In a surprising twist amid rising climate concerns, new research shows that scorching soil temperatures during extreme heatwaves do not necessarily boost the uptake of toxic elements like arsenic in rice crops. This finding, from a real world experiment during China's record breaking 2022 heatwaves, challenges fears that global warming will poison staple foods.
"Our study reveals that soil warming alone, decoupled from air temperature rises, does not inevitably ramp up arsenic or heavy metal accumulation in rice grains," said Sha Zhang, lead researcher at the Chinese Academy of Sciences' Institute of Urban Environment and corresponding author of the paper. "This provides reassurance for food safety under future climate extremes, as plant biology and seasonal factors play bigger roles than previously thought."
Published today in Environmental and Biogeochemical Processes, the study used innovative outdoor tanks to mimic paddy fields and test soil heating from direct sunlight. Researchers grew a special type of rice called ratoon rice, which yields two harvests from one planting: a main crop and a second ratoon crop from the stubble. This setup allowed them to observe effects over an extended 143 day growing season.
The experiment took place in Suzhou, China, using large aboveground tanks filled with local paddy soil low in contaminants. Each tank had a sun exposed south side and a shaded north side, creating a natural soil warming gradient of about 5.65 degrees Celsius on average at 5 to 10 centimeters depth. Crucially, air temperatures above the plants stayed identical across sides, isolating soil effects from canopy heat stress.
Three intense heatwaves hit during the 2022 season, with air temperatures topping 36 degrees Celsius for days at a time. Soil on the warmed sides baked even hotter due to solar radiation. Despite this, porewater arsenic levels – a key measure of availability to plants – showed no significant differences between warmed and control plots in either crop (p > 0.05). While arsenic in soil water jumped tenfold from main crop (average 6.9 micrograms per liter) to ratoon crop (576.6 micrograms per liter), rice grain levels rose only modestly, from 89.8 to 123.7 micrograms per kilogram.
The team analyzed 16 elements, including toxins like arsenic, cadmium, antimony, thallium, and lead, plus nutrients such as iron, zinc, and magnesium. Warming did not spike most heavy metals in grains or porewater. Arsenic translocation from plant nodes to grains was higher in the ratoon crop, but overall accumulation stayed in check, likely due to root uptake limits early in growth.
"This decouples soil mobilization from grain contamination," noted Zheng Chen, co corresponding author at Xi'an Jiaotong Liverpool University. "Flooded conditions buffered short term heat impacts, and rice physiology acted as a natural safeguard."
Previous studies linked warming to higher arsenic via air heated setups or lab simulations, raising alarms for rice eating billions in Asia. But those often mixed soil and air effects, or ignored plant defenses like root barriers and internal storage.
Ratoon rice, popular in southern China for efficiency, faces extra scrutiny as the second crop grows in late summer heat. Yet here, soil heat did not worsen risks. Magnesium in grains even dropped on warmed plots, while other nutrients held steady.
The findings urge refined climate models distinguishing soil from air warming. "Soil heat extremes can outpace air ones, so we need field relevant tests," Zhang added. Funded by China's National Key Research Program, the work calls for multi year studies across sites.
As heatwaves intensify – with 2022's Yangtze events among the worst – this research eases some food safety worries. Rice supplies over half the calories for 3.5 billion people, and arsenic threatens health from cancer to child development.
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Journal reference: Huangfu Q, Zhang S, Guo Y, Wang L, Chen Z, et al. 2025. Elevated soil temperatures during a heatwave year do not necessarily increase metal(loid) mobilization or accumulation across two harvests of semi-perennial rice: evidence from mesocosm experiments. Environmental and Biogeochemical Processes 1: e017
https://www.maxapress.com/article/doi/10.48130/ebp-0025-0017
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About the Journal:
Environmental and Biogeochemical Processes (e-ISSN 3070-1708) is a multidisciplinary platform for communicating advances in fundamental and applied research on the interactions and processes involving the cycling of elements and compounds between the biological, geological, and chemical components of the environment.
