Citric Acid Secreted by Rice Roots Boosts Arsenic Levels in Grain

A single, unassuming molecule that rice roots release into the soil may be quietly deciding how much arsenic ends up in the world’s most important staple grain. New research published in Plant and Soil shows that citric acid, one of the most abundant low-molecular-weight organic acids in the rhizosphere, can dramatically enhance the mobilization, transformation, and bioavailability of arsenic in flooded paddy soils. In carefully controlled pot experiments, additions of citric acid at concentrations as low as 0.1 millimolar—and most powerfully at 5 millimolar—set off a cascade of chemical and microbial changes that ultimately raised arsenic concentrations in rice grain from 1.55 to 2.51 milligrams per kilogram, an increase of more than sixty percent. The findings expose a previously underappreciated lever in the soil–plant transfer of one of the most concerning food-chain contaminants, and they carry weighty implications for rice paddies across Asia and beyond, where arsenic-laden groundwater and soils already threaten food safety for billions of people.

The study, led by Chengcheng Jia and Shuqiong Kong of Yangtze University, together with colleagues at the China University of Geosciences, the University of Maryland, and Henan University, set out to resolve a persistent uncertainty. Citric acid is a routine product of root exudation and microbial metabolism, and decades of work have established that organic acids can pry metals loose from soil minerals. Yet precisely how citrate behaves in the waterlogged, oxygen-starved environment of a paddy field—where iron oxides, microbes, and arsenic engage in a constantly shifting chemical dance—had remained poorly quantified. The team filled flooded pots with arsenic-contaminated paddy soil, amended them with citric acid at 0, 0.1, 1, or 5 millimolar, and then tracked the fallout across every layer of the system: porewater chemistry, soil-phase arsenic fractions, bacterial community composition, and finally the arsenic that accumulated in the rice grain itself.

The chemical signature of citrate’s influence emerged quickly and unmistakably. Porewater citrate concentrations rose, and with them came a pronounced drop in both pH and Eh, the redox potential that governs which oxidation states are stable in solution. Simultaneously, dissolved iron and dissolved arsenic surged in the porewater, with the strongest responses recorded at the highest citrate dose. This is the fingerprint of ligand-mediated dissolution: citrate ions, with their three carboxyl groups, form stable complexes with iron at mineral surfaces, stripping iron atoms out of oxide lattices and releasing whatever arsenic was bound alongside them. Because arsenic’s fate in soils is tightly coupled to that of iron—most arsenic in aerated soils is sorbed onto or coprecipitated with iron and manganese oxides—dissolving those oxides sets the poison loose.

Sequential extraction of the soil confirmed this wholesale redistribution. Total soil arsenic fell from 17.82 to 8.98 milligrams per kilogram over the course of the experiment, a striking depletion of the solid-phase reservoir. But the story is more nuanced than simple loss. The fractions of arsenic bound to humic substances and to strongly organic material approximately doubled, while arsenic associated with iron and manganese oxides declined by roughly nine percent. In other words, citric acid did not merely release arsenic into solution; it also shifted a substantial share of it into organic-associated pools, many of which are more labile under the reducing conditions that prevail in flooded paddies. Arsenic, in effect, was shuffled from a relatively stable mineral jail into compartments from which escape into porewater—and then into rice roots—is considerably easier.

Speciation measurements added a further and more sinister dimension. As the redox potential declined and ferrous iron, Fe(II), accumulated in the porewater, arsenic in its reduced form, As(III), progressively built up. This matters because As(III) is generally more mobile and more readily taken up by rice than its oxidized counterpart, arsenate or As(V). Rice, grown in anaerobic soil, transports As(III) through its silicon uptake pathways, so the more As(III) present near the root surface, the more efficient the contamination route into the plant. The citric acid amendments, by driving Eh downward and fueling the reduction of iron phases, effectively steered the soil’s arsenic chemistry toward the form most dangerous to the crop. The researchers observed that the reductive transformation of Fe-bearing minerals—accelerated both by citrate complexation and by the rising tide of microbial iron reduction—was central to this speciation shift.

