DOM influences Hg methylation in paddy soils across a Hg contamination gradient

Rice paddies provide optimum conditions for Hg methylation, and paddy soil is a hot spot for Hg methylation and the predominant source of methylmercury (MeHg) accumulated in rice grains. The role of dissolved organic matter (DOM) in controlling Hg bioavailability and methylation in rice paddy systems remains unclear. Paddy soils from eight various cultivation sites in China were chosen to investigate the variations in soil DOM and the influence of DOM concentration and optical characteristics on Hg methylation in rice paddy systems. In the present study, 151 rhizosphere soil samples were collected, and UV-Vis absorption and fluorescent spectroscopy were used to identify the optical properties of DOM. The relationship between MeHg and DOM's optical property indices revealed the production of MeHg consumes lower molecular weight DOM. Moreover, the correlation between DOM concentration and its optical characteristics highlighted the significant role of humic components on MeHg variability in paddy soil. Variation and correlation results demonstrated the allochthonous origin of DOM in the Hg-contaminated soil, with a higher molecular weight and humic character of DOM, as well as the dominant role of autochthonous DOM in promoting Hg methylation in uncontaminated soil. The current study indicated that soil organic matter and its dissolved fractions tend to limit Hg bioavailability and subsequently diminish MeHg production in contaminated paddy soils. Furthermore, the leading roles of allochthonous DOM in protecting MeHg from degradation and autochthonous DOM signatures in enhancing MeHg production in paddy soils. Overall, these findings provide insight into the correlative distributions of DOM and Hg along a Hg concentration gradient in paddy soil, thereby highlighting their potential role in controlling Hg bioavailability and regulating Hg methylation in the soil ecosystems.

nov., isolated from rice paddy soil

Three bacterial isolates, Gram-stain-negative, non-motile, coccobacilli-shaped bacteria, strains OP-27^T, OP-5 and OP-30, were isolated from rice paddy soil. Phylogenetic analyses based on 16S rRNA gene sequences revealed that three isolates belonged to the genus Roseomonas, showing the highest sequence similarities to Roseomonas sediminicola FW-3^T (98.1%) and Roseomonas lacus TH-G33^T (98.0%). The genome size of strain OP-27^T was 5.2 Mb in a single contig with DNA G+C content of 71.2%. The genome included 5164 predicted protein-coding genes, as well as 48 tRNA, 4 rRNA and 4 mRNA genes. The average nucleotide identity value between strain OP-27^T and type strains of related species of the genus Roseomonas were 81.1-83.1%, and the digital DNA-DNA hybridization values of strain OP-27^T and the related strains were 24.6-26.8%, respectively. The DNA-DNA hybridization values between strains OP-27^T, OP-5 and OP-30 were 84-100% and its closest relative, Roseomonas sediminicola KACC 16616^T was 21.1%. The major fatty acids were C_18:1 ω7c, C_18:1 2-OH and C_16:0 and predominant quinone was Q-10. Based on its distinctive phenotypic, phylogenetic, and chemotaxonomic characteristics, the three strains are considered to represent novel species of the genus Roseomonas, for which the name Roseomonas rosulenta sp. nov. is proposed. The type strain is OP-27^T (=KACC 21501^T= NBRC 114497^T).

Inhibition of methanogenesis leads to accumulation of methylated arsenic species and enhances arsenic volatilization from rice paddy soil

Flooded soils are important environments for the biomethylation and subsequent volatilization of arsenic (As), a contaminant of global concern. Conversion of inorganic to methylated oxyarsenic species is thought to be the rate-limiting step in the production and emission of volatile (methyl)arsines. While methanogens and sulfate-reducing bacteria (SRB) have been identified as important regulators of methylated oxyarsenic concentrations in anaerobic soils, the effects of these microbial groups on biovolatilization remain unclear. Here, microcosm and batch incubation experiments with an Arkansas, USA, rice paddy soil were performed in conjunction with metabolic inhibition to test the effects of methanogenic activity on As speciation and biovolatilization. Inhibition of methanogenesis with 2-bromoethanesulfonate (BES) led to the accumulation of methylated oxyarsenic species, primarily dimethylarsinic acid (DMAs(V)), and a four-fold increase in As biovolatilization compared to a control soil. Our results support a conceptual model that methanogenic activity suppresses biovolatilization by enhancing As demethylation rates. This work refines understanding of biogeochemical processes regulating As biovolatilization in anaerobic soil environments, and extends recent insights into links between methanogenesis and As metabolism to soils from the mid-South United States rice production region.

