Effects of microbial processes on the fate of arsenic in paddy soil

Abstract

Arsenic (As) is a metalloid toxic to organisms including humans. Arsenic in rice represents a significant exposure pathway for the general population, particularly for those subsisting on rice. Arsenic transformation, namely reduction, oxidation and methylation, in soil-rice systems has fundamental impacts on its mobility and toxicity. In addition to soil chemical properties (pH, Eh, metallic oxides, organic matter), microorganisms play critical roles in As transformation and mobility in paddy soil, such as through ArsM (As(III) S-adenosylmethyltransferase) and interactions with iron oxides or organic matters. Arsenic species in paddy soil directly influence As speciation in rice grain because the methylated As species in rice are mainly derived from microbial methylation in paddy soil. This paper aims to provide an overview on the status of the knowledge and gaps on the chemical aspects of As transformation in soil-rice system in conjunction with microbial ecology and functional genes. In addition, potential pathways (manipulation of microorganisms in paddy soil and genetic engineering) to decrease total As and/or inorganic As in rice grain are proposed.

Speciation of antimony and arsenic in the soils and plants in an old antimony mine

“…This finding indicates that the rhizosphere may favor Sb methylation in soils similar to the As methylation in the rhizosphere (Jia et al, 2013;Zheng et al, 2013). Similarly, methylantimony has been identified in soils involved in various land uses, including in situ farm land and garden soils, which suggests that plant roots have an effect on the methylation of Sb in soils (Amereih et al, 2005;Duester et al, 2005;Murciego et al, 2007).…”

Mitigating heavy metal accumulation into rice (Oryza sativa L.) using biochar amendment — a field experiment in Hunan, China

“…This was likely attributed to elevated Si level in rhizosphere competing with As for transporters and preventing As entering into rice root. Arsenite is taken up by rice roots mainly through the Si uptake pathway (Ma et al 2008;Zheng et al 2013b). The only increase in shoot As concentration in BBC indicated the influenced translocation of As in plant by biochar amendment in keeping with our previous study .…”

A buried Neolithic paddy soil reveals loss of microbial functional diversity after modern rice cultivation

“…However, the soil moisture, total reducing substances and active reducing substances were similar in both currently cultivated soil and buried soil, indicating that they were under similar anaerobic conditions (Table S3), which would reduce the impact of oxygen level on the soil microbial community. The Neolithic paddy soil were deeply buried under the mulberry field and were separated from current cultivated paddy soil, suggesting that it was barely affected by farming activities, which is supported by 14 C dating and 15 N abundance measurement (Table S3). In addition, over 40 % of 16S rRNA gene sequences in buried paddy soil remains unclassified (Fig.…”

“…Gray-colored genes were not targeted by this GeoChip, or not detected in those samples specific biogeochemical cycles after long-term cultivation of rice by comparing a pair of currently cultivated paddy soil and buried ones. The 14 C data showed that the buried soil could be dated to ca. 6000 cal.…”

“…Flooded rice cultivation is considered as one of the most ancient crop production systems in human history [7], and is often cited as an example of a sustainable system [10]. Various studies have investigated the microbial communities in paddy soils, including bacterial diversity [11], methanogenic community [12], iron-reducing microorganisms [13], arsenic transformation [14], ammonium oxidizing microbes [15] and transcriptional activity [16]. Although these studies have increased our knowledge about microbial processes in paddy soils, they represented only a small fraction of microbial community.…”