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 frequently affects the productivity of farming systems more than any other single management factor, but often the nitrogen is used inefficiently. Much of the fertilizer nitrogen applied to the soil is not utilised by the crop: it is lost either in solution form, by leaching of nitrate, or in gaseous forms as ammonia, nitrous oxide, nitric oxide or dinitrogen. The leached nitrate can contaminate rivers and ground waters, while the emitted ammonia can contaminate surface waters or combine with atmospheric sulfur dioxide to form aerosols which affect visibility, health and climate. There is also concern that increased evolution of nitrous oxide will deplete the protective ozone layer of the stratosphere. The possibility of a link between the intensity of agricultural use of nitrogen, nitrous oxide emissions and amounts of stratospheric ozone has focussed attention on these interactions.

Origem and distribution of nitrogen in soil. Inorganic forms of nitrogen in soil. Organic forms of soil nitrogen. Retention and fixation of ammonium and ammonim in soils. Biochemistry ammonification. Mineralization and Immobilization of soil nitrogem. Nitrogen trasport processes in soil. Nitrogen transformations in submerged soil. Advances in methodology for research on nitrogen transformations. Soil nitrogen budgets. Crop nitrogen requirements, utilizations, and fertilization. Nitrogen management for maximum efficiency and minimum pollution. Assesment of soil nitrogen availability. The effect of pesticides on nitrogen transformation in soils. Modeling of the soil nitrogen cycle. Economic implications of controls on nitrogen fertilizer use. Recycling of nitrogen through land application of agricultural, food processing, and municipal wastes. Energetics of nitrogen transformations. Nitrogem transfers and mass balances.

Volume 73 contains 4 reviews on contemporary and important topics in the agronomic sciences. In Chapter 1, "Interactions among Root-Inhabiting Fungi and Their Implications for Biological Control of Root Pathogens," fungi are defined, their distribution and abundance are discussed, and their role in agroecosystems is present. Chapter 2, "Dwarfing Genes in Plant Improvement," discusses advances in the role of dwarfing genes in plant improvement. Emphasis is placed on breeding and genetic aspects. Chapter 3, "A Review of the Effect of N Fertilizer Type on Gaseous Emissions," covers a topic that is of great environmental interest — the effect of nitrogen fertilizers on gaseous emissions. Chapter 4, "Rhizobia in the Field," is a comprehensive review of rhizobia including diversity, systematics, natural populations, and field introduction of rhizobia.