A thorough understanding of the physical and chemical processes involved in NH 3 emission from inorganic N fertilizers and fertilized crops is required if reliable and operational NH 3 emission factors and decision support systems for inorganic fertilizers are to be developed, taking into account the actual soil properties, climatic conditions and management factors For this reason, the present review focuses on processes involved in NH 3 volatilization from inorganic nitrogen fertilizers and the exchange of ammonia between crop foliage and the atmosphere The proportion of nitrogen lost from N fertilizers due to NH 3 volatilization may range from ≈0 to >50%, depending on fertilizer type, environmental conditions (temperature, wind speed, rain), and soil properties (calcium content, cation exchange capacity, acidity) The risk for high NH 3 losses may be reduced by proper management strategies including, eg, incorporation of the fertilizer into the soil, use of acidic fertilizers on calcareous soils, use of fertilizers with a high content of carbonate-precipitating cations, split applications to rice paddies or application to the soil surface beneath the crop canopy The latter takes advantage of the relatively low wind speed within well-developed canopies, reducing the rate of vertical NH 3 transport and increasing foliar NH 3 absorption Conversely, NH 3 is emitted from the leaves when the internal NH 3 concentration is higher than that in the ambient atmosphere as may often be the case, particularly during periods with rapid N absorption by the roots or during senescence induced N-remobilization from leaves Between 1 and 4% of shoot N may be lost in this way

Ammonia Emission From Mineral Fertilizers and Fertilized Crops

Anthropogenic NH3 emissions in europe

1. Volatilization of Ammonia.- 2. Biological denitrification.- 3. Chemodenitrification.- 4. The measurement of denitrification.- 5. Micrometeorological methods for measuring gaseous losses of nitrogen in the field.- 6. Gaseous nitrogen losses from plants.- 7. Nitrogen loss from sewage sludges and manures applied to agricultural lands.- 8. Ammonia loss from fertilizer applied to tropical pastures.- 9. Gaseous nitrogen exchanges in grazed pastures.- 10. Fate of fertilizer nitrogen applied to wetland rice.- 11. The fate of nitrogen compounds in the atmosphere.- 12. Agronomic technological approaches to minimizing gaseous nitrogen losses from croplands.

Gaseous loss of nitrogen from plant-soil systems

Ammonia is ubiquitous in Nature, being formed from the biological degradation of proteins in soil organic matter, plant residues and animal wastes. Its presence is readily detectable near barns, stables and feedlots where plant and animal residues are concentrated but it is also formed in many other situations from less concentrated sources, e.g. in fields and forests (Lemon and Van Houtte 1980). It is constantly being formed in soils at rates which depend on the level of microbial activity and the susceptibility of organic N compounds to biological attack. It is also being added to soils in increasing amounts as fertilizer.

Volatilization of ammonia

Between 10 and 20%of the N in fertilizers applied as urea is lost to the atmosphere as ammonia (NH 3 ). In contrast only small ( 3 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 NH 3 emissions in Europe. However, because of the greater susceptibility of nitrate- (NO 3 − ) based fertilizers to denitrification, the replacement of urea by AN may lead to increased emissions of nitrous oxide (N 2 O). There was a need therefore to critically review the evidence for substantially greater emissions of NH 3 − from urea than from other N fertilizers and also to appraise the effect of fertilizer-N type on emissions of N 2 O. Ammonia emissions from N fertilizers are consistent with their known effects on soil chemistry. Those that increase soil solution pH, for example, by increasing HCO 3 concentration or by reducing the concentration of Ca 2+ , have the greatest potential for NH 3 emission. In consequence the greatest emissions of NH 3 are from urea applied to any soil and from ammonium sulfate (AS) applied to soils of pHs > 7.0. Losses of NH 3 from AN were confirmed to be consistently less than from urea. Emissions of NH 3 from solutions composed of urea and AN were found to be intermediate between the two fertilizers. Thus applying urea in solution will not reduce NH 3 emissions. However, NH 3 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 NO 3 − -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 N 2 O. For summer applications, the substitution of AN for urea is likely to decrease N 2 O 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 N 2 O 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 NO x , 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.

J. E. Matocha

Papers

Ammonia Losses from Surface-Applied Nitrogen Fertilizer as Controlled by Soluble Calcium and Magnesium: General Theory1