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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

View larger version (12K): [in a new window]   Fig. 1. Schematic of the two models used: (a) considers linear local equilibrium and reversible NH4+ sorption, (b) considers NH4+ sorption and desorption as independent kinetics processes. Both models consider first-order kinetics for urea hydrolysis and pseudo-first order kinetics process for ammonia volatilization. KD is a distribution coefficient and kV, kN, kHU, kads, kdes are rate coefficients (volatilization rate, nitrification rate, urea hydrolysis rate, adsorption rate, and desorption rate, respectively).

View larger version (28K): [in a new window]   Fig. 2. Schematic of the experimental system.

View larger version (25K): [in a new window]   Fig. 3. Evolution of the different nitrogen forms described with the model assuming local equilibrium adsorption and the presence of an activation (lag) time for urea hydrolysis. Parameters used for this example are for the loamy sand (LS) soil at 28°C and = 0.15 cm3/cm3: U0 = 9.30 mg N/cm3, kV = 0.0095/h, KD = 0.025, tact = 200 h, kHU (hydrolysis rate constant) = 0.03/h. The value of kN (nitrification rate constant) was set to 0.001/h in order to reproduce the nitrification curve in the graph.

View larger version (24K): [in a new window]   Fig. 4. Evolution of the different nitrogen forms described with the model assuming ammonium adsorption-desorption kinetics and the presence of an activation (lag) time for urea hydrolysis. Parameters used for this example are for the loamy sand (LS) soil at 28°C and volumetric moisture content () = 0.15 cm3/cm3: initial concentration of urea (U0) = 9.30 mg N/cm3, volatilization coefficient (kV) = 0.0095/h, adsorption rate coefficient (kads) = 0.055/h, desorption rate coefficient (kdes) = 10–5/h, effective activation/adaptation time for the microbial population (tact) = 200 h, and hydrolysis rate constant (kHU) = 0.03/h. The value of kN (nitrification rate constant) was set to 0.001/h in order to reproduce the nitrification curve in the graph.

View larger version (23K): [in a new window]   Fig. 5. Experimental results for volatilized ammonia in the Sandy Clay Loam (SCL) soil at 28°C (circles) and 18°C (squares). Open symbols represent experiments carried out at volumetric moisture content () = 0.20 cm3/cm3; solid symbols represent experiments at = 0.33 cm3/cm3. Lines show model response with parameters presented in Table 3.

View larger version (18K): [in a new window]   Fig. 6. Experimental results for the loamy sand soil at 28°C using two different initial levels of urea. Squares represent volatilized amounts of ammonia for experiments using an initial urea concentration of 2 g/kg, and circles represent volatilized amounts of ammonia for experiments using an initial urea concentration of 0.2 g/kg. Open symbols show experiments carried out at volumetric moisture content () = 0.15 cm3/cm3, and solid symbols represent experiments at = 0.22 cm3/cm3. Lines show model response using the parameters presented in Table 3.