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SPECIAL SECTION: ZNS'03 VADOSE ZONE RESEARCH

Characterization of Nitrogen Transformations, Sorption and Volatilization Processes In Urea Fertilized Soils

^a Dep. de Ingeniería Química, Univ. de Valladolid, Spain
^b Inst. Tecnológico Agrario de Castilla y León, Valladolid, Spain
^* Corresponding author (silvia{at}iq.uva.es)

Received 23 June 2004.

The fate and transport of nitrogen compounds in urea-fertilized soils has historically been a topic of great interest. At present, several models exists for simulating the various transformations involved. A major constraint in the application of these models is the assignment of numerical values to the coefficients that quantify these transformations. This study attempts to characterize the processes linked to the hydrolysis of urea, adsorption and volatilization of ammonia, and nitrification in agricultural soils. We followed the temporal sequence of volatilization of ammonia in two different soils fertilized with urea under controlled conditions in the laboratory. The amounts of nitrate and ammonia in soil were also measured at the end of each trial. Several modeling alternatives were analyzed in order to accurately reproduce the results. Among these alternatives, the most successful approaches included the kinetics of urea hydrolysis and ammonia adsorption and desorption processes. Urea hydrolysis required a parameter accounting for an activation time, which was found to be greater at relatively high urea concentrations and low experimental temperatures. The adopted methodology in this study can provide rate data for N transformation and coupled sorption processes critically needed in N fate and transport studies. Values were estimated for kinetic coefficients which may be useful in general-purpose models. Rate data presented in this work quantify the effects of temperature, soil moisture, and initial concentration of applied urea on the dynamics of urea hydrolysis, nitrification, and ammonia sorption and volatilization.

Abbreviations: LS, loamy sand • SCL, sandy clay loam