Soil salinity impairs soil microbial activity, nutrient availability, plant nutrient uptake, and yield of onion (Allium cepa L.)

Background Onion ( Allium cepa L.) is highly sensitive to salinity, which can severely affect soil microbial activity, nutrient availability, plant nutrient uptake, and bulb yield. Methods A pot experiment was conducted to evaluate the effects of graded salinity levels (0.15–5.00 dS m - ¹) on four onion cultivars (Bhima Shakti, Bhima Red, Bhima Kiran, and Bhima Swetha) in a factorial completely randomized design with three replications, wherein soil organic carbon, fungal and bacterial population, soil enzyme activity, nutrient availability, plant nutrient uptake and bulb yield were assessed. Results Increasing salinity significantly reduced bacterial and fungal populations, fungal-to-bacterial ratio, enzymatic activities, microbial biomass, and nutrient availability. At 5.00 dS m - ¹, bacterial and fungal populations declined by 17.8% and 44.8%, respectively, while dehydrogenase and alkaline phosphatase activities decreased by 52.3% and 39.1%. Available N, P, S, and B also decreased notably, with P and B showing declines of up to 19.1% and 38.1%. Consequently, plant nutrient uptake declined sharply, with N, P, K, and S decreasing by 34–57%, and micronutrients (Zn, Mn, Cu) by over 80% at 3.55 dS m - ¹. The cumulative effects of reduced microbial activity and nutrient uptake led to substantial yield losses. Bhima Swetha, Bhima Red, and Bhima Shakti showed 3–10% yield gains at 0.49 dS m - ¹, whereas onion yield declined sharply above 1.85 dS m - ¹ and yield losses exceeded 80% at 3.55 dS m - ¹ across all cultivars. Conclusion Bhima Shakti, and Bhima Red increased onion yield slightly at salinity level of 0.49 dS m - ¹, maintaining higher microbial activity, nutrient uptake, and bulb yield. The study demonstrates that even moderate salinity can substantially impair onion growth and productivity, emphasizing the need for cultivar selection and soil management to mitigate salinity stress.