Conservation of threatened plant species requires more than effective nitrogen reduction policy

Through nitrogen policy, the EU wants to reduce nitrogen emissions to conserve plant biodiversity. Researchers Martin Wassen and Jerry van Dijk from the Copernicus Institute for Sustainable Development (Utrecht University) and their collaborators from the Universities of Göttingen and Zürich discovered that many rare and threatened plant species will suffer because of this policy. Their results were published in Nature Ecology and Evolution.

Targeting the high levels of nitrogen deposition in Europe is essential for halting biodiversity loss in grasslands, but may fail to protect the large pool of threatened plant species that persist under low phosphorus availability, like this meadow thistle (Cirsium dissectum) and the other plant species shown on this page.

Nitrogen is an important nutrient for plant growth, but an overabundance can be harmful to plant biodiversity. This is because the plants that thrive on nitrogen can take over plant species that do better with low nitrogen concentrations. “But merely reducing nitrogen is not enough,” says Martin Wassen, first author of the study. “Such policies can even backfire if you don’t include other nutrients too.”

Ratios are important

In addition to nitrogen, plants also need phosphorus and potassium to grow. The ratio between those nutrients in the soil are important, the researchers discovered. They showed that when the concentration of nitrogen is reduced in the soil, without simultaneous reductions of phosphorus as well, plant species that are already rare and threatened will disappear.

Sensitive plants

“Many threatened plant species live in areas where phosphorous concentrations are low,” Wassen and van Dijk illustrate. Because the relative phosphorus concentration increases when nitrogen concentrations decrease in response to effective environmental policy, these species become even more threatened. Those species are extra sensitive to changes in nutrient concentrations, and should be prioritized in conservation according to the researchers .

The study’s results have major consequences for the current nitrogen policy: the authors therefore argue for the introduction of a European Phosphate Directive, in addition to the existing Nitrate Directive.

Article

Martin Joseph Wassen, Julian Schrader, Jerry van Dijk and Maarten Boudewijn Eppinga, ‘Phosphorus fertilization is eradicating the niche of northern Eurasia's threatened plant species’, Nature Ecology and Evolution, https://www.nature.com/articles/s41559-020-01323-w