Wetland is one of the main sources for N₂O emission. Accurate assessments and modelling of N₂O emissions from wetlands are thus important but have remained elusive. A major reason is the high temporal and spatial variability in N₂O emission rates, the kinetics and controlling factors are largely unknown. Although many studies found N₂O emissions in wetlands were related to pH, temperature and C/N, the impact of vegetation on N₂O emission from wetland and its controlling factor are still unclear. In fact, vegetation can affect the physical and chemical conditions and the microbes in soil, thus strongly control N₂O emission in wetland. This study monitored the kinetics of N₂O emissions from wetland soils sampled from different vegetation zones, and tried to answer the following questions 1) what’s the main physical and chemical factors for N₂O emissions in different vegetation zones, 2) what microorganisms are responsible for N₂O emission. The results showed C/N may be the main controlling factor for N₂O emission in different vegetation zones, Proteobacteria seemed to be responsible for the majority of N₂O emission. These results increase our understanding on the kinetics and controlling factor of N₂O emission from wetland in different vegetation zones, and are useful in assessments and predictions of N₂O emissions in wetlands.

Arsenic(As) is a pervasive environmental toxin and carcinogenic metalloid. Arsenic pollution in aquatic environments is a concern due to its high toxicity and chronic effects on human health. This concern has generated increasing interest in the use of different treatment technologies to remove arsenic from contaminated water. Wetlands are a cost-effective natural system successfully used for removing various pollutants, and they have shown capability for removing arsenic. In this study, small-scale microcosm experiments were performed to simulate a natural wetlands system contaminated with dimethyl arsenate. Monitoring of As distribution in the aqueous and solid phases demonstrated that the amended dimethyl arsenate was completely removed from the aqueous phase after 23 days and most of the arsenic species associated with sediment were identified as inorganic As(V). These results imply that demethylation occurs at the interface between sediment and water, and the transformation of As speciation may be attributable to the microbial activity. The demethylated As(V) seemed to be adsorbed on wetland sediments, and the As(V) was reduced to As(III) in the deep layers of the wetland sediment with predominantly reducing conditions. Further, mass balance of arsenic in the microcosm was calculated based on the total concentrations of arsenic measured in water, sediments, and biomass, demonstrating an important role of wetland sediment as an efficient sink of As.

Despite presenting in natural environments at trace level, elevated concentration of antimony (Sb) would still occur through anthropogenic activities. High concentration of Sb in soils and waters is considered as environmental contaminant and human health hazard. Once entered the environment, Sb would undergo various processes and result in its speciation. The toxicity of Sb highly depends on the species and is regulated by redox reactions. Research since early 2000s has established an initial understanding of the biotic and abiotic redox of Sb in soils and water. In the case of biotic Sb redox studies, multiple strains of Sb(III)-oxidizing strains of bacteria with various oxidation rates have been identified. However, to date only four species of Sb(V)-reducing species were listed in literatures. The research on discovering novel Sb(V)-reducing strains possess critical values in expanding the knowledge in Sb cycle, as well as in designing Sb bioremediation strategies. Our preliminary study with microbial community enriched from soils collected from Sb refinery sites showed complete reduction of 2 mM Sb(V) within 10 days in the presence of 2 mM acetate as an electron donor. 16S rDNA analysis indicated that the enriched community is mainly composed of anaerobic bacteria closely related to Rhodoferax spp (20%)., Shewanella spp (65%)., etc. During the enrichment, white precipitates, likely to be Sb(III) oxides (e.g., Sb2O3), were also observed, and may serve as the evidence of Sb(V) reduction. In this presentation, a specific strain responsible for Sb(V) reduction, the Sb(V) reduction products, and the mechanism of Sb(V) reduction by the strain will be discussed.

