Nitrogen is one of the most important elements for life on Earth: It’s the main component of the air we breathe, a key building block for proteins, and the magic ingredient in agricultural fertilizers that are essential to feeding the global human population.
But in recent decades, experts across a range of different disciplines have begun to flag a worrying decline of available nitrogen in ecosystems around the world. This trend is linked to human-caused climate change and could have “far-reaching consequences,” according to a new review summary published on Thursday in Science.
Spearheaded by senior author Andrew Elmore, a professor of landscape ecology at the University of Maryland Center for Environmental Science and at the National Socio-Environmental Synthesis Center (SESYNC), the article pulls together multiple lines of evidence about worldwide nitrogen shortfalls and their potential implications.
“This has been on the scientific radar for a long time,” said Rachel Mason, lead author of the paper and former postdoctoral scholar at SESYNC, in a call. “In experiments, we've seen some evidence of this, but it was scattered around the community and in different fields.”
“So, it was really a question of someone deciding—that someone being Andrew Elmore—to bring these people together to just say: ‘Hey, what do we all know?” she continued. “If we actually look for observations out in the wild, aside from experiments and theory, what can we see is happening? And what is it going to mean for us?”
The growing awareness of declining nitrogen availability has been overshadowed somewhat by efforts to counter the opposite problem: An excess of anthropogenic nitrogen pollution. Industrial and agricultural activities have doubled the global abundance of reactive nitrogen, and as a result, the element often leaches into ecosystems. This elevates the risk of low-oxygen dead zones, harmful algal blooms, and reduces biodiversity, among other adverse effects.
Much attention has been paid to mitigating the problem of surplus nitrogen in certain ecosystems, but the new article aims to signal-boost the reverse trend, also caused by human pressures, that is driving declines in access to the vital element in other niches. Mason emphasized that this is not “a fight between the ‘too much nitrogen’ people and the ‘too little nitrogen’ people” calling it a “yes, and” problem.
“Yes, there is too much [nitrogen], and there is this other problem that we need to pay attention to, also,” she said.
There are many open questions about the dwindling nitrogen supply, but it’s clear that the problem, like so many others, is linked to human consumption of fossil fuels. Greenhouse gas emissions from human activity have soaked into Earth’s atmosphere, elevating atmospheric carbon dioxide (CO2) levels to the highest point in millions of years.
As a result, terrestrial plants are awash in about 50 percent more CO2 than they were just 150 years ago. Since CO2 fuels plants, many botanical species are growing faster under these new conditions. But the supply of nitrogen is not necessarily keeping pace with this accelerated development, according to experiments and field studies, which can cause nitrogen to become diluted in plants—a disruption that has ripple effects across entire ecosystems.
“We are adding carbon to ecosystems, and it can act as a natural fertilizer,” Mason explained. “Plants take up carbon, and they photosynthesize more. But except in some areas that are affected by agriculture and industry, we're not necessarily adding similar amounts of nitrogen, so you end up with this imbalance between carbon and nitrogen.”
“We like it that plants absorb the CO2 that we emit, because they offset some of our emissions and that hopefully dampens climate change a bit,” she continued. “But if plants are actually going to do this extra photosynthesis that is brought about by CO2 emissions, they need nitrogen to do things like make the chlorophyll that makes photosynthesis work in the first place. If they don't have that nitrogen, they can't do that and then potentially all our calculations about CO2 emissions are off.”
This imbalance is impeding the normal flow of nitrogen through habitats, reducing its overall availability to the lifeforms that need it. But the issue is not limited to off-kilter ratios of nitrogen and CO2. Wildfires, which are linked to human-driven climate change, can also straight-up delete nitrogen from ecosystems. For instance, the article refers to a 2017 study that found frequent burning in savanna grasslands and broadleaf forests resulted in a loss of almost 40 percent of soil-based nitrogen over six decades.
While the macro effects of all these factors is not well-understood, the new article suggests that nitrogen-depleted plants are likely not able to provide adequate nourishment to the many species that rely on them, such as insects and herbivores, a shift that can introduce new instabilities into ecosystems that may already be reeling from other anthropogenic changes.
Nitrogen deficits could likewise affect human well-being in numerous ways. For instance, “lower protein concentrations in grazing livestock diets may disproportionately affect those who do not have the resources to acquire supplemental feed for their animals,” according to the study, providing yet another example of the social inequalities that so often undergird climate change and its disparate effects.
“If you're a farmer in the U.S. who is just not well-resourced, or in a developing country where you rely on cattle that haven’t been strongly selected to grow faster, or do well on low-protein forage, then this could be a real livelihood issue,” Mason said.
While it’s clear that the most necessary action for addressing nitrogen declines is reducing greenhouse gas emissions, the new article also advocates for more targeted research efforts into the myriad impacts of this problem worldwide.
“We’ve got the big picture together—the outlines of what's happening—but we've also identified loads of uncertainties and areas where we need more research,” Mason said.
“It's another manifestation of how humans are perturbing the Earth system, and how it's responding in very complex and counterintuitive ways,” she concluded.