The study, conducted by scientists at the Institute of Natural Resources and Agrobiology of Seville (IRNAS-CSIC) together with colleagues at the Université Marie et Louis Pasteur and the CNRS in France, was published in the Spanish Journal of Soil Science. It forms part of the European EDAPHOS project, funded under the Horizon Europe program, which aims to develop practical strategies for restoring degraded soils across the continent. The work is a greenhouse prelude to a future field experiment planned on a site within the active Ríotinto mine, which today operates as an open-pit copper mine processing 15.5 million tons of ore each year.

The researchers assembled an unusual botanical cast. Five poplar varieties were tested: a parental hybrid called P10, derived from a cross between Populus nigra, prized for wood quality, and Populus maximowiczii, known for rapid growth, plus four backcross hybrids—P148, P149, P152 and P153—bred by crossing P10 with Populus alba, a species valued for drought and pathogen resistance and its ability to survive on nutrient-poor soils. Alongside the trees, two herbaceous companions were trialed: Indian mustard, Brassica juncea, a classic accumulator species in the phytoremediation literature, and Lablab purpureus, the hyacinth bean, a nitrogen-fixing legume better known as a forage crop than as a mine-soil pioneer.

The chemistry of the soil posed the central challenge. Untreated Ríotinto soil, sampled from fifteen points across roughly 600 square meters of the future field site, was so acidic that virtually nothing could germinate in it. The team therefore mixed in sugar lime, a by-product of sugar refining containing 70 to 80 percent calcium carbonate, at a rate equivalent to 30 tons per hectare. The amendment worked dramatically, raising the pH of the top ten centimeters from about 3.1 to 8.15. Because most toxic metals in soil carry positive charges, raising the pH causes them to bind to soil particles and drop out of solution. The bioavailable fractions of cadmium, copper, lead and zinc, measured with a mild calcium chloride extraction, fell sharply in the amended soil compared with the raw mine soil.

But the amendment produced a twist. Arsenic, unlike the metals, exists in soil mainly as a negatively charged anion, and its solubility increases as pH rises. Available arsenic in the amended soil climbed from 0.004 milligrams per kilogram in the untreated mine soil to 0.30 milligrams per kilogram after liming. The authors caution that the calcium chloride method is only an indicative measure of arsenic availability, but the finding underscores a broader lesson: remediation chemistry is not one-size-fits-all, and fixing one problem can aggravate another.

The poplar results were striking in their variation. After three months in pots, every hybrid had survived in the amended contaminated soil, but their vigor differed enormously. Hybrid P152 produced green leaves on 80 percent of its cuttings, the best performance in contaminated soil, followed by the parental P10 at 60 percent and P153 at 50 percent. Hybrids P148 and P149 fared dismally, with only 10 and 30 percent of cuttings leafing out—and their poor showing even in clean control soil suggests their weaknesses go beyond metal tolerance. Notably, leaf chlorophyll readings taken with a SPAD meter showed no significant differences between poplars grown in contaminated and clean soils, hinting that the surviving hybrids were not visibly stressed once the acidity had been neutralized.

Chemical analysis of the leaves revealed that hybrids P152 and P153 accumulated the highest concentrations of most trace elements, including arsenic, cadmium, copper, iron, mercury, manganese, lead and zinc. Arsenic, cadmium and lead levels in the leaves exceeded normal plant thresholds for all hybrids. Because the liming had locked most cadmium and zinc away in the soil, the researchers concluded that these hybrids would function mainly as bioindicators rather than extractors under current conditions—though in soils where those metals are more available, they could become genuine phytoextraction tools. Lead, present at nearly 12 grams per kilogram in the amended soil, remains the open question; a recent meta-analysis suggests poplars can accumulate substantial lead in leaves, and the team calls for deeper investigation into the uptake mechanisms.

The two herbaceous species told sharply different stories. Brassica juncea failed to germinate at all in the untreated acidic mine soil, and even in the amended soil its biomass was barely a third of what it achieved in clean agricultural soil—1.00 gram versus 2.86 grams of dry weight per pot. Yet what little mustard grew packed a punch: it accumulated the highest trace element concentrations of any species tested, with arsenic, copper, lead and zinc levels in its shoots exceeding those found in poplar leaves after just four weeks. This is the classic accumulator’s dilemma—high uptake, but too little biomass to matter much in the field.

Lablab purpureus was the quiet revelation. The legume germinated and grew in every soil treatment, including the raw, unamended mine soil at pH 3, where nothing else could establish. Its biomass declined along the contamination gradient—from 0.81 grams per plant in clean soil to 0.18 grams in untreated mine soil—but it survived everywhere, and its shoots contained the lowest trace element levels of all species tested, staying within normal plant thresholds except for lead in the harshest soil. The researchers attribute this resilience partly to its large seeds, which supply nutritional reserves during the vulnerable early growth stages. As a nitrogen-fixing legume tolerant of both acidity and metal contamination, the authors argue it could serve as a pioneer species, improving soil nitrogen and structure to pave the way for other plants.

The study’s practical message is one of matched strategies. If the goal is phytoextraction—using plants to pull metals out of the ground—amendment rates should be calculated to raise pH only enough to support growth, avoiding overly alkaline conditions that immobilize the very metals the plants are meant to harvest. If the goal is phytostabilization—locking contaminants in place—heavier liming is preferable, keeping metals bound and out of the food chain. Hybrids P152 and P153 emerge as the most promising poplar candidates for either strategy, while Lablab purpureus offers a low-cost biological entry point for the most degraded patches. As the team prepares to move from greenhouse pots to the mine itself, the work offers a template for restoring the thousands of metal-scarred landscapes left behind by centuries of mining: choose the right plant for the right purpose, and let the chemistry decide the rest.

Subject of Research: Phytoremediation of trace element contaminated mine soils using poplar hybrids and herbaceous companion species

Article Title: Evaluating Poplar Hybrids and Companion Herbaceous Species for Phytoremediation of Riotinto Mine Soils: Survival and Trace Element Accumulation Assessment

Article References: Madejón, P., Ciadamidaro, L., Chalot, M., Navarro-Fernández, C. M., de Sosa, L. L., & Madejón, E. (2025). Evaluating Poplar Hybrids and Companion Herbaceous Species for Phytoremediation of Riotinto Mine Soils: Survival and Trace Element Accumulation Assessment. Spanish Journal of Soil Science, 15, Article 14555. https://doi.org/10.3389/sjss.2025.14555

Image Credits: AI Generated

DOI: 10.3389/sjss.2025.14555

Keywords: phytoremediation, poplar hybrids, Ríotinto mine, trace elements, soil amendment, sugar lime, Brassica juncea, Lablab purpureus, heavy metals, soil pH, phytostabilization, phytoextraction