Enhancing cowpea quality and soil biochemistry through residual nickel and nitrogen in Vertisol

Nickel (Ni) plays a vital role in plant nitrogen (N) metabolism, yet its use is limited and its residual effects on subsequent rotational crops remain underexplored. The present study evaluated the effect of varying N levels and residual Ni, previously applied to soybean, on the growth, yield, nutritional quality, and soil biochemical properties of cowpea cultivated in Ni-deficient Vertisol. A pot experiment was conducted in a net-house using soil previously treated with Ni (2.5, 5, 10, and 20 mg Ni kg− 1) under soybean. Cowpea was subsequently grown with three N levels (50%, 100%, and 150% of recommended dose i.e., 30, 60, and 90 kg N ha⁻¹) along with recommend fertiliser practice. Growth and yield attributes, nutrient uptake, post-harvest soil properties, and urease activity were assessed. Optimal residual Ni (10 mg kg⁻¹) combined with 100–150% N significantly improved plant height, greenness index, number of branches, pod count, grain and stover yield of cowpea, and enhanced N and micronutrient (except Ni) content in seeds and stover. The maximum stover yield (13.7 g pot− 1) and grain yield (6.80 g pot− 1) of cowpea were observed in treatment 150% recommended doses of N (RDN) i.e., 90 kg N ha⁻¹ + 30 kg P ha⁻¹ + 30 kg K ha⁻¹ with residual Ni @ 10 mg kg⁻¹, reflecting increases of 12.3% and 12.4%, respectively, compared to the recommended practice. Urease activity and available soil N were positively correlated with residual Ni @ 10 mg kg⁻¹. Residual Ni (up to 10 mg kg− 1) paired with 100–150% recommended dose of N in crop rotations can sustain cowpea productivity and nutrient use efficiency in Ni-deficient Vertisol ensuring food security and sustainable fertiliser utilisation. Further, long-term field experiments with different crops and soil conditions are needed to better understand the effects of residual Ni on crop growth and soil properties. Residual Ni at 10 mg kg⁻¹ combined with 100–150% N significantly improved cowpea growth and yield, whereas higher Ni (20 mg kg⁻¹) caused yield depression. Enhanced cowpea productivity under optimal residual Ni was closely associated with increased soil urease activity and improved N availability. Strategic management of residual Ni in soybean–cowpea rotations can improve productivity and nutrient-use efficiency in Ni-deficient Vertisols.