Revitalizing Soybean Plants in Saline, Cd-Polluted Soil Using Si-NPs, Biochar, and PGPR

Excessive irrigation of saline-alkaline soils with Cd-contaminated wastewater has resulted in deterioration of both soil and plant quality. To an investigate this, a study was conducted to explore the effects of biochar (applied at 10 t ha^-1), PGPRs (<i>Bradyrhizobium japonicum</i> (USDA 110) + <i>Trichoderma harzianum</i> at 1:1 ratio), and Si-NPs (25 mg L^-1) on soybean plants grown in saline-alkali soil irrigated with wastewater. The results showed that the trio-combination of biochar with PGPRs, (as soil amendments) and Si-NPs (as foliar spraying), was more effective than individual or coupled applications in reducing Cd bioavailability in the soil, minimizing its absorption, translocation and bioconcentration in soybean tissues. The trio-combination reduced Cd bioavailability in the soil by 39.1% and Cd accumulation in plant roots, shoots, and seeds by 61.0%, 69.3%, and 61.1%, respectively. Physiological improvements in soybean plants were also observed, including 197.8% increase in root growth, 209.3% increase in chlorophyll content, and 297.4% increase in carotenoid levels. The trio-combination significantly improved soil physicochemical characteristics, enhanced soil microbial indicators and boosted soil enzymes activity, which in turn facilitated nutrient uptake and increased antioxidant enzymes activity. These positive outcomes enhanced photosynthesis, improved productivity and increased seed nutritional value. Overall, the trio-combination of biochar with PGPRs and Si-NPs are considered a reliable approach not only for revitalizing soybean growth but also for immobilizing Cd and improving soil health under wastewater irrigation.