A new study has shown that the type of microorganisms present in rice fields plays a key role in determining how much arsenic accumulates in rice grains.
Arsenic Toxicity in Agriculture
- Arsenic (As) is a toxic heavy metal and a known carcinogen, found naturally in groundwater and soils of rice-growing regions.
- Rice plants absorb arsenic more easily than other crops, especially in flooded paddy fields, where oxygen is limited.
- Arsenic toxicity leads to health risks in humans and serious yield losses in agriculture.
Mechanism in Flooded Paddy Fields
- In anaerobic (oxygen-poor) conditions, certain microbes transform inorganic arsenic into soluble and bioavailable forms, which are easily taken up by rice roots.
- These toxic organic compounds include dimethylarsinic acid (DMA) and dimethylated monothioarsenate (DMMTA).
- They cause “straighthead disease”, resulting in sterile, upright panicles and up to 70% yield loss.
Soil Microbes and Arsenic Behaviour
- Research by Nanjing Agricultural University (2025) revealed that soil age controls the type of microbial community present:
- Young Soils (<700 years): Dominated by arsenic-methylating bacteria that increase toxicity by forming DMA and DMMTA.
- Old Soils (>700 years): Rich in demethylating archaea that detoxify arsenic by breaking down harmful compounds.
- A global survey of 801 paddy soils found 11 methylators and 6 demethylators influencing arsenic levels.
- Toxicity Risk: When the ratio of methylators to demethylators exceeds 1.5, the chance of straighthead disease rises sharply.
Hotspot Regions in South Asia
- High arsenic contamination reported in West Bengal, Bihar, and Bangladesh, due to irrigation with arsenic-rich groundwater.
Mitigation Measures
- Mid-season drainage: Introduces oxygen to limit arsenic solubility.
- Silicon fertilization: Competes with arsenic uptake in rice roots.
- Microbial management: Promoting demethylating microbes restores soil redox balance and reduces arsenic accumulation.
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