Influence of pH and Partial Oxygen Pressure on the N₂O-N to N₂ Ratio of Denitrification

Abstract

It has been widely accepted that the relative amount of N₂O from soils may be favoured by acid rather than neutral or alcaline conditions, and/or by increasing the oxygen tension of the system concerned. The former observation has been ascribed to the relative sensitivity of N₂O-reductase to an increased proton activity, whereas the latter suggests that O₂ becomes increasingly a competitive electron acceptor to nitrate resulting in an incomplete denitrification. Interest in conditions that affect the production of N₂O has been stimulated by the concern about the possible destruction of stratospheric ozone as a result of the increased use in mineral nitrogen fertilizers. In soil and waters denitrification losses depend on amount and type of energy source, temperature, oxygen diffusion rate, nitrate availability and pH, all factors that are subject to continuous changes and interactions. However, among these factors, pH and oxygen partial pressure (a function of moisture tension) are thought to be key factors that enhance the relative amounts of N₂O particularly at high amounts of nitrate (Focht, 1974; Knowles, 1981, 1982). Estimates of the total quantity of gaseous nitrogen (N₂O-N + N₂) released annually from soil and water sources are still scattered and tentative, but in situ field measurements of N₂O (with and without acetylene inhibition) will contribute increasingly to the knowledge of denitrification (Focht, 1978; Knowles, 1982). Results from the acetylene inhibition technique, however, should be considered with care, since C₂H₂ may reach only a part of the N₂O-reductases (Yeomans and Beauchamp, 1978), partially inhibit N₂O-reductase itself (Lensi and Chalamet, 1979; 1982), accelerate the reduction of nitrate into N₂O (Lensi and Chalamet, 1982) or even stimulate denitrification by acting as an energy source (Yeomans and Beauchamp, 1982; Haider et al., 1983). Further, N₂O release from soil varies with time, climatic and envrionmental conditions (Letey et al., 1980; Ottow and Fabig, 1984) which make long-term measurements inevitable. Before accurate and reliable predictions are made on the possible effects of agricultural fertilizer practices, more basic information is needed on (1) the long-term rate of N₂0 production (relative to total denitrification) from both natural and fertilized soils and (2) on those environmental conditions that determine essentially incomplete transformation of nitrate into N₂0. In order to elucidate the effect of one single ecological factor on the relative amount of N₂0, model experiments under defined conditions are quite appropriate. In the present paper, such model experiments were made both with soils and liquid cultures in order to determine the role of pH and p0₂ on the relative amounts of N₂0 in the presence of high nitrate concentrations.