Microbes influence the fractionation of arsenic in paddy soils with different fertilization regimes

Potentially toxic elements in the environment - a review of sources, sinks, pathways and mitigation measures

Pollution of ecosystems with potentially toxic elements (PTEs) has become a global problem with serious consequences for public health. The PTEs are hazardous to humans owing to their longevity, toxicity, and ability to accumulate in the biotic environment. As most PTEs cannot be degraded microbially or chemically, they can persist in soils for a long time. Besides posing a threat to landsphere, they may be transported to surrounding environmental spheres through movement of water, atmospheric circulation, and biological transmission. This can severely affect the ecological equilibrium. Accumulation of PTEs in soils pose serious health hazards to higher organisms leading to various diseases and disorders and significant relationships exist between the occurrence of PTEs and the toxic effects in humans. In natural soils, PTEs accumulate due to weathering of rocks and ores. Furthermore, locally or regionally significant accumulation of PTEs in soils may occur from industrial goods, pesticides and paints, municipal and industrial waste, fertilizer application, mining activities and atmospheric deposition. In response to the growing need to address PTE contamination, remediation methods have been developed employing mechanical, physico-chemical or biological based technologies. In this review, we discuss sources, sinks, pathways and mitigation measures related to natural and anthropogenic PTEs. We focus on As, Cd, Cr, Hg and Pb which are highly toxic and perform no physiological functions in biota. Further, these are the most widely studied PTEs.

Arsenate Adsorption on Different Fractions of Iron Oxides in the Paddy Soil from the Karst Region of China

Adsorption of arsenate, a common soil contaminant, on natural occurring iron oxides controls arsenate adsorption on soils, especially on iron rich soils. However, soils contain multiple iron minerals and their contributions for arsenate adsorption on the bulk soil are unclear. In this study, Wenzel's sequential extraction procedure (SEP) was used to characterize different fractions of iron oxides in the paddy soils from Guangxi, China. Soils treated with different extractants in the SEP (i.e. soils contained different iron fractions) were used to study arsenate adsorption on different iron fractions in the soil. Langmuir model was used to assess arsenate adsorption capacities of those iron fractions. The results showed that though amorphous iron made up only 16% of the total iron in the soil, it accounted for 82% of the soil adsorption capacity. Besides, arsenate adsorption capacity on the soils could be estimated by linear addition of arsenate adsorption capacity on the amorphous iron oxides fraction and that on the other iron oxides fraction. This study implied a possibility to use iron fractions characterized by Wenzel's SEP to evaluate arsenate adsorption on the bulk soils.

Co-selection of antibiotic-resistant bacteria in a paddy soil exposed to As(III) contamination with an emphasis on potential pathogens

The increased acquisition of antibiotic resistance by pathogens is a global health concern. The environmental selection of antibiotic resistance can be caused by either antibiotic residues or co-selecting agents such as toxic metal(loid)s. This study explored the potential role of As(III) as a co-selecting driver in the spread of antibiotic resistance in paddy soils. By applying high-throughput sequencing, we found that the diversity and composition of soil microbial communities was significantly altered by As(III) exposure, resulting in an increased proportion of potential pathogens (9.9%) compared to the control soil (0.1%). Meanwhile, a total of 46 As(III)-resistant isolates were obtained from As(III)-exposure soil, among which potential pathogens accounted for 54.3%. These As(III)-resistant bacteria showed a high incidence of resistance to sulfanilamide (100%) and streptomycin (88-93%). The association between antibiotic and As(III) resistances was further investigated in a potentially pathogenic isolate by whole-genome sequencing and a transcription assay. The results showed that As(III) and antibiotic resistance genes might co-occur in a mobile genomic island and be co-regulated by As(III), implying that antibiotic resistance could be co-selected by As(III) via co-resistance and co-regulation mechanisms. Overall, these results suggest that As(III) exposure provides a strong selective pressure for the expansion of soil bacterial resistome.

Arsenic Release from Foodstuffs upon Food Preparation

In this study the concentration of total arsenic (As) and arsenic species (inorganic As, arsenobetaine, dimethylarsinate, and methylarsonate) was monitored in different foodstuffs (rice, vegetables, algae, fish, crustacean, molluscs) before and after preparation using common kitchen practices. By measuring the water content of the foodstuff and by reporting arsenic concentrations on a dry weight base, we were able to distinguish between As release effects due to food preparation and As decrease due to changes in moisture content upon food preparation. Arsenic species were released to the broth during boiling, steaming, frying, or soaking of the food. Concentrations declined with maxima of 57% for total arsenic, 65% for inorganic As, and 32% for arsenobetaine. On the basis of a combination of our own results and literature data, we conclude that the extent of this release of arsenic species is species specific, with inorganic arsenic species being released most easily, followed by the small organic As species and the large organic As species.

The response of ammonia-oxidizing microorganisms to trace metals and urine in two grassland soils in New Zealand

An incubation experiment was conducted to investigate the response of ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and the nitrification rate to the contamination of Cu, Zn, and Cd in two New Zealand grassland soils. The soils spiked with different concentrations of Cu (20 and 50 mg kg^-1), Zn (20 and 50 mg kg^-1), and Cd (2 and 10 mg kg^-1) were incubated for 14 days and then treated with 500 mg kg^-1 urine-N before continuing incubation for a total of 115 days. Soils were sampled at intervals throughout the incubation. The nitrification rate in soils at each sampling period was determined, and the abundance of AOB and AOA was measured by real-time quantification polymerase chain reaction (qPCR) assay of the amoA gene copy numbers. The results revealed that moderate trace metal stress did not significantly affect the abundance of AOB and AOA in the two soils, probably due to the high organic matter content of the soils which would have reduced the toxic effect of the metals. Nitrification rates were much greater and the observable nitrification period was much shorter in the dairy farm (DF) soil, in which the AOB and AOA abundances were greater than those of the mixed cropping farm (MF) soil. AOB were shown to grow under high nitrogen conditions, whereas AOA were shown to grow under low N environments, with different metal concentrations. Therefore, nitrogen status rather than metal applications was the main determining factor for AOB and AOA growth in the two soils studied.

