Nitrification inhibitors, with emphasis on natural products, and the persistence of fertilizer nitrogen in the soil

Abstract

Nitrogenous fertilizers have been major contributers to increases in agricultural productivity worldwide. However, there is continuing need to improve the efficiency of N fertilizer use to achieve more efficient production of food crops and minimize fertilizer-related pollution of the environment. Poor efficiency of fertilizer N use is largely due to loss of N by denitrification, ammonia volatilization or leaching. With the exception of ammonia volatilization, these losses are associated with and follow the nitrification of ammonium to nitrate. The nitrification process converts the relatively immobile cationic ammonium to nitrate, which is very mobile and generally not retained by soil particles and subject to loss by leaching and denitrification. Nitrogen use efficiency can be increased by use of nitrification inhibitors. These chemicals retard nitrification in soils by slowing the rate of conversion of ammonium to nitrite, but have no effect on the rate of oxidation of nitrite to nitrate. This reduces the loss of N through leaching and denitrification in situations where such losses are normally high. Retardation of nitrification maintains a relatively higher proportion of inorganic N in the ammonium form which is retained by soil minerals and thus less liable to transport and movement in the soil system. Nitrification inhibitors can also increase the persistence of N by incorporating ammonium in the organic N fraction or by its migration to fixed or nonexchangeable sites on clay minerals. However, surface application of nitrification inhibitors in conjunction with ammonium or ammonium-forming fertilizers to light-texured soils may enhance volatile loss of ammonium. The present status and scope of research on the effects of nitrification inhibitors on the persistence of N in the soil is presented with emphasis on indigenous materials. Future research needs relating to the use of nitrification inhibitors are considered.