Increased rainfall may place saline/sodic soils on the tipping point of sustainability

Abstract

Increased rainfall is increasing the risk of the capillary movement of sodium (Na⁺) and other salts from buried marine sediments to the soil surface in the North American northern Great Plains. These salts reduce productivity and resilience while increasing their effect on the environment. Understanding the interactions among management, climate, cropping system, and soil is the first step toward implementing effective management plans. Unfortunately, while much work has been conducted, little is known about the effective management of semiarid dryland saline/sodic soils. This study determined the influence of soil depth on hydraulic conductivity and changes in soil Na⁺ (mg Na⁺ kg^–1 soil) to electrical conductivity (EC_1:1) (dS m^–1) ratio following high spring rainfall in 2019 in three soils. The soil parent materials were Glaciolacrustrine underlaid at a depth of approximately 15 m by marine sediments that contained Na⁺ and other salts. The landscape positions included in the study were a well-drained shoulder, moderately well-drained backslope, and a poorly drained toeslope soil. Based on the soil classification, shoulder and backslope subsoils were not predicted to be salt affected, while the toeslope soil was predicted to contain a natric soil horizon. Rainfall in 2018, 2019, and 2020 was 46, 76, and 37 cm, respectively, and soil cores were collected prior to and following the 2019 high rainfall. Soil cores were separated into the 0 to 7.5, 50 to 57.5, 82.5 to 90, 92.5 to 100, and 105 to 112.5 cm segments. Samples from 2018 were analyzed for soil EC_1:1, pH, ammonium acetate extractable cations, soil particle size, available water at field capacity, drainable porosity, soil bulk density, and saturated hydraulic conductivity. Samples from 2019 were analyzed for EC_1:1 and ammonium acetate extractable Na⁺. Across the sampling sites, shoulder and backslope soils had higher saturated hydraulic conductivities than the toeslope soils. Saturated hydraulic conductivities were negatively correlated to pH (<i>r</i> = –0.55, <i>p</i> &lt; 0.01), the Na⁺ to EC_1:1 ratio (<i>r</i> = —0.66, <i>p</i> &lt; 0.01), extractable Na⁺ (<i>r</i> = —0.56, <i>p</i> &lt; 0.01), and sand content (<i>r</i> = —0.66, <i>p</i> &lt; 0.01), and positively correlated to the silt content (<i>r</i> = 0.65, <i>p</i> &lt; 0.01). A comparison between the saturated hydraulic conductivity and the Na⁺ to EC_1:1 ratio suggests that saturated conductivities approached 0 cm h^–1 when the Na⁺ (mg Na⁺ kg^–1) to EC_1:1 (dS m^–1) ratio exceeded 600. The 2019 high rainfall increased the risk of soil dispersion in the lower soil depths (&gt;82.5 cm). For example, in the shoulder soil at the 105 to 112.5 cm depth, EC_1:1 decreased 0.936 ± 0.254 dS m^–1 from 2018 to 2019, whereas the exchangeable Na⁺ increased 688 ± 283 mg kg^–1 soil. Our findings suggest that a climate change-induced shift in rainfall patterns can increase salinity and sodicity risks in northern Great Plains subsurface soils. Salinity and sodicity risks are expanding into zones not previously identified as at risk, and improving or maintaining the productivity of these soils requires careful planning.