Two-phase remediation of extreme textured soils: Integrating stabilized sludge and plant-assisted strategies for enhanced water-nutrient supply and nitrogen fixation

The continuous degradation of soil texture has severely suppressed soil productivity. Here, we present a two-phase remediation strategy aimed at improving extreme textured soils (ETS), specifically clay (CL) and sandy (SA) soils, by integrating stabilized sludge (SS) amendment and nitrogen-fixing plant (alfalfa) cultivation. In Phase I, the application of SS improved soil texture, as indicated by the optimized mechanical composition and aggregate structure. Water-stable aggregates contributed to organic carbon accumulation by protecting particulate organics. In addition, the improved aggregate spatial arrangement enhanced water dynamics. In Phase II, alfalfa cultivation restructured the diazotroph community via plant-rhizobium symbiosis, upregulating nitrogenase genes (nifKDH) and boosting nitrogen fixation. The synergy between SS and alfalfa established a resilient microbial ecosystem, offering a sustainable solution for ETS remediation and efficient sludge utilization, addressing both environmental and resource management goals.