Assessment of heavy metal pollution and the effect on bacterial community in acidic and neutral soils
Introduction
As metal mining and smelting activities continue to increase, environmental concerns have been raised due to amounts of heavy metals entering surface water, soil, and sediment (Kodirov et al., 2017, Slack and Voulvoulis, 2006). Heavy metals, as non-biodegradable factors, can persist in the soil for a long time and present persistent toxicity to ecosystems (Khan et al., 2009, Tang et al., 2019, Xiao et al., 2017). The serious metal pollution on soil ecosystems has increased the urgency of heavy metal remediation in the environment (Li et al., 2017b).
Microorganisms play a key role in promoting soil ecosystem services and functions, as they participate in organic matter decomposition and nutrient cycling (Giller et al., 1998). However, an increasing body of evidences suggest that microorganisms respond quickly and sensitively to environmental changes and disturbances, especially in the presence of widespread contaminants (Ivshina et al., 2014, Tang et al., 2019). Heavy metals have a profound impact on the microbial biomass, activity, diversity, and microbial community structure (Ivshina et al., 2014, Liao and Xie, 2007, Wang et al., 2007). However, the acidic wastewater discharged during the mining process causes more stress for microorganisms, as they have to survive in an environment polluted with both heavy metals and acidic wastewater, thus impacting more on the soil nutrient cycling mediated by microorganisms (Pereira et al., 2014). It has been suggested that the toxicity of heavy metals is affected by both their concentrations in soils and their bioavailability. Soil pH affects the adsorption of metals by substances in the soil, such as organic matter, by changing the surface charge and ionizability of heavy metal adsorbents (Bang and Hesterberg, 2004), which then affect the bioavailability and toxicity of heavy metals to microorganisms.
Heavy metals, soil, and bacteria interact in complex ways. The microbial community structure is strongly affected by heavy metals in moderately, and severely contaminated soils (Li et al., 2017a). Bang and Hesterberg (2004) characterized trace element dissolution in soil (pH 4–8) and found that the content of dissolved copper, zinc and lead in soil increased significantly with the decrease of pH value. Less Hg was absorbed by humic substances under lower pH values due to proton competition. Furthermore, the soil bacterial community is also highly correlated with the soil properties. Jiang et al. (2018) reported that the soil properties were more influential, especially the soil pH, than heavy metal concentrations. However, most previous studied on the relationships among heavy metals, soil properties, and microorganisms have focused on normal soils or extremely acidic environments. Less is known about the differences between microbial communities in acidic and neutral soils and the different responses of microorganisms to heavy metals.
The Xikuangshan (XKS) mine area in Hunan has been severely damaged by mining activities (Zhang et al., 2016). In this study, we investigated acidic and neutral areas in XKS by measuring the bacterial community and aimed to (1) assess the heavy metal potential ecological risk in XKS, (2) investigate differences between the impacts of heavy metals on bacterial communities in acidic and neutral soils and the main stressors on soil bacteria, and (3) explore the differences in bacterial resistance to heavy metals in acidic and neutral environments to provide a reference for heavy metal remediation in different pH levels.
Section snippets
Sample collection
The research area is an Sb mine located north of Lengshuijiang City (27°30′49″–27°50′38″N, 111°18′57″–111°36′40″E), Hunan Province, China. The mine, which covers approximately 30 km2 of mountainous area, is one of the largest Sb mines in the world and has been referred to as the “World’s antimony capital” (Tan et al., 2018). This Sb mine was discovered in the late Ming dynasty and currently has 40 million tons of Sb reserves (Qi et al., 2011). Mining and ore smelting have produced a large
Environmental characteristics
The heavy metal contents (Table 2) and physicochemical properties (Table 3) of soil were determined at all sampling sites. The concentrations of Sb (14.17–383.62 mg/kg), As (61.00–184.23 mg/kg), Cr (144.80–302.44 mg/kg) and Zn (102.93–230.45 mg/kg) exceeded the background values in China (CNEMC, 1990) at all sampling sites. Sb and As were the two most abundant heavy metal pollutants, with minimum concentrations exceeding the background values in China (CNEMC, 1990) by 9.14 and 5.16 times and
Heavy metal contamination in Xikuangshan
Due to mining activity, the soil around XKS has been severely polluted not only with Sb, and As, but also with Pb, Cu, Cr, Zn and Cd. We assessed the ecological risk posed by these heavy metals, and investigated the structure and diversity of bacterial communities under conditions of heavy metal contamination in acidic and neutral environments.
The XKS mine was heavily contaminated with heavy metals. The levels of Sb and As varied greatly among sites, ranging from 14.17 to 383.62 and
Conclusion
The environment around XKS mine has been seriously polluted by heavy metals through hundreds of years of mining activity. The potential ecological index of heavy metals was used to evaluate the heavy metal pollution in XKS mine, and the microbial community structure variations in acid and neutral soils under heavy metal pollution were investigated. We found that, due to the differences between the properties of heavy metals and the physical and chemical properties of the soil, the effect of
CRediT authorship contribution statement
Yongjiao Ma: Conceptualization, Writing - original draft, Formal analysis. Yating Wang: Formal analysis, Software. Qi Chen: Software. Yongshuang Li: Writing - review & editing. Dongchu Guo: Writing - review & editing. Xiaohong Nie: Investigation. Xiawei Peng: Conceptualization, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (2017YFC0505500, 2017YFC0505504) and Beijing Municipal Science and Technology Commission Program (Z151100002115006).The authors are grateful for the support from Hunan Academy of Forestry. The authors also appreciated Fangping Tong for technical support.
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