Irrigated agricultural areas with high levels of soil Se face the dilemma of elevated Se levels in drainage and groundwater systems, which will adversely affect the environment and the wildlife. The release of Se from soils to water systems is dependent on the speciation of Se, which is primarily governed by the redox potential. This study describes the dynamics of redox transformations of naturally occurring Se in a soil matrix as a continuous function of time during a sequence of oxic‐anoxic‐oxic transition. An experimental setup where a solution stream is continuously passed through a soil column in a closed loop to reach an equilibrium between the soil and the solution was used. The pH and redox potential (Eh) were recorded and the solution was analyzed over time for various Se forms and Mn2+. During soil reduction, the total soluble Se and SeO2−4 decreased, while the SeO2−3 and other Se forms (organic Se, elemental Se, and selenide) increased initially, then decreased. Both soluble Se and SeO2−4 decreased during the anoxic phase, slowly at first, then more rapidly, and were characterized by first‐order rate constants. During reoxidation, the total soluble Se and SeO2−4 increased, SeO2−3 initially increased followed by a decrease, and other Se forms decreased. Decrease in SeO2−3 during reduction may be partly due to the precipitation of MnSeO3. This study showed that the soil Eh plays an important role in mobilizing Se into water systems.
Ammonia volatilization from flooded rice (Oryza sativa L.) is a major mechanism for N loss and poor fertilizer use efficiency. Ammonia volatilization is influenced by five primary factors: NH4‐N concentration, pH, temperature, depth of floodwater, and wind speed. This NH3‐volatilization model is based on chemical and volatilization aspects. The chemical aspects of the model deal with the NH4/NH3(aq) equilibrium in floodwater. Ammonium ions undergo dissociation with a first‐order rate constant, while NH3(aq) and H undergo a diffusion‐controlled association reaction with a second‐order rate constant. The transfer of NH3 across the water‐air interface of flooded soil systems is characterized by a first‐order volatilization rate constant. By utilizing the chemical dynamics of the NH4/NH3(aq) system in association with transfer of gaseous NH3 across the interface, an equation was derived to determine the rate of NH3 volatilization from flooded systems as a function of the five primary factors. The chemical aspects of the model include the derivation of association and dissociation rate constants. The volatilization aspects of the model, which is based on the two‐film theory, allows it to compute the volatilization rate constant for NH3. Expressions are derived to compute the Henry's law constant, gas‐phase and liquidphase exchange constant, and the overall mass‐transfer coefficient for NH3.
Our theory to describe the process of NH3 volatilization from flooded systems is that the rate of NH3 loss is principally a function of two parameters, floodwater NH3(aq) concentration and the volatilization rate constant for NH3, kvN. These parameters are governed by five primary factors, floodwater NH4‐N concentration, pH, temperature, depth of floodwater, and wind speed. The NH3‐volatilization model is executed with five primary factors as input variables. With the input of time, it predicts the NH3 loss for a specified period. The interactive effects of these factors were studied by individually varying one factor while maintaining the four other factors at their mean values; the same factor was also studied by maintaining a second factor at its highest and lowest values while the other three factors were kept constant at their mean values. It is seen that, by changing the existing conditions, the NH3‐volatilization losses are increased or decreased appreciably. The sensitivity analysis shows that pH is the most sensitive and temperature and water depth are the least sensitive determinants affecting NH3 volatilization.
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