Nitrogen is a very important nutrient element in agriculture. In soils it occurs in organic and inorganic forms. Inorganic N occurs primarily as nitrate in arable soils. Nitrate is subject to various processes such as plant uptake, leaching from soils among others. Leaching of nitrates from soils is a global phenomenon. Although a lot of attention has been paid world over on this phenomenon, its importance is being felt recently in developing countries like India where the emphasis has been on the problems related to increased food production from limited cultivable land. However, maintaining delicate agro-ecosystems in order to achieve sustainable agricultural productivity while protecting the environment has attracted the attention of scientists and policy makers. This article discusses the phenomenon of nitrate leaching from soils, its impact on man and animals and means to minimize the leaching. Nitrate leaching could be a major threat to environment in different agricultural situations. By proper management of agricultural systems, these leaching losses could be reduced and pollution problems can be minimized.

Nitrogen conversions in soil–plant systems

Nitrogen occurs in soil in several forms and inter conversion between these forms is the net result of a large number of dynamic processes (Figure 1). Many of these processes are mediated by micro-organisms. While incorporation of ammonium into organic compounds by microbial assimilation is known as immobilization, the reverse process where micro-organisms oxidize organic matter to produce energy and convert organic nitrogen into inorganic nitrogen is known as mineralization and both these processes occur simultaneously. In most soils, ammonium is rapidly converted to nitrate via nitrite by a process called nitrification, where ammonium is oxidized to nitrite and then to nitrate by the action of the aerobic bacteria such as  Nitrosomonas, Nitrosospira, Nitrosococcus  or Nitrosovibrio and Nitrobacter, Nitrospina or Nitrococcus, respectively. Ammonium is adsorbed on clay minerals and therefore is less mobile but nitrate is highly mobile. Plants take up nitrogen in mineral form (ammonium or nitrate). Nitrate is very soluble and unless intercepted and taken up by plant roots, leach down in the soil along with irrigation or rain water or it is carried away by runoff. Under some conditions, depending on the availability of organic carbon  and anaerobic conditions nitrate may undergo bacterial conversion to molecular nitrogen or nitrous oxide, by a process called ‘denitrification’. Unlike nitrifying bacteria, denitrifying bacteria include a wide range of bacteria. Presence of nitrates, excess water and available carbon source are important  factors that affect the denitrification process. Denitrification may result in the liberation of nitrogen (N2), nitrous oxide (N2O) or nitric oxide (NO). While nitric oxide easily gets converted to nitrate and is brought down again by precipitation, nitrous oxide escapes to the stratosphere and destroys ozone. It also produces a powerful greenhouse effect. Nitrate is also immobilized by microbes but to a lesser extent than ammonium.