A World Awash with Nitrogen

摘要

For most of the history of the biosphere, nitrogen has swirled tantalizingly out of reach: In the form of inert N_2 gas (78% of the modern atmosphere), it was available only to certain bacteria and cyanobacteria capable of producing the nitrogenase enzyme that breaks the strong N-N triple bond. Even these nitrogen fixers cannot liberally fix nitro gen because of the high energy costs of run ning nitrogenase and the high demands for other elements needed to produce nitroge nase. It is for this reason that plant and algal biomass and productivity of many ecosystems are limited by nitrogen (1) and that the supply of nitrogen plays a major role in structuring plant communities (2). A report by Holtgrieve et al. on page 1545 of this issue (3) and other recent studies (4,5) shed light on the extent to which human activities have changed nitrogen availability, with implications for ecosystems around the world.