Effects of planting alfalfa on soil water-salt transport characteristics, physical properties, and organic matter turnover in coastal saline-alkali soils

<p indent="0mm">Alfalfa (<italic>Medicago sativa</italic>) is widely cultivated in saline-alkali areas and demonstrates significant potential for soil improvement. However, systematic research on its amelioration effects on saline-alkali soils remains limited. This study was conducted at the coastal saline-alkali experimental base in Huanghua, Hebei, using an in situ field experiment to investigate the dynamic changes in soil volumetric water content, temperature, electrical conductivity (EC), bulk density, porosity, aggregates, saturated hydraulic conductivity (Ks), and organic matter (SOM) in alfalfa grassland (ALF) and saline-alkali wasteland (CK). The results showed that ALF treatment reduced soil volumetric water content in the <sc>0–20 cm</sc> layer during spring and decreased soil temperature in the same layer during autumn. During the growing season, soil EC was significantly lower in both the <sc>0–100 cm</sc> alfalfa root zone and <sc>0–20 cm</sc> surface soil. ALF treatment suppressed surface soil salinization in spring and promoted leaching of salts during the rainy season. Specifically, in April and September, the EC in the <sc>0–20 cm</sc> soil layer decreased by 85.10% and 90.91%, respectively, accompanied by reductions of 94.33% and 87.28% in the Na⁺ content and 86.94% and 92.07% in the Cl^‒ content. The ALF treatment also improved soil physical properties. In the <sc>0–10 cm</sc> soil layer, total porosity, capillary porosity, and non-capillary porosity increased by 0.16-fold, 0.16-, and 0.15-fold, respectively. In the <sc>10–20 cm</sc> layer, soil bulk density decreased by 8.44%, while total porosity, capillary porosity, and non-capillary porosity increased by 0.28-, 0.21-, and 1.55-fold (<italic>P</italic>​&lt;0.05). Additionally, the content of large aggregates <sc>(&gt;0.25 mm)</sc> in the <sc>0–20 cm</sc> soil layer increased by 4.68-fold (<italic>P</italic>​&lt;0.05), and saturated hydraulic conductivity increased by 2.14-fold (<italic>P</italic>​&lt;0.05). The SOM content under the ALF treatment increased significantly by 8.14%–36.6%, which was attributed to the synergistic effect of alfalfa roots and rhizosphere microbes in reducing the decomposition of SOM during summer. Planting alfalfa on coastal saline-alkali land is an effective approach to stimulating the self-regulation capability of the ecosystem. By utilizing the synergy of water-soil-forage-microorganisms, this method optimizes the transport characteristics of water-salt in saline-alkali soils, improves physical properties, and promotes SOM turnover and accumulation, thereby enhancing soil fertility and ensuring sustainable use of saline-alkali land.