Nitrogen: regional contributions to the global cycle

N itrogen is critical for life on Earth: it is an integral component of all amino acids, the basic constituents of protein. Although elemental nitrogen is abundant-the Earth's atmosphere is 79 percent nitrogen-the availability of nitrogen often limits the productivity of plant communities, whether natural vegetation or agricultural crops. This is because only certain forms of nitrogen are available to most organisms, and these forms are generally in short supply. Much effort has been devoted to understanding these forms of nitrogen-in particular, their abundance and the rates at which they are transformed from one form to another. We now have an excellent qualitative understanding of most parts of the overall nitrogen cycle, including the ways in which anthropic (human-induced) influences can affect major portions of it. (See also Robert B. Cook, "Man and the Biogeochernical Cycles: Interacting with the Elements," Environment, September 1984.) Such influences can have important consequences at many geographic scales: at the local level, anthropic changes in the cycle can affect crop productivity and perhaps surface-