Ecological drivers of methanotrophic communities in paddy soils around mercury mining areas.

Methanotrophs play a crucial role in mitigating methane (CH4) emission by oxidizing produced CH4 in paddy soils; however, ecological drivers of methanotrophic community in the soils around heavy metal contaminated areas remain unclear. In this study, we evaluated the effects of heavy metal pollution and soil properties on the abundance, diversity and composition of methanotrophic community in paddy soils from two typical mercury (Hg) mining regions in southwest China. The results of random forest and structure equation models suggest that both heavy metal content and soil nutrients greatly influenced the attributes of methanotrophic community. In general, the abundance and diversity of methanotrophs were negatively related to soil Hg content, but showed positive correlation with soil organic carbon content. However, the other metals (cadmium (Cd), nickel (Ni), lead (Pb), arsenic (As), zinc (Zn)) had inconsistent associations with the microbial indexes of methanotrophic community in the soil. Elevated levels of heavy metal and nutrients in the soils shifted the community composition of methanotrophs. For example, Pb, As and Zn contents had negative associations with the relative abundance of Methylocaldum. In addition, changes in the relative abundance of ecological clusters within the co-occurrence network of methanotrophs were related to metal contents and soil properties. Together, our findings provide novel insights into understanding ecological drivers of methanotrophic community in paddy soils around Hg mining regions, with important implications for mitigating CH4 emissions in terrestrial ecosystems.

[Adsorption Properties of Fluorine onto Fulvic Acid-Bentonite Complex].

Fulvic Acid-Bentonite (FA-BENT) complex was prepared using coprecipitation method, and basic properties of the complex and sorption properties of fluorine at different environmental conditions were studied. XRD results showed that the d001 spacing of FA- BENT complex had no obvious change compared with the raw bentonite, although the diffraction peak intensity of smectite in FA-BENT complex reduced, and indicated that FA mainly existed as a coating on the external surface of bentonite. Some functional groups (such as C==O, −OH, etc. ) of FA were observed in FA-BENT FTIR spectra, thus suggesting ligand exchange-surface complexation between FA and bentonite. Higher initial pH values of the reaction system were in favor of the adsorption of fluorine onto FA-BENT, while the equilibrium capacity decreased with the increase of pH at initial pH ≥ 4.50. The adsorption of fluorine onto FA-BENT was also affected by ionic strength, and the main reason might be the "polarity" effect. The adsorption of fluorine onto FA-BENT followed pseudo-second-order kinetic model and was controlled by chemical process ( R² = 0.999 2). Compared with the Freundlich model, Langmuir model was apparently of a higher goodness of fit (R² > 0.994 9) for absorption of fluorine onto FA-BENT. Thermodynamic parameters indicated that the adsorption process of fluorine was an spontaneously endothermic reaction, and was an entropy-driven process (ΔH 32.57 kJ · mol⁻¹, ΔS 112.31 J · (mol · K)⁻¹, ΔG −0.65- −1.76 kJ · mol⁻¹).