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The processes of NH3 emission from field-applied slurry are reviewed and their relative importance assessed. In achieving this objective, the study served to focus on a number of features that have not previously been highlighted. These include the effect of the size of the area to which slurry is applied, the interaction between solar radiation input and wind speed, the role of the solid chemistry and the interaction between slurry NH4+ and the slurry/soil cation exchange capacity (CEC). The most important processes controlling NH3 volatilisation were considered to be turbulent and molecular diffusion in the atmosphere, meteorological processes controlling evaporation and surface temperature, the ion production and buffering processes controlling the pH of the slurry/soil liquid, the solid chemistry that determines precipitation of NH4+ to slurry dry matter, the physical processes controlling the movement of slurry liquid into and within the soil, and the interaction of slurry liquid with soil CEC. [Copyright &y& Elsevier]

Abstract: Ammonia and pesticide volatilization in the field is a surface phenomenon involving physical and chemical processes that depend on the soil surface temperature and water content. The water transfer, heat transfer and energy budget sub models of volatilization models are adapted from the most commonly accepted formalisms and parameterizations. They are less detailed than the dedicated models describing water and heat transfers and surface status. The aim of this work was to assess the ability of one of the available mechanistic volatilization models, Volt''Air, to accurately describe the pedo-climatic conditions of a soil surface at the required time and space resolution. The assessment involves: (i) a sensitivity analysis, (ii) an evaluation of Volt''Air outputs in the light of outputs from a reference Soil–Vegetation–Atmosphere Transfer model (SiSPAT) and three experimental datasets, and (iii) the study of three tests based on modifications of SiSPAT to establish the potential impact of the simplifying assumptions used in Volt''Air. The analysis confirmed that a 5mm surface layer was well suited, and that Volt''Air surface temperature correlated well with the experimental measurements as well as with SiSPAT outputs. In terms of liquid water transfers, Volt''Air was overall consistent with SiSPAT, with discrepancies only during major rainfall events and dry weather conditions. The tests enabled us to identify the main source of the discrepancies between Volt''Air and SiSPAT: the lack of gaseous water transfer description in Volt''Air. They also helped to explain why neither Volt''Air nor SiSPAT was able to represent lower values of surface water content: current classical water retention and hydraulic conductivity models are not yet adapted to cases of very dry conditions. Given the outcomes of this study, we discuss to what extent the volatilization models can be improved and the questions they pose for current research in water transfer modeling and parameterization. [Copyright &y& Elsevier]