Prolonged alkali water irrigation:impacts of treatment strategies on soil health and microbial dynamics

Background The extent of natural salt-laden groundwaters used for irrigation is increasing worldwide, which is a fast-emerging threat to agroecosystems and global food security. The salt buildup in the soil is linked to deteriorated soil chemical, physical, and biological health and decreased land productivity. Alkali waters with high residual sodium carbonate（RSC） is one of the severe poor-quality waters that deteriorate soil. We evaluated soil microbial dynamics and soil health at critical growth stages of rice crop receiving two-decade-long irrigation with three levels of alkali water and two reclamation strategies. These included good-quality water（GQW）, alkali water（ALKW） with an RSC concentration of 5 me L-1（ALKW1）, ALKW with an RSC concentration of 10 me L-1（ALKW2）, ALKW2 treated to neutralize RSC to 5 me L-1 using gypsum（Ca SO4·2H2O; ALKW2 + GYP）, and ALKW2 treated with sulfuric acid（H2SO4; ALKW2 + SA）. Eleven microbial parameters were used to develop a soil microbial activity index（SMAI）, and eight soil health indicators were correlated with changes in SMAI and crop productivity.Results The SMAI peaked under good-quality water（GQW） conditions（0.84–0.89）, while the lowest values were recorded under ALKW2（0.06–0.18）. Neutralized alkali waters, ALKW2 + SA and ALKW2 + GYP, significantly improved SMAI with corresponding values of 0.25–0.35 and 0.13–0.32, respectively. SMAI across all stages correlated positively（R2 = 0.91–0.98） with rice yield. Microbial activity varied with the crop growth stage, peaking at tillering. Gypsum application alone, aimed at neutralizing alkalinity from an RSC of 10 to 5 me L-1, proved insufficient in bringing the SMAI up to the ALKW1(RSC level of 5 me L-1).Conclusions The application of dilute sulfuric acid demonstrated better results in restoring the soil microbial activity index than gypsum amendment; however, sulfuric acid treatment depends on native calcium carbonate（CaCO3） dissolution for its effectiveness. It may not suffice for soil stability improvement in the long term, especially when native CaCO3 is low. Therefore, integrating gypsum and dilute sulfuric acid for RSC neutralization is worthwhile; however, further assessment is needed to confirm their combined impact on soil biochemical and physical properties.