Mystery in the nitrogen cycle. (Update).

Polluted and pristine forests may use nitrogen in radically different ways, according to a recent report by Steven Perakis and Lars Hedin, two ecologists studying remote South American forests. Their research, which appeared in the January 24 issue of the scientific journal, Nature, could substantially alter our understanding of how forests respond to nitrogen pollution--and how they are likely to respond to climate change.

Nitrogen is a major plant nutrient and one of the most common elements on Earth--78 percent of the atmosphere consists of nitrogen in its elemental form ([N.sub.2]). But plants cannot metabolize elemental nitrogen; the substance doesn't become a nutrient until it is combined with certain other elements in a process called fixation. In nature, nitrogen fixation is the result of lightning strikes, volcanic eruptions, and the activities of certain soil microbes. But a great deal of additional nitrogen is now fixed artificially, especially in the production of synthetic fertilizer and the combustion of fossil fuels. During the past century, these and other human activities have more than doubled the amount of fixed nitrogen in circulation.

To understand how nitrogen is flowing through a forest, scientists monitor the quantities and types of nitrogen compounds that are being "leaked" from forest soils into streams and rivers. Scientists have only been doing this kind of work for a few decades, and nearly all of their attention has focused on North American and western European forests. In these forests, typically around three-quarters of the leaked nitrogen occurs in an inorganic form known as nitrate ([NO.sub.3]). ("Inorganic" means that the molecule does not include carbon.) Many of these northern forests are known to have been subjected to substantial long-term nitrogen pollution; nevertheless, the preponderance of nitrate was assumed to be a natural condition.

This is the assumption that the new research appears to undermine. Perakis and Hedin trekked to several remote forests in southern Chile and Argentina--areas that receive no significant nitrogen pollution. There, they sampled 100 forest streams, and found that nitrate accounted for only around 5 percent of the nitrogen content. About 70 percent of the stream nitrogen was in organic form--in the carbon-containing molecules that plants make from the nitrogen that they absorb. In North American forests, by comparison, only about 2 percent of the leaked nitrogen is organic.

It has generally been assumed that except for some unusual plant species, plant growth only occurs by metabolizing inorganic nitrogen, which is then converted to organic nitrogen in plant tissues. And there is no doubt that the process works this way in cropland (fertilizer supplies nitrogen in inorganic form). But the new research suggests that in pristine ecosystems, organic nitrogen may be far more important than the inorganic forms. And that, in turn, suggests that humans are somehow responsible for the overwhelming predominance of nitrate in northern forest ecosystems.

A great deal of additional research will be needed to understand the implications of that conclusion, assuming it holds up. "If we aren't able to clearly understand which aspects of the nitrogen cycle are natural versus induced," says Perakis, "then it will be difficult to determine exactly what condition the world is changing from, and where that change is headed in the future." Since the amount of fixed nitrogen is a major factor in determining rates of plant growth, and since growth rates determine how much climate-changing carbon dioxide plants can absorb, understanding the nitrogen budget is critical not just for the health of the forests, but for the health of the planet in general.

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