Nitrogen in Agriculture: An Overview and Definitions of Nitrogen Use Efficiency (Paperback)

The increased use of nitrogen (N) fertilizers in agriculture in the last five decades is justified by the increased yield potential of the crops grown and the judicious use of agrochemicals to control weeds, pests and diseases to protect that potential. Nitrogen use-efficiency can be assessed in a number of ways, discussed here with examples, but is usually determined as percent recovery of added N in the harvested product when calculated by the difference method. Assessed in this way, N-use efficiency is frequently only about 50%. But N-use efficiency of applied N should include that taken up by roots to produce the above-ground plant, where N plays a vital role in photosynthesis, the fixation of carbon dioxide to produce sugars, and the root system, which takes up nutrients and water. The fate of this N after harvest is important, but the total amount in the crop should be included in any estimate of percent recovery of applied N. Within the plant-soil system, the soil microbial population competes with the plant for fertilizer N applied to increase plant growth. Hence any fertilizer N remaining in the soil should also be included in an estimate of the fate of applied N. The use of 15N-labelled fertilizer allows the fate of the applied N to be determined. At harvest up to 70%, and sometimes more than this, of the applied N can be accounted for in the above-ground crop and in the soil to 100 cm. The fate of the N which is not accounted for should be a major research topic.