Of all the human-induced changes to Earth’s chemical cycles, our use of nitrogen is among the most profound. Usually this is evident when there’s too much of it, as when fertilizer-induced algae blooms extinguish life in the Gulf of Mexico — but there’s another, overlooked way that nitrogen has been disrupted. Across many parts of the terrestrial world, plants may soon contain too little of it, with potentially disastrous consequences.
In a study published in Nature Ecology & Evolution, a team of 38 researchers led by Joseph Craine, an ecologist with Jonah Ventures, describe their analyses of more than 40,000 vegetation samples collected across 37 years on every continent but Antarctica. They wanted to learn whether the availability of nitrogen — one of life’s chemical building blocks — is changing.
It certainly seems to be. According to the researchers, plants outside of agricultural settings now contain 9 percent less nitrogen than they did in 1980. And that, says Craine, may be only the latest stage of a planetary decline that started a century ago. It “could change how terrestrial ecosystems function,” he says.
It’s an unusual way of thinking about nitrogen. Thanks to nitrogen-rich fertilizers and nitrogen-belching fossil fuel combustion, many regions are grappling with an environmental excess of the element. But at the same time, explains Craine, elevated levels of atmospheric carbon dioxide have accelerated plant photosynthesis, leading to the production of more vegetable matter even as the amount of nitrogen available to their roots remains constant.
The result is an ecosystem-scale dilution. Nitrogen concentrations on a leaf-by-leaf basis decline. And that could have cascading effects.
After all, nitrogen is a key element in proteins. Creatures who eat plants may become protein-limited, leading to dietary stress and changes in metabolism in reproduction. The researchers suggest that nitrogen declines may be implicated in massive, as-yet-unexplained modern declines in insect populations. There just isn’t as much protein for them as there used to be.
To be sure, that’s still just a hypothesis. More research is needed to show whether it’s correct. Still, it’s a troubling possibility. It’s also possible that vertebrate herbivores could eventually be similarly affected, leading to population collapses.
“The one eater-of-plants that benefits,” says Craine, “is fire. It doesn’t care what the nitrogen concentrations of plants are. Less of the world is eaten. More is burned.”
Having identified the problem: what about solutions? Craine suggests that people start thinking about how to fertilize ecosystems, at least temporarily, until atmospheric CO2 levels are brought under control. As with so many climate change issues, that’s the ultimate solution — and the possibility of global plant nitrogen declines only adds to the urgency.
Source: Craine et al. “Isotopic evidence for oligotrophication of terrestrial ecosystems.” Nature Ecology & Evolution, 2018.
About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.