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

Authors: MEHMET, BUDAK - South Dakota State University; CLAY, DAVID - South Dakota State University; RESSE, CHARYL - Chinese Academy Of Sciences; WESTHOFF, SHAINA - South Dakota State University; OWNES, RACHEL - Missouri State University; Birru, Girma; WANG, ZHICHUN - Chinese Academy Of Sciences

Submitted to: Journal of Soil and Water Conservation Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/24/2021
Publication Date: 7/1/2022
Citation: Mehmet, B.E., Clay, D.E., Resse, C.L., Westhoff, S., Ownes, R., Birru, G.A., Wang, Z. 2022. Increased rainfall may place saline/sodic soils on the tipping point of sustainability. Journal of Soil and Water Conservation Society. 77(4):418-425. https://doi.org/10.2489/jswc.2022.00131.
DOI: https://doi.org/10.2489/jswc.2022.00131

Interpretive Summary: This study investigated the effects of increasing rainfall on temporal changes in salinity and sodicity across a landscape. The soils in the shoulder area were well drained, whereas in the footslope, the soils were poorly drained. Even though soils in two of the three landscape positions were not expected to have sodium enriched subsoil all three soils contained had %Na > 10. Similar findings were observed by Birru et al. (2019) where salinity and sodicity problems were observed in the Harmony (fine, smectitic, frigid, Pachic Argiudolls) and Houdek (fine-loamy, mixed, superactive, mesic Typic Argiustolls) series. These finding suggest that while providing useful information, the Web-Soil Survey should not be the only information source considered when identifying salt effected soils. A comparison between hydraulic conductivity and the ratio between Na+ and EC1:1 suggest that the tipping point between soils that will conduct water and those with near zero hydraulic conductivity is approximately 600 (mg Na/kg)(dS/m)-1. These results are attributed to high Na+ causing the soil to dispersion followed by the clogging of the soil pores and/or increased swelling of the soil clays that reduces drainable porosity. From 2018 to 2019, the Na+ to EC1:1 increased above 600 (mg Na/kg)(dS/m)-1 in the 105 to 112.5 cm depth in the shoulder zone and the 82.5 to 90 cm depth in the backslope soil. Cihacek et al. (2020) summarized this problem and stated that “Once soils disperse due to subsurface drainage, attempting to remediate the soils to near their original internal drainage condition is extremely difficult and costly.” For example, Sharma et al. (1974) conducted a long-term study in Illinois and reported that mixing a high amount of gypsum into the surface 90 cm when combined with tile drainage may increase annual crop yields in 13 to 15 years. The findings from Sharma et al. (1974) suggest that remediation techniques that rely on tile drainage and chemical amendments are needed for these fragile soils. However, the use of gypsum in the NGP may not be effective because the soils may already contain gypsum (Birru et al. 2019). Fielder et al. (2022) evaluated an approach that did not rely on drainage or chemical amendments. In Fielder et al. (2022), perennial grasses were dormant seeded in 2018 and 2019 into productive, transition, and saline/sodic soils. Across the three years of the study, perennial plants had similar or higher biomass production than corn (Zea mays) and soil health gradually improved. Given that saline/sodic soils are often found in riparian zones, restoring the productivity of these soils have the added benefits of reducing erosion and nitrous oxide emissions (Fiedler et al. 2021). Fielder et al. (2022) also showed that salinity and sodicity risks do not remain constant during a drying cycle, with ions being retransported to the surface soil with capillary water. In conclusion, during a wet cycle, salts including Na+ can be flushed from surface soils to subsurface soils. In the subsoil, these ions can increase the Na+ to EC1;1 ratio and reduce soil hydraulic conductivity. During a dry cycle, a related project on a similar soil showed that subsurface salts including Na+ are transported to the surface soil through capillary movement. Over the long-term, improving the drainage characteristics of these soils is very difficult and may require returning these lands to native prairie (Birru et al. 2019; Fiedler et al. 2022).

Technical Abstract: Predicted increases in spring rainfall in the North America’s northern Great Plains region may increase soils salinity, soil erosion, and greenhouse gas emissions while reducing soil productivity and resilience. Understanding the complex interactions among the climate, soils, and management is the first step toward implementing effective management plans. This study determined the influence of soil depth on hydraulic conductivity and changes in the soils Na+ to EC1:1 ratio following the 2019 high spring rainfall on three soils across a landscape in South Dakota. The soils parent materials were Glaciolacrustrine underlaid by marine sediments and the landscape positions included a well-drained shoulder, moderately well drained backslope, and a poorly drained toe slope soil. Based on the soil classification, shoulder and backslope subsoils were not predicted to be salt affected while the toeslope soil contained a natric soil horizon. Soil cores to a depth of 112.5 cm were collected and 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 prior to and following 770 mm of precipitation. Samples from 2018 were analyzed for soil electrical conductivity (EC1;1), pH, ammonium acetate extractable cations, soil particle size, available water at field capacity, drainable porosity, soil bulk density, and saturated hydraulic conductivity and samples from 2019 were analyzed for EC1:1 and ammonium acetate extractable Na+. Across the sampling sites, shoulder and backslope soil had higher saturated hydraulic conductivities than footslope soils. Saturated hydraulic conductivities were negatively correlated to pH (-0.55, p<0.01), the Na+ to EC1;1 ratio (r= -0.66, p<0.01), extractable Na+ (r= -0.56, p<0.01), and sand content (r = -0.66, p<0.01), and positively correlated to the silt content (r=0.65, p<0.01). A comparison between the saturated hydraulic conductivity and the Na+ to EC1:1 ratio suggests that saturated conductivities approached 0 cm h-1 when the Na+ to EC1:1 ratio exceeded above 600 (mg Na/kg)(dS/m)-1. After the high 2019 spring rainfall, the Na+ to EC1:1 ratio increased in the 105- to 112.5- cm shoulder soil zone from 493 ±31 in 2018 to 998 ±119 (mg Na/kg)(dS/m)-1 in 2019. A similar increase occurred at soil depths > 82.5 cm in the backslope. Our findings suggest that shift in rainfall patters caused by climate change can impact the salinity and sodicity levels of soils which are not suspected of being salt-affected.