Biochar amendment reduced methylmercury accumulation in rice plants

There is growing concern about methylmercury (MeHg) accumulation in rice grains and thus enhanced dietary exposure to MeHg in Asian countries. Here, we explored the possibility of reducing grain MeHg levels by biochar amendment, and the underlying mechanisms. Pot (i.e., rice cultivation in biochar amended soils) and batch experiments (i.e., incubation of amended soils under laboratory conditions) were carried out, to investigate MeHg dynamics (i.e., MeHg production, partitioning and phytoavailability in paddy soils, and MeHg uptake by rice) under biochar amendment (1-4% of soil mass). We demonstrate for the first time that biochar amendment could evidently reduce grain MeHg levels (49-92%). The declines could be attributed to the combined effects of: (1) increased soil MeHg concentrations, probably explained by the release of sulfate from biochar and thus enhanced microbial production of MeHg (e.g., by sulfate-reducing bacteria), (2) MeHg immobilization in soils, facilitated by the large surface areas and high organosulfur content of biochar, and (3) biodilution of MeHg in rice grains, due to the increased grain biomass under biochar amendment (35-79%). These observations together with mechanistic explanations improve understanding of MeHg dynamics in soil-rice systems, and support the possibility of reducing MeHg phytoaccumulation under biochar amendment.

Selenium inhibits sulfate-mediated methylmercury production in rice paddy soil

There is increasing interest in understanding factors controlling methylmercury (MeHg) production in mercury-contaminated rice paddy soil. Sulfate has been reported to affect MeHg biogeochemistry under anoxic conditions, and recent studies revealed that selenium (Se) could evidently reduce MeHg production in paddy soil. However, the controls of sulfate and Se on net MeHg production in paddy soil under fluctuating redox conditions remain largely unknown. Microcosm experiments were conducted to explore the effects of sulfate and Se on net MeHg production in rice paddy soil. Soil was added with 0-960 mg/kg sulfate, in the presence or absence of 3.0 mg/kg selenium (selenite or selenate), and incubated under anoxic (40 days) or suboxic conditions (5 days), simulating fluctuating redox conditions in rice paddy field. Sulfate addition moderately affected soil MeHg concentrations under anoxic conditions, while reoxidation resulted in evidently higher (18-40%) MeHg levels in sulfate amended soils than the control. The observed changes in net MeHg production were related to dynamics of sulfate and iron. However, Se could inhibit sulfate-mediated MeHg production in the soils: Se addition largely reduced net MeHg production in the soils (23-86%, compared to the control), despite of sulfate addition. Similarly, results of the pot experiments (i.e., rice cultivation in amended soils) indicated that soil MeHg levels were rather comparable in Se-amended soils during rice growth period, irrespective of added sulfate doses. The more important role of Se than sulfate in controlling MeHg production was explained by the formation of HgSe nanoparticles irrespective of the presence of sulfate, confirmed by TEM-EDX and XANES analysis. Our findings regarding the effects of sulfate and Se on net MeHg production in rice paddy soil together with the mechanistic explanation of the processes advance our understanding of MeHg dynamics and risk in soil-rice systems.

Evaluating changes in cellulolytic bacterial population to explain methane emissions from air-dried and composted manure treated rice paddy soils

Compost application recorded ~20% reduction in methane (CH4) emission during rice cultivation as compared to air-dried manure treatment. The objective of this study was to evaluate the dependence of methanogens on cellulolytic bacteria (CB) to produce CH4 in organic-amended rice paddy soils. The presence of more decomposable organic C in manure was probably the key factor for higher CH4 emission from manure-treated soils as compared to compost application. Manure application facilitated anaerobic CB abundance in rice paddy soils, and that in turn increased concentrations of dissolved organic C compounds like carbohydrates in soil. Soluble organic C compounds are converted into acetate and/or carbon dioxide, which act as initial energy source for methanogens. Therefore, it could be concluded that CB positively influenced methanogen activity and methanogenesis and stabilized organic substrates like compost are more rational treatment to mitigate CH4 emission from rice paddy soil than cattle manure application.

Fractionation and characterization of humic acids from organic amended rice paddy soils

Humic acids (HAs), mixture of several complex-structured aromatic and aliphatic compounds, are ubiquitous in the terrestrial environment. In this study, HAs were fractionated on the basis of their polarity (degree of humification). The fractions namely 200P, 300P and 400P were comparatively more humified than fractions like 800P and 800S. The application of organic amendments significantly increased concentrations of HA fractions especially the more humified fractions in rice paddy soils. Among the different fractions, 300P fraction was the most abundant in rice paddy soils and its concentration was significantly increased due to compost application. The compounds like methyl ketone and 3-phenylprop-2-enal contributed almost 90% of 300P fraction.