Evaluation of Ammonia Volatilization in the Field

Abstract

Until about 10 years ago, NH, volatilization had been studied primarily under controlled laboratory conditions. However, the recent development of methods to measure NH3 losses under field conditions has led to a much improved understanding of NH3 volatilization and the factors which influence it. In most cases, the magnitude of NH3 loss under laboratory conditions is quite different from that measured in the field because laboratory conditions are not very representative of the dynamic field environment. The objective of this review was to evaluate data related to NH3 volatilization under field conditions. Soil factors that are important in determining the magnitude of NH, volatilization in the field include initial soil pH, pH buffering capacity, cation exchange capacity, amount of soluble and exchangeable Ca2+, and urease activity. However, environmental factors such as temperature, soil water content, and air exchange at the soil surface are overriding factors in determining the magnitude of NH3 loss. Soil water content and air exchange at the soil surface are of particular importance. In the field, these factors fluctuate widely on a diurnal basis due to the combined effects of dew formation and evaporation and are difficult to mimic under laboratory or simulated field conditions. It is apparent from recent studies that, on a diurnal basis, high NH3 loss rates are generally associated with periods of rapid soil drying, when wet (near field capacity) surface soil is followed by several days with little or no rainfall (<0.5 in.). It is also apparent that NH3 loss is related to windspeed under field conditions, and that low air exchange rates at the soil surface, such as might be expected in a crop canopy or in no-tillage production systems, might limit NH3 loss. Management factors such as N source, N rate, application method, crop residue management, and fertilizer modification also influence NH3 losses under field conditions, although these have not been widely measured. Additional research needs include: (i) a better understanding of urea hydrolysis under field conditions, (ii) better quantification of soil moisture effects under field conditions, (iii) better quantification of effect of air exchange at the soil surface, (iv) evaluation of NH3 loss in the field as affected by management techniques, (v) additional field measurements using appropriate methodology, and (vi) development of computer-based simulation models of NH3 volatilization.