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Using cover plants to remove pollutants from arable soil

Nitrate, pesticides, metals and plastic agricultural soils often contain pollutants. But are there sustainable and climate-friendly ways to restore and promote soil health in agricultural land? Yes, says a research team from the Helmholtz Centre for Environmental Research (UFZ). Specific plant species could be used as cover plants for phytoremediation, i.e. to relieve agricultural land from adverse pollutant impacts.

In their article in Trends in Plant Sciences, the researchers summarize the results of more than 100 scientific studies and present which plants, according to current knowledge, are suitable for removing pollutants from agricultural soils or trapping them in their root systems.

Farmers often grow so-called cover plants between main crops. They are used for purposes such as for animal feed or remain on the field as green manure. In this way, they supply the soil with nutrients before the next planting. However, cover plants also protect against erosion, stabilize the soil's water, nutrient and carbon balance, regulate soil temperature, promote humus formation, sequester carbon dioxide and increase biodiversity above and below ground.

"Cover plants are actually a kind of miracle tool in agriculture," says Prof Marie Muehe, head of the Plant Biogeochemistry working group at the UFZ and senior author of the publication. However, their potential for removing soil contaminants has yet to be recognized.

Using plants to remediate contaminants in the soil is nothing new. For example, contaminated soil on industrial sites is already being remediated in this way. But agriculture could also benefit from this method, says Muehe. "The use of selected cover plants for phytoremediation is a natural, climate-neutral way to improve and maintain soil health. We should also apply this in the interests of sustainable agriculture."

But which plants are suitable for phytoremediation in agriculture? And which pollutants could be managed with which plants? The UFZ team investigated these questions and analyzed the current status of research.

"For example, we researched whether there are already studies that indicate which of the frequently used cover plants have the ability to break down contaminants. We also looked for plants that can break down or fix the pollutants in six categories—nitrate, salts, metals, pesticides, plastics and antibiotic resistance genes," explains first author Dr. Pooja Sharma, who is also a researcher in the Plant Biogeochemistry working group at the UFZ.

Based on the results of the literature review, the research team developed concepts for phytoremediation for the respective pollutant categories. For example, rye and sunflower could be used as cover plants to prevent excess nitrate in arable soil from being washed out and polluting the groundwater.

The plants absorb the nitrate from the soil to grow and can remain on the field as green manure. However, cover plants that remove unwanted metals such as cadmium from the soil should be removed. Various types of clover, rye or rape could be used for this.

"The cover plants used to remove metals are not generally suitable as animal feed. But they could play a role in the production of biogas," says Sharma. "Sunflowers are also good at removing metals from the soil. Potentially metals mainly accumulate in the leaves, so that the seeds could be harvested." The same applies to the seeds of mustard, which, as an intercrop, removes pesticides from the soil in the same way as grass or sunn hemp.

It was difficult to identify cover plants that are particularly suitable for phytoremediation targeting the contaminant categories of plastic or antibiotic resistance genes. The researchers discuss that the interactions between soil microorganisms and cover plants also play an important role in whether and how well pollutants can be fixed, degraded or removed by phytoremediation.

"A lot of research still needs to be done here—working together with farmers. This is the only way to develop effective and practicable strategies for phytoremediation, tailored to different locations, soils and pollutant problems," says Muehe. "From our perspective, using cover plants to manage soil pollutants could be an efficient future concept for healthier soils and more sustainable agriculture."

A UFZ research team will be launching a field study together with farmers as part of the SmartManure project in the summer of 2025. Their aim is to more closely investigate different cover plants and their remediation performance and to test the practicability of phytoremediation in agricultural practice.