Nitrogen Fertilization I: The Nitrogen Balance

Nitrogen is usually the most limiting nutrient for plants in agroecosystems. The natural input of N in agricultural soils comes from the atmosphere by N fixation which implies the transformation of gaseous N2 in NH4+. Fixation is mainly a biological process, and the most relevant organisms involved are Rhizobium bacteria that infect the roots of legumes. Organic N becomes inorganic through mineralization, and then inorganic N is absorbed by plants. Soil microorganisms may capture temporarily inorganic N when residue with high C/N ratio decomposes (immobilization). Ammonium in the soil is converted to NO3− through nitrification, a microbial process greatly reduced in waterlogged soils. In the latter, denitrification generates gaseous N forms that are lost. Nitrate leaching 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.