A growing interest has been shown recently in the dymanics of nitrogen in agricultural and natural ecosystems. This has been caused by increasing demands for food and fibre by a rapidly expanding world population, and by a growing concern that increased land clearing, cultivation and use of both fertilizer and biologically fixed nitrogen can have detrimental effects on the environment. These include effects on water quality, eutrophication of surface waters and changes in atmospheric composition all caused by increased cycling of nitrogenous compounds. The input and availability of nitrogen...