Decades under the Clean Air Act have greatly reduced the amounts of nitrogen and sulfur falling to the Earth's surface.

But levels remain high enough to threaten biodiversity.

As nitrogen and sulfur fall to the ground via acid rain or particulates, they make soil more acidic.

Nitrogen unbalances soil nutrients, boosting populations of pests and invasive species, while sulfur restricts plant sprouting and worsens frost sensitivity.

Estimates of harmful amounts of nitrogen or sulfur tend to focus on ecosystems, so Heather Throop of ASU and her colleagues decided to drill down and look at nearly 350 plant species.

"Many of the species are vulnerable, although they differ in whether they have positive or negative influences of this nitrogen and sulfur deposition," she said. 

Overall, nitrogen and sulfur increases tended to favor invasive species.

The research appears in the journal Nature Plants.