Modeling Ammoniacal Nitrogen Fate in an Alkaline Soil: Degradation and Leachate Potentiality

Abstract

Determining the behavior of inorganic nitrogen compounds at plot scale is a useful tool to predict the consequences of the discharge of nitrogen-enrich effluent in the vadose zone and groundwater at a larger scale. In this work, field data were collected from an experimental plot located on the periphery of an ammonia and urea fertilizer manufacturing industry. Water flow and solute transport were simulated with HYDRUS 2D/3D. Numerical simulation allowed estimating the main processes and transformations that ammoniacal nitrogen underwent during its transport in the soil. Soil pH was in the range of 8.8 and 9.8 being both species of ammoniacal nitrogen (N-NH3 and N-NH4) present in the soil. The process that was most relevant in the fate of nitrogen was volatilization (33.7% of total nitrogen applied). Most of the nitrogen that remained in the soil was nitrified (30.6% of total nitrogen applied). Leaching of nitrogen to the aquifer accounted for 4.5% of total nitrogen applied, mainly as ammoniacal nitrogen. Nitrification rates in the industrial zone were very low; thus, ammoniacal nitrogen had longer residence times in the vadose zone facilitating its leaching into the aquifer. As the predominant specie of ammoniacal nitrogen was N-NH3 (at pH > 9.3), its great solubility in water grants a higher mobility than NH4+, due to the minimization of retardation. In the future, further data collection is needed to estimate robust and representative input parameters for a larger scale numerical model of reactive nitrogen transport. Solute transport parameters applied in this research may vary considerably for other sites with different conditions.