The microbially mediated redox reaction has a close association with elemental cycling in the natural environment however, the mineralogical assessments on the biomineralization and its mechanism have not been fully understood. The sediment and hot-spring water were collected from active acid-sulfate-chloride geothermal spring of Norris Geyser Basin area and incubated at in-situ temperature and pH condition to investigate the biomineralization associated with biotic/abiotic Fe and As redox cycle. It is interesting to note that oxyanion of arsenic (As) would be rapidly reduced abiotically to arsenate via As-S complex at low pH condition (~4.0). Nonetheless, the concentration of arsenate increased only for the enriched samples for the first 6-month of incubation suggesting that the oxidative phase of As was supplied by microbial As oxidation. Furthermore, the neo-formed mineral pharmacosiderite (KFe₄(AsO₄)₃(OH)₄·6-7H₂O) has arsenate as well as ferric Fe that abiotic Fe(II) oxidation is slow in acidic condition, but could be mediated by chemolithoautotrophs. These reactions are supported by pyrosequencing analysis results. The mechanism and implications to the natural environments will be further discussed.

Liquid fuel products leaked from underground storage tanks (UST) and pipelines have led to increasing contamination of soils and groundwater. Total petroleum hydrocarbons (TPH) are of particular concern and composed of many compounds including alkanes, aromatic hydrocarbons, methyl t-butyl ether, ethanol, butanol, sulfur, and nitrogen. Many of these compounds are toxic to microbes, plants, and animals, and can be a source of long-term soil and groundwater contamination. The regulatory guidelines governing TPH in soil and groundwater near residential areas in South Korea are 500 mg kg^-1 and 1.5 mg L^-1, respectively. As a remedial option for TPH contaminated soils, aerobic TPH biodegradation (e.g., landfarming) is well-known, but anaerobic TPH degradation in situ is largely unknown. This study investigates physicochemical and microbiological factors contributing to natural attenuation of TPH in subsurface environments. Three soil cores with 6 m length were collected from the closed military base in South Korea where TPH has been released from UST since 1960s. Soil TPH, total Fe(II), and bacterial abundance were determined at each one meter interval. TPH concentrations in soil samples varied with depth and were observed up to 3,000 mg kg^-1. Strong correlations between total Fe(II) and TPH concentrations (r² = 0.886, p<0.001) were observed. TPH concentrations were also significantly correlated with the number of total bacteria (r²=0.494, p<0.05) and oil degrading bacteria (r²=0.561, p<0.01), respectively, suggesting TPH was utilized by indigenous bacteria. These results imply that TPH oxidation coupled with O₂ reduction as well as dissimilatory Fe(III) reduction has been occurred in the subsurface environment of the military base. Microbial community and quantitative PCR analysis will confirm the natural attenuation of TPH by microbial Fe(III) reduction in subsurface environments.

Distinct microbial community diversity and structure has been observed in various Earth’s critical zones (i.e., near-surface environments) with soil depth. Here we investigated microbial community compositions and geochemical properties along a soil vertical profile spanning 0 to 40 m depths including unsaturated zone, capillary fringe, and saturated zones in a testbed site formerly used as a farmland for several decades. Based on the results of 16S-rRNA community analyses, microbial diversity showed unique patterns along the soil vertical profile. Alpha-diversity was highest in the top soil layer and was also significantly higher in the capillary fringe than in unsaturated and saturated zones. At the phylum level, Proteobacteria, Actinobacteria, Acidobacteria, and Chloroflexi were more abundant in the top soil layer, while Bacteroidetes, Firmicutes, Actinobacteria, and Proteobacteria showed relatively high abundance in the capillary fringe zone. Redundancy correlation analyses (RDA) showed that the community in the soil samples was significantly affected by NO₃ and SO₄ in the unsaturated zone; chromium in the capillary fringe; vanadium in the saturated zone (P<0.05, respectively). Microbial numbers and community compositions were also significantly different in between soil and groundwater samples likely because microorganisms attached to the solid particles might better access to nutrients than those suspended in aquifer. The abundance of Acidobacteria, Firmicutes, Gemmatimonadetes, Betaproteobacteria, and Gammaproteobacteria was much higher in soil samples compared to in groundwater samples. Microbial numbers in soil samples by Most Probable Numbers were 30 ~ 400 times higher than those in groundwater samples. This study will discuss potential roles of microbial community structure and functional diversity on geochemical properties in each critical zone.