The microbial dimension of the study reveals that chemistry alone does not explain the outcome. Using 16S rRNA gene sequencing of the bacterial communities, the team documented a marked restructuring of the soil microbiome in response to citrate. Proteobacteria, a phylum rich in metabolically versatile organisms including many iron-reducers, expanded dramatically from 18 to 33 percent of the community. Citrate is an excellent carbon source and electron donor, and its arrival in an anaerobic paddy soil provides fresh fuel for microorganisms that respire iron oxides and, in doing so, release their sorbed arsenic cargo. Previous work by some of the same groups has shown that dissimilatory iron-reducing bacteria can liberate arsenic from As-bearing iron minerals, and the new findings slot citrate neatly into that mechanism as a community-shaping substrate. The authors conclude that this microbial community restructuring likely contributed substantially to the observed iron and arsenic transformations, acting in concert with the purely chemical complexation reactions.

The final, and most consequential, endpoint was the rice itself. Grain arsenic concentrations climbed from 1.55 to 2.51 milligrams per kilogram across the citrate treatments, while the translocation factor—a measure of how efficiently arsenic moves from root to shoot and ultimately to grain—rose from 0.089 to 0.28, roughly a threefold increase. That the transfer efficiency changed even more than the grain concentration suggests citric acid did not just increase the amount of arsenic available at the root surface; it enhanced the whole soil–plant transfer chain, plausibly by sustaining higher dissolved arsenic in porewater over longer periods and by altering the iron plaque that forms on rice roots, a barrier that normally intercepts some arsenic before it can enter the plant. The result demonstrates that the rhizosphere’s organic chemistry is not a passive background condition but an active determinant of grain contamination.

The implications ripple outward in several directions. For rice-growing regions where arsenic in soil and irrigation water is a chronic problem, the study cautions that management practices which increase root exudation or organic acid inputs—certain fertilization regimes, organic amendments, or cultivar choices that exude more citrate—could inadvertently worsen grain arsenic. Conversely, the mechanism offers a double-edged opportunity: citrate-assisted mobilization is precisely the chemistry exploited in citrate-enhanced extraction remediation, and understanding its side effects could help design cleanup strategies that mobilize arsenic for removal without transferring it into crops. The work also strengthens the case that porewater chemistry—pH, Eh, dissolved iron, and dissolved arsenic—should be central to predictive models of rice grain contamination, replacing or supplementing static soil total arsenic measurements that miss the dynamic mobilization triggered by root-derived compounds.

More broadly, the research is a vivid reminder that the boundary between plant and soil is a chemically engineered interface, not a passive filter. Every exuded molecule of citrate is a small agent of geochemical change, dissolving minerals, feeding microbes, reshaping redox gradients, and deciding whether arsenic stays locked away or rides the water into the grain. In a world where rice supplies roughly a fifth of humanity’s calories, decoding these molecular conversations in the rhizosphere is not an academic luxury. It is one of the clearest routes toward keeping a silent, naturally occurring toxin out of the daily meals of billions, and toward farming practices that work with—rather than against—the restless chemistry of flooded soils.

Subject of Research: How citric acid in flooded paddy soil rhizospheres promotes arsenic migration, transformation, and bioavailability in rice

Article Title: Citric acid promoted arsenic migration, transformation, and bioavailability in flooded paddy soils

Article References: Citric acid promoted arsenic migration, transformation, and bioavailability in flooded paddy soils. (n.d.). https://doi.org/10.1007/s11104-026-09108-z

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09108-z

Keywords: citric acid, arsenic mobilization, flooded paddy soil, arsenic speciation, rice grain arsenic, iron oxides, rhizosphere chemistry, microbial community, Proteobacteria, soil redox potential, As(III), soil–plant transfer