Published online 18 August 2008 | Nature | doi:10.1038/news.2008.1046

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Trees eat pollution products

Leaves can absorb organic nitrates and turn them into amino acids.

Forest canopies can suck up organic nitrogen compounds produced from pollutants and turn them into useful amino acids.

Paul Shepson and his colleagues at Purdue University in West Lafayette, Indiana, believe they are the first to show that trees can take in organic nitrates through their leaves and then process the nitrogen.

forestForests can turn organic nitrates produced from pollutants into useful amino acids.Punchstock

Plants are already known to use their leaves to absorb inorganic airborne nitrogen molecules, such as ammonia or nitrogen dioxide, and turn them into amino acids. And a relatively reactive compound called peroxyacetyl nitrate can be absorbed by leaves, although it's not clear whether plants actually use it. "There's a difference between [nitrogen species] going into the leaves and that process being useful," says Shepson.

The organic nitrates in question are created from the plant's own chemicals. Many trees emit reactive molecules known as volatile organic compounds (VOCs), the most common of which is called isoprene. These VOCs are so reactive that they quickly get consumed in the atmosphere, and some react with nitrogen oxides (NOx) — emitted from combustion engines — to form longer-lived organic nitrate compounds, more stable than peroxyacetyl nitrate.

Shepson's team studied how seedlings of the trembling aspen, a widespread North American native and an isoprene emitter, reacted to an analogue of these compounds called 1-nitroxy-3-methyl butane. This compound had been radiolabelled with a specific stable nitrogen isotope — nitrogen-15 — which makes up less than half a per cent of Earth's nitrogen.

Shepson et al. then mushed up the leaves and measured how much nitrogen-15 was in them. The researchers tracked two amino acids: glutamate, the first amino acid the tree makes from absorbed nitrogen compounds, and aspartate, which is formed further downstream in the biochemical process. In test leaves, both amino acids contained low but significant levels of the nitrogen isotope, whereas those in control leaves contained almost none.

The team also measured how fast the labelled organic nitrate was taken up, and found that it was absorbed at up to half the rate of NO2, and one tenth the rate of peroxyacetyl nitrates. The work is published in Geophysical Research Letters1.

Pollution implications

Shepson says he was surprised to find any nitrogen-15 in the leaves at all. Organic nitrates are relatively unreactive, so the leaves must have a specific biochemical pathway for breaking them down. The team estimates that these organic nitrates may contribute about 1% of the total nitrogen used by trees.

Knowing that the forest canopy can take up and use organic nitrogen is really important, says Jed Sparks of Cornell University in Ithaca, New York, who is an expert on the role of nitrogen in the atmosphere and in plants. "[This work] definitively demonstrates that organic nitrates can be incorporated into plant proteins," Sparks says.

It also has potential implications for polluted areas where NOx is reacting with the VOCs from plants, says Bill Collins from the UK's Meteorological Office. If plants really can use the by-product — organic nitrates — then this will ultimately help to remove NOx from the atmosphere, he says. The process would be an efficient way to transport nitrogen away from polluted areas, he suggests. "If plants can take up these organic nitrates, human activity has a larger effect on plant fertilization than we thought."

The suggestion that atmospheric nitrogen from pollution could be sequestered and used by trees has been hypothesized before, says Shepson, and his team's work shows it to be likely — although the significance of the reaction may not be huge. "The bottom line is that while we found this is bio-mechanistically possible, it is small compared with the atmospheric source of nitrate and ammonium, and also small compared with what is utilized from the soil." And more nitrogen isn't always better for the plant: "Stated simply, plants can be over fertilized," he says. 

  • References

    1. Lockwood, A. L., Filley, T. R., Rhodes, D. & Shepson, P. B. Geophys. Res. Lett. 35, L15809 (2008) | Article |
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