Nitrogen Fixation by Non-leguminous Plants

ALL STUDENTS OF BIOLOGY learn about legumes, including such agriculturally essential plants as peas, beans, and alfalfa that form a symbiosis with nitrogenfixing root nodule bacteria. But most biology students never learn about another abundant, widespread, and perhaps equally important group of plants that have nitrogen-fixing root nodules quite different from those of the legumes. This group of non-leguminous nitrogenfixing plants includes alder trees and shrubs (Alnus sp.), bayberry and sweet gale (Myrica sp.), and sweet-fern (Comptonia peregrina). These plants are rarely mentioned in basic biology or ecology texts, despite the fact that their nitrogen-enriching ability makes them important components of their ecosystems and potentially very useful to farmers and foresters. The so-called nitrogen-fixing plants are of special interest to me because I study vegetation tolerant of nutrient-poor soils. These plants have an advantage in such soils since they associate with bacteria that can convert or "fix" nitrogenous gas to ammonium (table 1). The plant harbors the bacteria in nodules that develop on its roots; the bacteria obtain respiratory carbon substrates from the plant, and the plant obtains nitrogen containing amino acids from the bacteria. Most of the nitrogen-fixing plants growing in the habitats I have been studying-sand dunes, burned sites, and eroded soils in the Northeast-are not legumes. But several years ago when I attempted to learn more about nitrogen fix-