You probably don’t spend much time thinking about nitrogen. If you do, maybe you think of it as a pollutant. Fertilizer running off farm fields, livestock manure giving off ammonia, and burning fossil fuels all add nitrogen to the environment. This nitrogen contributes to the ocean ‘dead zones’ that have formed downstream from agricultural regions, among many other environmental problems. It’s a case of too much of a good thing. Plants need nitrogen to make essential components such as proteins, and plant-eating animals need plant proteins. But too much nitrogen in ecosystems is a recipe for trouble. 

 

There’s another side to the nitrogen story, though. Humans are now disturbing the Earth’s ecosystems in ways that could reduce how much nitrogen is available to plants and the creatures that eat them. Higher levels of carbon dioxide (CO₂) in the atmosphere and longer, warmer growing seasons can increase the growth of plants, which may then need extra nitrogen to support that growth. Plants grown in elevated CO₂ levels in experiments tend to have lower nitrogen concentrations in their leaves. At the same time, it’s likely that more and more nitrogen is being lost from ecosystems in disturbances like wildfires, or when we export protein-rich cattle from grazing lands. Nitrogen levels are too high in many locations from agricultural and industrial activities, but could the gap between nitrogen supply and demand be quietly widening elsewhere? 

 

Until recently nobody had systematically gathered all the evidence that might show what is happening ‘in the wild’, across ecosystems like forests and grasslands that are not part of controlled experiments. We aimed to fill that gap by examining published measures of nitrogen availability over time in natural ecosystems (as opposed to, say, croplands and urban areas). Scientists have several ways of measuring nitrogen availability, and some of these techniques allow us to explore surprisingly far back in time. For instance, nitrogen concentrations have been measured in preserved plant samples collected as far back as the 1700s, while nitrogen isotopes (atoms that are a bit heavier than most) in tree rings provide records that can extend back well over a century. We also found some… interesting... sources of information. For example, the US Department of Agriculture has collected cattle manure samples since the 1990s, and one study had used those samples to deduce the nitrogen content of the grass the cows had been grazing on each year. 

 

A consistent picture emerged from the very diverse studies we examined. From sediments in remote Arctic lakes, to pollen from around the USA and southern Canada, and tree leaves across Europe, almost all the measurements indicated a decline in nitrogen availability. (One exception was a study in China, where nitrogen emissions from industry and agriculture may be increasing the amount of nitrogen in plants). Putting it all together, we concluded that nitrogen availability was generally fairly level until around the start of the 20th century, when it began a steady decline. 

 

If excess nitrogen is a problem, could declining nitrogen availability be a good thing? And might it simply be the result of reductions in nitrogen pollution in some regions since the 1990s? In some cases, both may be true. However, the fact that we found evidence of declining nitrogen availability dating back more than a century means that there must be more to it than recent pollution regulations. Moreover, grazing mammals and leaf-eating insects are very sensitive to low protein in their diets. So we suspect that declining nitrogen availability is a factor behind the ‘insect apocalypse’ that you may have read about. Finally, humanity relies on plants to take up (sequester) some of the carbon we emit, reducing global warming to some degree. If plants have access to less nitrogen than we expect, their ability to absorb CO₂ could be less than we think. 

 

At this point we can see the outlines of the declining nitrogen picture, but there are many gaps to fill in. We need to better understand which ecosystems are most affected, and which driving forces (such as rising CO₂ and wildfires) are influential in different situations. Researchers must also investigate how declines in nitrogen availability will affect ecosystem food webs, and people who depend on grazing animals for their livelihoods. One thing, though, is clear: declining nitrogen availability adds to the long list of reasons for reducing our CO₂ emissions before our problems become even harder to solve.