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Attempts to limit climate change by planting trees to absorb carbon dioxide from the atmosphere could be futile, according to two studies published this week.

It had previously been suggested that rising concentrations of the gas would boost plant growth and, with it, the amount of carbon dioxide plants absorb.

But two US-based teams — led by Johan Six of the University of California at Davis and Peter Reich at the University of Minnesota — say this is a false hope.

Insufficient amounts of nitrogen gas, they say, will limit plant growth regardless of how much extra carbon dioxide is available.

Nitrogen levels, also essential for plant growth, are not rising as fast as those of carbon dioxide. This means there is a limit to how fast plants can grow, and therefore how much carbon dioxide they can absorb, say the researchers.

Reich's team undertook one of the longest -ever studies of how plants respond to different levels of nitrogen.

Over the course of six-years, they found that the amount of nitrogen naturally available to plants gradually limited plant growth. Adding nitrogen to the soil reversed this trend.

For their part, Six and colleagues used data from 80 previous studies to see how nitrogen levels affect the way carbon is stored in soil.

Carbon dioxide that plants absorb from the atmosphere ends up in the soil they grow in, for instance when dead leaves and branches drop to the ground and decompose.

Six's team concluded that high levels of atmospheric carbon dioxide only leads to an accumulation of carbon in soil if nitrogen is added "at rates well above typical atmospheric inputs".

Six told SciDev.Net that because his study looked at data from a range of ecosystems, its results reflect general trends.

The scientists also found that even when nitrogen was not the limiting factor, other nutrients that were in short supply had the same restrictive effect.

The bottom line, says Six, is that we "cannot rely on nature to clean up" industrial carbon dioxide emissions.

Link to full paper in Nature (Reich and colleagues) 

Reference: Nature 440, 922 (2006)

Link to abstract of paper in Proceedings of the National Academy of Sciences (Six and colleagues)
[link will be made live by 16 April]

Reference: Proceedings of the National Academy of Sciences doi:10.1073/pnas.0509038103 (2006)