ABSTRACT
Soil nitrogen is derived primarily from atmospheric nitrogen gas, N2. The decomposition of organic matter converts some organic N into mineral-nitrogen, a term applied to ammonium, NH4+, nitrite, NH2- and nitrate, NH3-. Denitrification is the loss of nitrogen and nitrous oxide gas from the soil under anaerobic conditions. Nitrate and nitrite are reduced to these gases by micro-organisms NO3-and NO2-N2O N2. Nitrogen fixation is the conversion of N2 in the soil atmosphere into NH4+ by specialized groups of micro-organisms. Laboratory studies to examine the factors controlling mineralization rates have led to proposals for routine methods which measure the amount of mineral-N released during the incubation of soil under standard conditions. Plants take up mineral-N. Thus under natural vegetation, mineralized N with small amounts of mineral-N from the atmosphere supply plant needs and thus have a controlling effect on the productivity of natural systems. In agricultural systems crops also rely on these sources together with inputs from fertilizers and manures.
