Effects of acid mine drainage on photochemical and biological degradation of dissolved organic matter in karst river water

Dissolved organic matter(DOM)can be removed or transformed by photochemical and biological processes,producing the negative effect of transforming organic carbon into inorganic carbon,which plays a vital role in the karst carbon cycle.However,acid mine drainage(AMD)will affect this process,so the degradation of DOM in karst river water(KRW)needs to be studied in this context.In this study,to reveal the evolution processes of DOM under photochemical and biological conditions in AMD-impacted KRW,AMD and KRW were mixed in different ratios under conditions of visible light irradiation(VL),biodegradation(BD),ultraviolet irradiation(UV)and ultraviolet irradiation+biodegradation(UV+BD).The average DOC concentrations in samples after mixing AMD and KRW in different proportions decreased significantly(by 23%)in UV+BD,which was 1.2–1.4 times higher than under the other conditions and would lead to a significant release of inorganic carbon.Further analysis of the fluorescence parameters via parallel factor analysis(PARAFAC)revealed that the DOM fluorescence components in AMD comprised mainly protein-like substances derived from autochthonous components,while the DOM fluorescence components in KRW were mainly humic-like substances with both autochthonous and allochthonous sources.Therefore,AMD could promote both the photochemical and biological degradation of DOM in karst receiving streams,resulting in the conversion of DOC to inorganic carbon.The results showed that the synergistic effects of UV+BD and AMD accelerated the degradation of DOM and the release of inorganic carbon in KRW,thus affecting the stability of the karst carbon cycle.

Elevated methylmercury production in mercury-contaminated soil and its bioaccumulation in rice:key roles of algal decomposition

Algal-derived organic matter(AOM)regulates methylmercury(MeHg)fate in aquatic ecosystems,whereas its role in MeHg production and bioaccumulation in Hg-contaminated paddies is unclear.Pot and microcosm experiments were thus performed to understand the response characteristics of MeHg concentrations in soil and rice in different rice-growing periods to algal decomposition.Compared to the control,algal decomposition significantly increased soil water-soluble cysteine concentrations during the rice-tillering and grain-filling periods(P<0.05).It also significantly lowered the molecular weight of soil-dissolved organic matter(SDOM)during the rice-tillering period(P<0.05)and SDOM humification/aromaticity during the grain-filling period.Compared to the control,AOM input increased the abundance of potential Hg and non-Hg methylators in soil.Furthermore,it also greatly increased soil MeHg concentrations by 25.6%-80.2%and 12.6%-66.1%during the rice-tillering and grain-filling periods,with an average of 42.25%and 38.42%,respectively,which were significantly related to the elevated cysteine in soil and the decrease in SDOM molecular weight(P<0.01).In the early stage(within 10 days of microcosm experiments),the MeHg concentrations in decayed algal particles showed a great decrease(P<0.01),suggesting a potential MeHg source in soil.Ultimately,algal decomposition greatly increased the MeHg concentrations and bioaccumulation factors in rice grains,by 72.30%and 16.77%,respectively.Overall,algal decomposition in Hg-contaminated paddies is a non-negligible factor promoting MeHg accumulation in soil-rice systems.

Effects of fulvic acid and humic acid from different sources on Hg methylation in soil and accumulation in rice

Humus is often used as an organic modifier to reduce the bioaccumulation of heavy metals in plants, but the effects of different humus components from different sources on the fate of mercury(Hg) in paddy fields are still unclear. Here, fulvic acid(FA) and humic acid(HA) extracted from composted straw(CS), composted cow dung(CCD), peat soil(PM) and lignite coal(LC) were used to understand their effects on the methylation and bioaccumulation of Hg in paddy soil by pot experiments. Amendments of both FA and HA largely increased the abundance of Hg-methylating microbes and low-molecular-weight organic matters(e.g, cysteine) in paddy soil. They were also found to change the aromaticity, molecular size and Chromophoric DOM concentration of DOM, and resulted in heterogeneous effects on migration and transformation of Hg. All the FA-amended treatments increased the mobility and methylation of Hg in soil and its absorption in roots. Nevertheless, FA from different sources have heterogeneous effects on transport of Hg between rice tissues. FA-CCD and FA-PM promoted the translocation of Me Hg from roots to rice grains by 32.95% and 41.12%, while FA-CS and FA-LC significantly inhibited the translocation of inorganic Hg(IHg) by 52.65% and 66.06% and of Me Hg by 46.65% and 36.23%, respectively. In contrast, all HA-amended treatments reduced the mobility of soil Hg, but promoted Hg methylation in soil. Among which, HA-CCD and HA-PM promoted the translocation of Me Hg in rice tissues by 88.95% and 64.10%, while its accumulation in rice grains by 28.43% and 28.69%, respectively. In general, the application of some FA and HA as organic modifiers to reduce Hg bioaccumulation in rice is not feasible.