Bioenergy by-products enhance the soybean photosynthesis, antioxidant defense, and related gene expression by modulating metal bioavailability in acidic soil

Soil acidification regulates the mobility of aluminum (Al) and manganese (Mn), thereby affecting legumes growth. Bioenergy by-products (BBP) including biochar, bottom ash and biogas slurry, can mitigate soil metal toxicity in acidic soils; however, the precise impacts of these amendments in soil-plant system remains unknown. Therefore, different treatments of BBP namely Control (T1), Biogas slurry (T2), Bottom ash (T3), Biochar (T4), Biogas slurry with bottom ash (T5), Biogas slurry with biochar (T6), Bottom ash with biochar (T7), and Biochar along with bottom ash and biogas slurry (T8) were used to mitigate the bioavailability and toxicity of Al and Mn. Results revealed that T8 reduced Al and Mn content by 63 % and 78 % in soil and 64 % and 65 % in soybean plants, respectively. Notably, T8 mitigates oxidative damage and improves rubisco activity, photosynthetic efficiency, and antioxidant activities as compared to other treatments. Furthermore, Transmission electron microscopy (TEM) shows that cell structure restoration was obvious under T6 and T8 than that of other treatments. The antioxidant genes (GmSOD, GmCAT1, and GmPOD1) and photosynthesis genes (GmRbcS and GmRCAβ) expressions were upregulated in T7 and T8 than that of other treatments. Our correlations analysis shows that BBP improved soil organic matter and further enhanced the availability of NO3-, P, and K in the soil. Furthermore, increased soil pH by BBP significantly decreased the NH₄⁺ availability in the soil. In conclusion, our study demonstrates that BBP can enhance soybean physiological characteristics by modulating soil pH and improving nutrient availability.