Ammonia Emission from Mineral Fertilizers and Fertilized Crops

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, e.g., 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.