Nitrogen Fertilization I: The Nitrogen Balance

Abstract

Nitrogen is the most important nutrient in agricultural production. The natural input of N is due to N fixation, especially by Rhizobium bacteria that infect the roots of legumes. Organic N becomes inorganic through mineralization, and then inorganic N is absorbed by plants. Often soil microorganisms ``capture'' temporarily inorganic N when residue with high C/N ratio decompose (immobilization process). The ammonium in the soil is converted to NO3− through nitrification which is greatly reduced in waterlogged soils. In the latter denitrification generates gaseous N forms that are lost (oxides of N and N2). Major losses of N may occur by nitrate leaching which is proportional to deep percolation and to nitrate concentration in the soil solution. A Leaching Index may be calculated as a function of rainfall and soil type to quantify the risk of leaching.