Simulating Nonequilibrium Transport Processes of Ammonium Through Unsaturated Sandy Soil
2021
Hou, Chaoshuo | Xiu, Wei | He, Mengmeng
Transport of nitrogen compounds to groundwater, especially from agricultural fertilization, is a main problem in aquifer contamination. Ammonium (NH₄⁺-N) is one of the most common nitrogen fertilizer forms. In order to evaluate the risk of agricultural fertilizer pollution to aquifers caused by infiltration, a soil column experiment and breakthrough modeling simulation were employed to study NH₄⁺-N retention by a sandy soil profile (10–60 cm depth). The soil used was collected from an irrigated semi-arid cropland in Bulanghe Town, Yuyang District, Yulin City, Shaanxi Province of China, and recombined in a laboratory soil column. Column experiments were accomplished using aqueous solutions containing various concentrations of NH₄⁺-N and the breakthrough curves were determined. The linear soil-water partition coefficients (Kd) were determined from batch equilibrium experiments, and the retardation factor (Rd) for the saturated sandy soil was computed. The lowest NH₄⁺-N concentrations at the depth range of 10-20 cm suggested the strongest NH₄⁺-N sorption. The chemical nonequilibrium model in Hydrus-1D can better simulate the breakthrough of NH₄⁺-N through the soil column. The soil sorption capacity decreases as the irrigation flow rate increases. The results provide a scientific basis for optimization of fertilizer application in agricultural management under irrigation in desert oasis, and those semi-arid areas with similar soil texture and vadose zones of small thickness.
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