Abstract
Shallow groundwater contamination by nitrate is frequent in agricultural lands in Egypt because of the use of urea fertilizers. The urea transformation process in the vadose zone was simulated using a HYDRUS-2D model, Software package for simulations of 2D movement of water, heat, and multiple solutes in variably saturated media, for subsurface drip irrigation. The root water and nutrient uptake were assessed for three soil types (sandy loam, loam, and silty loam) with three emitter discharge levels (1.0 L h−1, 1.50 L h−1, and 2.0 L h−1), for a comparison of three fertigation strategies (A) at the beginning, (B) at the end, and (C) at the middle of the irrigation cycle. The extension of the wetted area mainly depends on soil hydraulic conductivity. The high emitter discharge with a short irrigation time is suitable for shallow-rooted crops. The cumulative flux was highest for silty loam soil and the lowest was for the sandy loam soil (1891, and 1824 cm3) for the 2 L h−1 emitter discharge within the 35 days simulation. The cumulative drainage significantly differs among soil types with little effect of emitter discharge. It recorded 1213, 295, 11.9 cm3 for sandy loam, loam, silty loam, respectively. Urea transformation is controlled by hydrolysis and nitrification as well as the adsorption coefficient of ammonium. Nitrate distribution is mainly governed by soil type rather than the emitter discharge where the sandy loam soil is more highly susceptible to nitrate leaching than to silty loam. Nitrate concentration has recorded the minimum possible level when applying the urea fertilizer at the beginning of the irrigation event for sandy loam and loam soil while for the silty loam soil, urea application at the middle of the irrigation event is more effective. Urea application at the end of the irrigation event gives the highest accumulated leached nitrate concentration below the root zone and should be avoided (the worst strategy).
Highlights
Agriculture in Egypt represents 80% of the total water budget while water resources are misused, and inefficient irrigation techniques are applied [1]
At the beginning of the irrigation each event, the soil moisture content started to increase close to the emitter until it reached the maximum by the end of the irrigation event, while the moisture content decreased as the distance from the emitter increased
The presented results in this study describe the effect of changing emitter discharge and the application of three fertigation strategies on urea transport and transformation and leached nitrate below the root zone of onion under subsurface drip irrigation for sandy loam, loamy, and silty loam soil
Summary
Agriculture in Egypt represents 80% of the total water budget while water resources are misused, and inefficient irrigation techniques are applied [1]. Subsurface drip irrigation, SDI can save 30% of the water resources and fertilizer usage [2]. Many government programs have been initiated and planned to rationalize water uses and obtain higher irrigation efficiency [3]. The optimum design and operation of the SDI system requires more understanding of water and nutrient root uptake and. Water 2019, 11, 1487 distribution [4]. In SDI, water discharge is controlled around the emitters and fertilizer is directed to the roots synchronizing with crop demands [5]. Urea is the most common nitrogen fertilizer used in agriculture in Egypt [6]. Nitrate (the most dominant contaminant) concentration in groundwater is directly related to these urea applications [7]
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