Changing soil hydraulic properties and water repellency in a pomegranate orchard irrigated with saline water by applying polyacrylamide

Soil wettability and water repellency are important physical properties which greatly affect soil-water relations. This study was conducted to investigate the effects of polyacrylamide (PAM) on soil water repellency and wettability in a pomegranate orchard drip irrigated with saline water in Isfahan Province, central Iran. The experiment was a randomized complete block design conducted within each “durations of drip irrigation” (considered as the environment) including control (uncultivated), and 8 and 15 years old trees under drip irrigation system (DIS). PAM concentrations were 0 (control), 10 and 20 mg l−1 arranged in each block at three replications. The PAM (solubilized in saline water with electrical conductivity of 6.2 dS m−1) with the mentioned concentrations was applied once during the irrigation time. A week after applying the PAM, soil samples were taken from 0 to 0.3, 0.3-0.6 and 0.6-0.9 m depths under the emitters. Water repellency index (WRI) and soil-water contact angle (β) were determined using intrinsic sorptivity method by measuring the water and ethanol sorptivities in all soil samples. Increasing the duration of irrigation with saline water increased electrical conductivity (ECe), concentrations of (Ca2+ + Mg2+) and Na +, and sodium adsorption ratio (SAR) of the saturated extract in the 0-0.3 m soil layer under the emitters. Increasing the duration of irrigation also increased WRI and β, and decreased water infiltration, especially in the 0-0.3 m soil layer, presumably through enhancing effects of long-term use of saline water on surface tension of water, soil aggregate stability, physical protection of organic matter in soil and stability of hydrophobic coatings. PAM application significantly reduced soil water repellency and increased soil water sorptivity. In the 0-0.3 m, 8 and 15 years of drip irrigation increased the WRI by 56 and 134 percent, respectively, compared with the control. PAM application at rates of 10 and 20 mg l−1 decreased WRI by 27 and 40 percent, respectively, when compared with the control. Application of 20 mg l−1 of PAM could completely ameliorate the degradative effects of drip irrigation with saline water for 8 years, whereas amelioration in the 15-year treatment remained incomplete.