Microbial Inoculant and Polyacrylamide Jointly Improve Cotton Root-Zone Function Under Alternating Brackish–Freshwater Irrigation

Alternating brackish–freshwater irrigation is a promising strategy for improving the utilization of marginal water resources in arid cotton (Gossypium hirsutum L.) production; however, its effectiveness is often limited by salt-induced physicochemical stresses, including sodium-induced soil structural degradation, osmotic stress, and reduced rhizosphere biological activity. This study investigated whether the combined application of microbial inoculant and polyacrylamide (PAM) could enhance root-zone functioning and plant performance under alternating brackish–freshwater irrigation. A controlled greenhouse pot experiment was conducted with five treatments, including conventional irrigation (CI), alternating irrigation (AI), AI combined with microbial inoculant (AI + B), AI combined with PAM (AI + PAM), and AI combined with microbial inoculant and PAM (AI + B + PAM). Soil water–salt conditions, physical properties, nutrient availability, microbial activity, root growth, and plant nutrient uptake were determined, and partial least squares path modeling (PLS-PM) was used to evaluate soil–root–plant interactions. Alternating irrigation reduced soil salinity and sodium accumulation compared with conventional irrigation, with electrical conductivity of the 1:5 soil–water extract (EC1:5), Na+, and sodium adsorption ratio (SAR) decreasing by 14.68%, 16.21%, and 14.27%, respectively; under AI conditions, PAM increased water-stable aggregates by 22.54%, while microbial inoculant increased microbial biomass carbon by 33.47%. The combined AI + B + PAM treatment produced the greatest improvement in plant performance, increasing biomass, N uptake, P uptake, and K uptake by 28.79%, 47.37%, 48.00%, and 60.80%, respectively, compared with AI alone. PLS-PM supported a hypothesized pathway in which PAM-associated physical conditioning and microbial inoculant-mediated biochemical activation converged on root development, which was positively linked to nutrient acquisition and plant growth. These findings indicate that integrating microbial inoculant with PAM has potential to enhance root-zone resilience and cotton growth under alternating brackish–freshwater irrigation conditions, providing insights for the development of amendment strategies in saline soils. Further field validation is required before broader agricultural application.