Geoengineering Redux: Fertilizing Trees with Nitrogen to Fight Climate Change

Image from Joshua Rappeneker

It's not quite ocean iron fertilization, but I have a feeling this new geoengineering proposed will still raise quite a few hackles. The idea, which originated with Federico Magnani of Italy's University of Bologna, is, as the name implies, fairly straightforward: You would fertilize, or "dope," as the New Scientist's Catherine Brahic put it, trees with nitrogen to stimulate their ability to absorb more carbon dioxide and, by increasing their albedo, to reflect more solar radiation back out into space. More nitrogen: higher carbon uptake and higher solar energy reflection
In a new study published in the Proceedings of the National Academy of Sciences, Scott Ollinger of the University of New Hampshire and several colleagues discovered that forest canopies with higher concentrations of nitrogen tended to take up more carbon dioxide and reflect more solar energy than those with little of the nutrient.

Magnani, who has spent many years studying carbon uptake in nitrogen fertilized trees, reported similar results in a study he published 2 years ago in Global Change Biology. Beginning in 1994, he and his colleagues simulated chronic nitrogen deposition (deposition that typically occurs because of atmospheric pollution) by adding 3 grams of nitrogen per square meter to four different northern hardwood forests in Michigan every year.

Ten-year study showed chronic N deposition boosted ecosystem C storage
After a decade of observation, they concluded that, compared to a control treatment, nitrogen deposition had greatly increased ecosystem carbon storage (i.e. carbon sequestration) in both the trees and the surrounding soil. Magnani used the study's results to argue that a program of annual nitrogen fertilization in the northern hemisphere could potentially create a new sink for carbon dioxide.

Many scientists, including Ollinger, aren't sure this would be such a good idea. For one thing, little is still known about the relationship between albedo and nitrogen; even if the nutrient does act as a switch that changes the leaves' structure to increase their albedo, only certain species would be able to take advantage of this property. As a result, if we wanted to apply this method on a sufficiently large scale to effect carbon emissions, we would have to plant entire forests made just out of those few species.

And then there are all the environmental downsides associated with high nitrogen concentrations: nitrous oxide emissions (a far more potent greenhouse gas), groundwater contamination and drying (trees that consume larger amounts of nitrogen need more water), just to name a few.

For his part, Magnani agrees that more research needs to be done before governments or businesses can seriously consider spraying large swathes of forest with nitrogen. He has just submitted a research proposal to the European Union to study the long-term effects of fertilization on carbon storage -- specifically the effects of artificial fertilization. A side objective would be seeing whether it makes sense to plant northern forest species in regions where nitrogen pollution levels are already high.

But at what risk?
Like iron fertilization before it, I doubt this plan will win many converts outside of the scientific community. With the London Convention recently imposing restrictions on commercial fertilization, it remains to be seen whether the EU will be willing to support another risky geoengineering scheme.

More about geoengineering
Fake Plastic Trees to Solve Climate Change
This Month In Wired: Geoengineering and Ken Caldeira


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