Poisoned soils could be healed with the right kind of cress

Poisoned soil is a common trope in sci-fi. Often it is fixed, slowly, through the work of robots. But farmers in a polluted landscape might be better off putting their faith in plants. Irina Drozdova and colleagues have been comparing the ability of Brassicaceae plant species to see how well they grow in heavy metal-rich soils. Interest in the results isn’t limited to fans of DS9 or Buck Rogers. Phytoremediation, the cleaning of soils by plants, has real-world application in the 21st century.

The botany of urban life can be distinctive because the soil of urban life is distinctive. The urban landscape often has a lot of renewal, through demolition, levelling and rebuild. The result is something called anthrosol. Also, pollution in urban sites is a common problem. Long-term development gives plenty of opportunity for metals to leach into the soil. Even in what seem to be small concentrations, they can be toxic.

Drozdova and colleagues describe the Brassicaceae as synanthropic. A synanthropic species is one that isn’t domesticated, but may be adapting to live alongside humans, whether the humans like it or not. So, the team looked to plants in this family to see how they responded to pollutants. Could plants related to mustards and cabbages pull toxic metals from the soil?

Drozdova and colleagues set up their experiment in the Botanical Garden in St. Petersburg. “The park of Botanical Garden of Komarov Botanical Institute is in the central part of St Petersburg and is therefore substantially influenced by anthropogenic activities,” said the team in their paper. “In the center of urban areas, road traffic is likely to make the most important contribution to the influence of potentially harmful elements to the biota. Emissions of potentially toxic metals Zn, Cu, Cd, and Pb to the environment are due to wear of brakes, tires, other vehicle components, and road surfacing also exhaust emissions…”

Sampling showed that toxic metals were indeed in the soil in the botanic garden at higher than natural concentrations. “Moreover, these soils were shown contamination with bioavailable concentration of potentially toxic metals, wherein concentrations of Pb and Zn were higher, than the Russian maximum permissible values (6.0 and 23.0 respectively),” said the authors.

The plants had mixed success in pulling the metals from the soil. Only a couple of species could extract more than one metal at a time. Brassica campestris was suitable for zinc, cadmium and copper, while Rorippa palustris, Marsh Yellow-cress, lifted zinc and cadmium into its leaves. “In addition, these had rapid growth, providing the possibility of harvest several times per season,” said Drozdova and colleagues. “Thus, they could be a suitable candidate species for phytoremediation to clean-up soil from polyelemental contamination. However, they did not show the characteristics of a hyperaccumulator species.” Additionally, they found Sinapis arvensis, charlock mustard, tackled zinc and Thlaspi arvense, field pennycress, could clean lead.

The most successful plant Brassica campestris appears to be the same species as B. rapa, and B. rapa comes in a variety of forms. Use of the name Brassica campestris suggests that Drozdova and colleagues were using field mustard.

While it might be possible to clean land with plants like cress and mustard, there are still some drawbacks. The plants will be accumulating the toxic metals in their bodies, so you wouldn’t want to put any of the cress into the salad of someone you like.