Bacillus subtilis Y16 and biogas slurry enhanced potassium to sodium ratio and physiology of sunflower (Helianthus annuus L.) to mitigate salt stress

Salinity harms crop productivity; thereby, the management of salt-affected soils is a prerequisite to obtaining optimum crop yields and achieving UN-SDGs. The application of bio-organic amendments is an eco-friendly and cost-effective technique for the management of salt-affected soils. Therefore, this study examined the effect of salt-tolerant Bacillus subtilis strain Y16 and biogas slurry (BGS) on growth, physiology, and yield of sunflower under salt-affected soil conditions. Three levels of soil salinity (original electrical conductivity (EC): 3 dS m⁻¹; induced EC: 6 dS m⁻¹ and 8 dS m⁻¹) were evaluated against three levels of BGS (0 kg ha⁻¹, 600 kg ha⁻¹, and 800 kg ha⁻¹) with and without bacterial inoculation. Soil salinity (EC = 8 dS m⁻¹) significantly (P < 0.05) increased Na⁺ contents (86%), which significantly (P < 0.05) reduced growth (17–56%), physiology (39–53%), and yield (58%) of sunflower. However, the combined application of BGS and B. subtilis alleviated salt stress and significantly (P < 0.05) improved sunflower growth (11–179%), physiology (10–84%), and yield (106%). The correlation analysis showed the superiority of B. subtilis for inducing salt-stress tolerance in sunflower as compared to BGS through homeostasis of K⁺/Na⁺ ratio. The tolerance indices and heat map analysis revealed an increased salt-stress tolerance in sunflower by the synergistic application of BGS and B. subtilis at original (3 dS m⁻¹) and induced (6 dS m⁻¹) soil salinity. Based on the results, we conclude that the combined application of B. subtilis and BGS enhanced growth and yield of sunflower by improving physiological processes and adjustment of K⁺/Na⁺ ratio in shoot under moderate salt-stress soil conditions.