THE ROLE OF NITROGEN IN CROP PRODUCTION AND LOSSES OF NITRATE BY LEACHING FROM AGRICULTURAL SOIL

Managing land to produce food, fibre or timber must have some environmental impact, the magnitude of which will depend on the cropping system and the intensity of management. Nitrogen is an indispensable input for modern agricultural systems, which not only aim to feed people but seek to sustain rural communities dependent on agriculture. In temperate regions there is a universal problem of nitrate leaching from agricultural land, and increases in nitrate concentrations in water bodies in recent years have been a cause for concern, especially the role of nitrate in the development of algal blooms, Nitrate invariably appears in drainage from agricultural land in the absence of any significant input of nitrogen as a result of the breakdown of soil humus or from aerial deposition of combined nitrogen in various forms. Where only inorganic nitrogen fertilizers are applied in amounts and at times to satisfy crop demand, they are apparently used efficiently, Where nitrate in drainage is a direct residue from applied nitrogen fertilizers, it can usually be associated with the use of excessive quantities or with the failure of a crop to achieve its expected yield. Most of the nitrate which appears in soil in autumn comes from the microbial mineralization of soil organic matter. The soil microbial population breaking down organic matter does not differentiate between soil humus or organic matter added to soil by ploughing in grass leys, forage legumes or large quantities of organic manures. Adding such organic materials to soil can lead to the release of much nitrate. Such microbial processes would be impossible to control in environmentally benign ways.