A review of the effect of N fertilizer type on gaseous emissions

Between 10 and 20%of the N in fertilizers applied as urea is lost to the atmosphere as ammonia (NH3). In contrast only small (<4% of N applied) emissions of NH3 have been measured following the application of ammonium nitrate (AN) fertilizer. In consequence the replacement of urea fertilizer with AN has been proposed as a cost-effective measure to reduce NH3 emissions in Europe. However, because of the greater susceptibility of nitrate- (NO3−) based fertilizers to denitrification, the replacement of urea by AN may lead to increased emissions of nitrous oxide (N2O). There was a need therefore to critically review the evidence for substantially greater emissions of NH3− from urea than from other N fertilizers and also to appraise the effect of fertilizer-N type on emissions of N2O. Ammonia emissions from N fertilizers are consistent with their known effects on soil chemistry. Those that increase soil solution pH, for example, by increasing HCO3 concentration or by reducing the concentration of Ca2+, have the greatest potential for NH3 emission. In consequence the greatest emissions of NH3 are from urea applied to any soil and from ammonium sulfate (AS) applied to soils of pHs > 7.0. Losses of NH3 from AN were confirmed to be consistently less than from urea. Emissions of NH3 from solutions composed of urea and AN were found to be intermediate between the two fertilizers. Thus applying urea in solution will not reduce NH3 emissions. However, NH3 emissions from urea may be reduced by the use of urease inhibitors. Nitrous oxide emissions are crucially dependent on the interaction between timing of N fertilizer application and weather. Conditions in spring are more likely to be wet so that emissions are greater from NO3−-based fertilizers than from AS. In the summer conditions may be dry or wet; under dry conditions emissions are usually smaller than under wet conditions. For urea the effect of pH appears to be important. Generally greater emissions can take place from urea, except where temperature (controlling the rate of urea hydrolysis) and rainfall (controlling the dispersion of alkalinity) limit this. Thus, the substitution of AN for urea for spring applications is likely to increase emissions of N2O. For summer applications, the substitution of AN for urea is likely to decrease N2O emissions providing conditions are relatively dry; when conditions are wet large emissions may occur from both AN and urea. At this stage it is difficult to say with any certainty whether a strategy based on urea or AN would result in the smaller N2O emissions. Nitric oxide (NO) may also be released from soils following N fertilizer application. While soil emissions of NO are small in comparison with other sources of NOx, it is worth considering the effect of fertilizer type on this gas as well. Insufficient data is available to predict the effect of urea substitution on NO emissions, but since these are mainly a consequence of nitrification then replacing urea with AN should also reduce NO emissions.