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Carbon dioxide and nitrogen: not such a double whammy

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Rising CO2 levels might reduce the damage from another source: nitrogen pollution.

Between the conference in Copenhagen and those e-mails that were leaked recently, carbon dioxide, the major culprit for global warming, has been getting even more press than usual. But ecologists are familiar with another human pollutant that’s already having huge effects on ecosystems, even though it doesn’t make headlines. I’m talking about nitrogen.

Nitrogen makes up nearly 80% of the atmosphere, but pure nitrogen (aka N2) isn’t very easy to get at. What counts is ‘reactive nitrogen’, especially dissolved nitrate (NO3) and ammonia (NH4+). Plants can readily take this up and use it, helping them to grow faster. Gardeners have probably met NPK fertiliser; the N stands for nitrogen. And we’re producing a lot of reactive nitrogen, from power stations, from cars, but mostly by making fertiliser. In fact, over half of the reactive nitrogen being made each year now comes from human activities. That’s a substantially greater share than our effect on CO2.

Ecologists know quite well that adding nitrogen often leads to fewer species living together. And a recent experiment neatly showed why that might be: when plants’ roots are battling it out for nutrients (like nitrogen), it’s a relatively fair fight. But add nitrogen, and the fight moves above the ground, for the light plants need to grow. Here, bigger plants can quickly shade out smaller ones and kill (some of) them off.

But it’s all too clear that we’re not just adding nitrogen: year by year, the air has a little more carbon dioxide in. Carbon dioxide is also very important to plants: they use the energy from sunlight to grab it and build themselves. So what effect do the two have together? Peter Reich tested this the obvious way: add nitrogen to one set of areas, add CO2 to another set, and add both to a third set. Plus a control, of course: some areas were left alone. Keep it up for ten years, and then see what’s happened. Yes, ten years—species don’t come and go in a hurry.

Bizarrely, although lots of species (16%) were lost by adding nitrogen, and some (2%) were lost by adding carbon dioxide, adding carbon dioxide on top of nitrogen only caused 8% of species to disappear. That is, extra CO2 cut the damage done by extra nitrogen in half.

Graph showing the number of species under different conditions

The average numbers of species in control plots, with added nitrogen, with added carbon dioxide, and with both added. Data from reference.

So what’s going on? Prof. Reich has measured some other things, and has a few ideas:

  • Adding CO2, the plants take up more nitrogen. This means there’s less of it left in the soil, so it reinvigorates the root battle.
  • With more nitrogen, plants leave less water in the soil. Adding carbon dioxide seems to counteract that, so it might affect the number of species.
  • Adding nitrogen gives one sort of grasses a helping hand in particular. Carbon dioxide seems to even the score a bit.

So perhaps it is a double-whammy, but with whams from opposite directions: carbon dioxide and nitrogen have different effects on plants, and they seem to balance out to some extent. Although the effect of light wasn’t clear cut in this experiment, I think it might also be important that higher carbon dioxide lets plants grow in deeper shade.

Of course, there’s always the possibility that, even after ten years, maybe the extra CO2 just slowed down species loss. Plus, this study was done in grasslands, so it might not pan out the same for forests, or wetlands. But it’s still a fascinating result.

Reich, P. (2009). Elevated CO2 Reduces Losses of Plant Diversity Caused by Nitrogen Deposition Science, 326 (5958), 1399-1402 DOI: 10.1126/science.1178820


Written by Thomas Kluyver

8 December, 2009 at 11:49 pm

Posted in Papers

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