Numerical simulation and influence factors analysis for infiltration characteristics of nitrate nitrogen under furrow irrigation with fertilizer solution

Abstract: This paper aims to study solute transport in soil under furrow irrigation and fertilization. Sandy loam and clay loam taken from Yangling district in Shaanxi province were repacked into laboratory tanks; 2D furrow irrigation and fertigation with KNO3 as fertilizer was conducted in attempts to investigate the movement of water and nitrate in the soils under different conditions. For each treatment, we inversely calculated the hydraulic and solute transport parameters of the soils by calibrating the simulated results against the measurements, and we then analyzed nitrate transport in the soils under different initial soil moisture and fertilizer concentration. The results showed that change in fertilizer concentration could alter soil hydraulic properties. In particular, we found that increasing fertilizer concentration led to an increase in saturated moisture content, the shape parameter n in the van Genuchten formula and saturated hydraulic conductivity Ks, while reducing the air-entry parameter α in the van Genuchten formula. The parameters obtained from the inverse method were used to simulate fertilizer movement under furrow irrigation and fertigation. The simulated cumulative infiltration and advancement of the wetting front were both consistent with the measurements, with their mean relative errors being less than 6.52%. The mean relative errors of the simulated solute transport were higher than that of the water but were less than 11.49%. Given other uncertainties involved in water flow and solute transport in the soils, the errors of the parameters and the model were tolerable and were representative of the soils. The simulated water flow and solute transport under different conditions revealed that NO3 distribution in the sandy loam and the clay loam were geometrically similar, and that the initial soil moisture and fertilizer concentration significantly impacted NO3 movement in which the spatial spreading of NO3 increased with initial soil moisture and fertilizer concentration. The depth of water in the furrow and the width of the furrow bottom were found to affect NO3 movement in horizontal direction more significantly than in vertical direction, and that the spreading of NO3 in horizontal direction increased with the water depth in the furrow and the width of the furrow bottom. These suggest that under furrow irrigation and fertigation, increasing water depth in the furrow and/or the width of the furrow bottom could ameliorate the risk of NO3 leaching by enhancing its horizontal movement; and NO3 leaching can be further reduced by not fertilizing when initial soil moisture is high and reducing the concentration of the fertilizer solution. Our results have a wide implication and they provide a general guidance for scheduling furrow irrigation and fertilization.