Sir

The News Feature “Fertilized to death” (Nature 425, 894–895; 2003) contains several inaccuracies about how reactive nitrogen is affecting ecosystems. As scientists working to understand these effects and to formulate methods for reducing them, we feel that the increase in reactive nitrogen is a serious and critical issue, which deserves informed discussion of the many complex issues involved.

First, although excess fertilizer use can have adverse environmental consequences, it contributes little to acid rain, whose principal causes are sulphur and NOx emissions from fossil-fuel combustion by power plants and vehicles. Moreover, although ammonia emissions from agriculture contribute to high rates of nitrogen deposition in some forests, indirectly increasing soil acidity, these contributions are primarily from livestock, not from fertilizer.

Second, although the feature does state that it is not clear how different types of forest will respond to increased nitrogen deposition, it should be emphasized more clearly that the effects of increased nitrogen deposition on forest growth and mortality are highly variable and complex, and even the direction of response (increased or decreased growth) remains uncertain.

Third, it is important to recognize that almost half the nitrogen applied to crop fields is lost through aqueous run-off and atmospheric emissions (not run-off alone as stated in the article) and that the environmental impact of these two processes are very different. Aqueous losses of fertilizer nitrogen (∼20%) are usually substantially less than atmospheric losses, although this run-off can still have serious environmental effects. Much of the loss to the atmosphere is through denitrification, which removes most of the reactive nitrogen from the ecosystem but can produce the greenhouse gas N2O.

Finally, not all potential solutions to this problem need to be expensive or result in increasing the costs of agricultural production. In the United States, at least, current levels of agricultural production can be met by reducing excess application of synthetic nitrogen fertilizer, potentially saving farmers money.