes of climate and land-use, a systematic study of modern trap and 100-year-long core sediments was conducted in Fuxian Lake, (Yunnan, SW China), the second-deepest plateau oligotrophic freshwater lake in China. It was found that (1) the autochthonous OC in the lake sediments was characterized by lower C/N ratios and higher δ13C org. By means of an n-alkanes compound calculation, the proportions of autochthonous OC were determined to be in the range, 60–68% of all OC; (2) increase in the autochthonous OC accumulation rate (OCAR auto) was accompanied by an increase in the inorganic carbon accumulation rate (ICAR) in both the trap and core sediments. In particular, the post-1950 OCAR auto was estimated to be about 6.9 times that for the period, 1910–1950; (3) OCAR auto in core sediments increased significantly with global warming and land-use change, from 1.06 g C m−2 yr−1 in 1910 to 21.74 g C m−2 yr−1 in 2017. The increasing carbon sink may act as a negative feedback on global warming if the trend holds for all lakes globally. This study is the first to quantify the burial flux of organic carbon generated by the BCP effect in lakes and may contribute to solving the problem of the missing carbon sink in the global carbon cycle. Unlabelled Image • The ratios of AOC were determined by n-alkanes compound calculation. • They ranged from 47 to 84%, with an average of 63%. • The post-1950 AOC burial rate was about 7 times that for the period before. • The sharp increase in the burial rate was chiefly related to land-use changes. • It may contribute significantly to the missing sink in the global carbon cycle. [ABSTRACT FROM AUTHOR]

Abstract Separating watershed evapotranspiration into its evaporation and transpiration components is important for calculating the carbon that is assimilated by terrestrial vegetation in carbon cycle studies. The key step in this separation is to quantify the evaporation component. The deuterium excess (d-excess) in meteoric water has been shown to be an important indicator of both the original source of the water vapor and the humidity at the vapor source area. It has also shown promise for use in investigating the evaporation losses. While many studies have used the δD/δ18O method to study watershed evaporation, few have discussed the differences between the δD/δ18O (single isotope system) and d-excess (dual isotope system) methods in quantifying watershed evaporation. Given the complexity of natural watersheds, the Shawan Test Site was established at Puding, China, to study the water cycle in five concrete tanks (simulated watersheds) with different land uses over one hydrologic year. There were no plants in two of the tanks (bare rock and bare soil), which allowed verification of evaporation calculations derived from the d-excess and δD/δ18O methods. δD or δ18O values of precipitation in the rainy season, when most of the groundwater recharge occurs, showed great variability. In contrast, the d-excess of the meteoric waters collected during the same rainy season was much more stable than the δD or δ18O values. We quantified the annual evaporative loss of the five watersheds using both methods. Comparison of the results indicated that the d-excess method is more acceptable than the δD/δ18O method due to the stability of d-excess. Calculated ratios of transpiration to evapotranspiration in three tanks planted with vegetation were 56.8% in cultivated land, 70.9% in shrub land, and 85.9% in grassland, demonstrating that in well vegetated watersheds, this component of the cycle is controlled chiefly by plant transpiration. Land use has an important impact on the hydrologic cycle in a watershed, and the d-excess calculations conducted in this study provide new insights for quantifying components of the cycle, especially in the East Asian monsoon region which has rainfall with a large range in δD or δ18O values. [ABSTRACT FROM AUTHOR]

• Diel lake hydrochemical variations and the coupled carbon cycling were observed. • These processes were controlled mainly by lake metabolic processes. • High DIC in a karst lake may support high primary production. • The carbon sink by the coupled carbonate weathering reached 650–704 t C km−2 yr−1. • A karst aquatic system can maintain net autotrophy. The precipitation of carbonate minerals does not invariably result in CO 2 emission to the atmosphere, because dissolved inorganic carbon (DIC) can be partially utilized by terrestrial aquatic phototrophs, thus generating an autochthonous organic carbon (AOC) sink. However, little is known about the potential effects of this mechanism on carbon cycles in DIC-rich lakes, mainly due to the lack of detailed documentation of the related processes, which limits our ability to accurately evaluate and predict the magnitude of this carbon sink. We conducted field observations in Fuxian Lake, a large and representative karst lake in the Yunnan-Guizhou Plateau, SW China. Continuous diel monitoring was conducted to quantitatively assess the coupled relationship between lake metabolism and DIC cycling and its influence on the carbonate weathering-related CO 2 sink. We found that the diel physicochemical variations and isotopic characteristics were mainly controlled by the metabolism of aquatic phototrophs, evidenced by a significant relationship between net ecosystem production and diel DIC cycling, and demonstrating the significance of DIC fertilization in supporting high primary production in karst lakes. The data showed that a reduction in photosynthesis occurred in the afternoon of almost every day, which can be explained by the lower CO 2 /O 2 ratio that increased the potential for the photorespiration of aquatic plants, thus reducing photosynthesis. We found that a net autotrophic ecosystem prevailed in Fuxian Lake, suggesting that the lake functions more as a sink than a source of atmospheric CO 2. Considering carbonate weathering, the estimated AOC sink amounted to 650–704 t C km−2 yr−1, demonstrating both the potentially significant role of metabolism in lacustrine carbon cycling and the potential of the combination of photosynthesis and carbonate weathering for carbon sequestration. Our findings may help to quantitatively estimate the future impact of lake metabolism on carbon cycling, with implications for formulating management policies needed to regulate the magnitude of this carbon sink. [Display omitted] [ABSTRACT FROM AUTHOR]