Role of nitrifier denitrification in the production of nitrous oxide

Nitrifier denitrification is the pathway of nitrification in which ammonia (NH3) is oxidized to nitrite (NO,) followed by the reduction of NO2- to nit ric oxide (NO), nitrous oxide (N2O) and molecular nitrogen (N-2). The trans formations are carried out by autotrophic nitrifiers. Thus, nitrifier denit rification differs from coupled nitrification-denitrification, where denitr ifiers reduce NO2- or nitrate (NO3-) that was produced by nitrifiers. Nitri fier denitrification contributes to the development of the greenhouse gas N 2O and also causes losses of fertilizer nitrogen in agricultural soils. In this review article, present knowledge about nitrifier denitrification is s ummarized in order to give an exact definition, to spread awareness of its pathway and controlling factors and to identify areas of research needed to improve global N2O budgets. Due to experimental difficulties and a lack of awareness of nitrifier denitrification, not much is known about this mecha nism of N2O production. The few measurements carried out so far attribute u p to 30% of the total N2O production to nitrifier denitrification. Low oxyg en conditions coupled with low organic carbon contents of soils favour this pathway as might low pH. As nitrifier denitrification can lead to substant ial N2O emissions, there is a need to quantify this pathway in different so ils under different conditions. New insights attained through quantificatio n experiments should be used in the improvement of computer models to defin e sets of conditions that show where and when nitrifier denitrification is a significant source of N2O. This may subsequently render the development o f guidelines for low-emission farming practices necessary. (C) 2001 Elsevie r Science Ltd. All rights reserved.