Nitrogen: how important is it?

May 16, 2008 – 11:13 pm

Today in the journal Science, a review article was published about human modification to the global nitrogen cycle. If you want to know about the environmental issues surrounding nitrogen and don’t mind a rigorous technical report, this is the best yet. You can read the abstract here and you can link to the full text if you have a subscription.

High-profile review papers in leading journals beg the question: exactly how important might the nitrogen issue become? Will it be the next inconvenient truth society has to grapple with? Although the global increase in reactive nitrogen is already acknowledged by some as an important environmental issue, it’s not clear whether nitrogen will ever rise to the fore the way climate change has. Will policy makers ever have to discuss their nitrogen policies as they now discuss their carbon policies? Right now, it doesn’t look like the shit (literally in the case of N-rich sewage) is about to hit the political fan. But like CFC’s in the 80’s and like climate change over the past decade, that could change as our quantitative knowledge of global N cycling improves and evidence emerges.

While there is public consciousness about climate change, invasive species, land cover change, and population growth, similar attention has not been paid to nitrogen although the effects on global ecology are arguably as dramatic. Click on this thumbnail (a figure from this paper) to see a graphic showing our species’ handiwork:


At issue is what has been termed by James Galloway, the most outspoken scientist on this issue, the nitrogen cascade. When reactive N (called Nr) is released into the environment, it can cause a cascade of environmental effects. A single molecule of N2 oxidized in your car’s engine to become 2NO can pollute the air in your city, drift out of town and fertilize an invasive plant species while acidifying the soil, and then end up in a stream or river where it encourages algal growth and finally contributes to a dead zone in the ocean before it is denitrified and sent back to the atmosphere, perhaps as N2O, a greenhouse gas. At the same time, nitrogen is important globally as fertilizer and a lot of people depend on it or need more of it to eat.

Scientists and a few other close observers are aware of the scope of the issue, but the general public is certainly not, and the educated public is only a little more aware. For example, take this recent article in The New York Times. It does a decent job reporting an increase in fertilizer demand, but it biffs when it talks about the environmental effects of nitrogen, mentioning only the water pollution and ignoring the other major parts of the cascade.

For whatever reason, and I can think of several, nitrogen has not yet gripped people the way climate change has. Perhaps because nitrogen-related issues are varied and complex, there is no main problem like rising global temperatures and sea levels to rally around. Perhaps it is just a smaller problem that doesn’t have room on the international stage. Perhaps the case to be made is still too murky and we need more information.

Whatever the reason for the lurking nature of this issue, I think that something more needs to be discovered for it move beyond the subject of wonky environmental policy discussions among experts. It doesn’t yet seem urgent enough for people to become mobilized. Although I can’t say the issue will continue to lurk or whether it will become more prominent, here are a couple scenarios that might turn attention to the nitrogen issue.

Right now, nitrogen is not included in global climate models. Why? Because spatially explicit global climate/carbon models that are coupled with nitrogen cycling models are too computationally intensive. We don’t have dedicated supercomputers that are fast enough. But climate scientists are trying to fund the next supercomputing facility for this type of analysis and more. If nitrogen turns out to be a key player in those models, we could find ourselves trying to reduce our nitrogen footprints as well as our carbon ones.

Another possibility that we should never discount is that someone could make some sort of black-swan-type scientific discovery that thrusts nitrogen to the forefront. Although this may not be the one, here’s an example of something unexpected that could be important. In Narragansett Bay in Rhode Island, researchers discovered that rising water temperatures, possibly due to climate change, have started to reduce the efficiency of microbes that scrub out nutrients and purify water flowing in from surrounding rivers:

If Narragansett is typical of other bays, they argue, it could be the harbinger of a new threat. Shifting the effect of anthropogenic nitrogen loading beyond the immediate coastal zone could destabilize ocean ecosystems by acidifying the waters, exacerbating harmful algal blooms, killing fish and shellfish, or perhaps even powering a vicious new cycle of global warming. The studies are currently hard to interpret and some say the system is poised to rebalance itself. But if they are wrong, global warming may do more to the oceans than make them rise.

The nitrogen issue is a lens with which to examine a lot of longstanding environmental issues like pollution, invasive species, and climate change. As recently as five years ago, it was difficult to imagine the public grasping this subtle approach. But now that the global carbon cycle has entered the public debate in a big way, we may be ready for nitrogen too. I recently saw a talk at a conference that ended with a slide that had a giant N on it. The speaker said that his message was that nitrogen is big. Time will tell if he’s right.


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