Impact of temperature on the aging mechanisms of arsenic in soils: fractionation and bioaccessibility

The present study focused on the influence of temperature variation on the aging mechanisms of arsenic in soils. The results showed that higher temperature aggravated the decrease of more mobilizable fractions and the increase of less mobilizable or immobilizable fractions in soils over time. During the aging process, the redistribution of both carbonate-bound fraction and specifically sorbed and organic-bound fraction in soils occurred at various temperatures, and the higher temperature accelerated the redistribution of specifically sorbed and organic-bound fraction. The aging processes of arsenic in soils at different temperatures were characterized by several stages, and the aging processes were not complete within 180 days. Arsenic bioaccessibility in soils decreased significantly by the aging, and the decrease was intensified by the higher temperature. In terms of arsenic bioaccessibility, higher temperature accelerated the aging process of arsenic in soils remarkably.

Evaluation of potential effects of soil available phosphorus on soil arsenic availability and paddy rice inorganic arsenic content

The transfer of arsenic from paddy field to rice is a major exposure route of the highly toxic element to humans. The aim of our study is to explore the effects of soil available phosphorus on As uptake by rice, and identify the effects of soil properties on arsenic transfer from soil to rice under actual field conditions. 56 pairs of topsoil and rice samples were collected. The relevant parameters in soil and the inorganic arsenic in rice grains were analyzed, and then all the results were treated by statistical methods. Results show that the main factors influencing the uptake by rice grain include soil pH and available phosphorus. The eventual impact of phosphorus is identified as the suppression of As uptake by rice grains. The competition for transporters from soil to roots between arsenic and phosphorus in rhizosphere soil has been a dominant feature.

Assessing the combined risks of PAHs and metals in urban soils by urbanization indicators

We quantitatively describe the impacts of urbanization on the accumulation of polycyclic aromatic hydrocarbons (PAHs) and heavy metals (HMs) in urban soils as well as their health risks to residents. Residential building age, population density, road density, and distance from urban center were used as urbanization level indicators. Significant correlations were found between those urbanization indicators and the amounts of PAHs, Cu, Cd, Pb, Zn and As in residential soils. The exposure time of soils to urban air was the primary factor affecting soil pollution, followed by local road density and population density. Factor analysis suggested that 59.0% of the elevated pollutant concentrations were caused by citywide uniform deposition, and 15.3% were resulted from short-range deposition and/or non-combustion processes. The combined health risks posed by soil PAHs and HMs were aggravated with time and can be expressed as functions of residence age, road density, and other urbanization indicators.

Occurrence of arsenic in brown rice and its relationship to soil properties from Hainan Island, China

The acquaintance of arsenic concentrations in rice grain is vital in risk assessment. In this study, we determined the concentration of arsenic in 282 brown rice grains sampled from Hainan Island, China, and discussed its possible relationships to the considered soil properties. Arsenic concentrations in the rice grain from Hainan Island varied from 5 to 309 μg/kg, with a mean (92 μg/kg) lower than most published data from other countries/regions and the maximum contaminant level (MCL) for As(i) in rice. The result of correlation analysis between grain and soil properties showed that grain As concentrations correlated significantly to soil arsenic speciation, organic matter and soil P contents and could be best predicted by humic acid bound and Fe-Mn oxides bound As fractions. Grain arsenic rises steeply at soil As concentrations lower than 3.6 mg/kg and gently at higher concentrations.

Effects of mercury on the activity and community composition of soil ammonia oxidizers

PURPOSE

Experiments were conducted to examine the effects of mercury (Hg) on soil nitrification activities and the microbial communities of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA).

METHODS

The soil samples spiked with different Hg concentrations were incubated for a period of 1, 2, 4, and 8 weeks in triplicate and the potential nitrification rate (PNR) of the samples was determined. The abundance of AOB and AOA was measured after an 8-week incubation by real-time polymerase chain reaction (PCR) assay of the amoA genes, while the community compositions by cloning and sequencing approaches.

RESULTS

The soil PNR differed with different incubation periods. It tended to decrease with increasing soil Hg concentrations at week 1, basing on which the half-maximal effective concentration (EC50) was 1.59 mg kg(-1). There was no significant difference in the abundance of AOB or AOA among the treatments. The AOB community was dominated by Nitrosospira-like sequences and more than 70% of the obtained clones were affiliated with the cluster 3a.2. The percentage of cluster 3a.1 in AOB community appeared to a consistent trend of decreasing with ascending soil Hg concentrations. While all the AOA sequences in the clone libraries were grouped into cluster S (soil and sediment origin).

CONCLUSIONS

This study revealed that Hg could inhibit soil potential nitrification and the extent varied with incubation periods. Soil Hg pollution changed the composition of soil AOB to some extent. These findings will be helpful to recognize the effects of Hg on the activity and community composition of soil ammonia oxidizers.