Mystery in the nitrogen cycle.

• Author: Nierenberg, Danielle
• Position: Update - Different use in polluted or pristine forests - Brief Article

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...