Nickel-mediated modulation of soil urease activity and nitrogen uptake in maize cultivated on Egyptian calcareous soil

Nitrogen (N) utilization in maize production is frequently constrained by nutrient interactions and the physicochemical properties of calcareous soils. Nickel (Ni), an essential micronutrient and structural component of urease, plays a critical role in nitrogen metabolism; however, its beneficial and toxic thresholds under Egyptian calcareous soil conditions remain poorly understood. Therefore, a greenhouse pot experiment was conducted to investigate the interactive effects of Ni and N on maize (Zea mays L., single hybrid 10 white pioneer) growth, nitrogen uptake, soil urease activity, and crude protein accumulation in an Egyptian calcareous soil. Four nitrogen levels (0, 30, 60, and 120 mg N kg⁻¹ soil as urea) and five nickel levels (0, 5, 10, 15, and 20 mg Ni kg⁻¹ soil as NiCl₂·6 H₂O) were arranged in a factorial completely randomized design with four replications. Soil urease activity, dry matter yield, nitrogen uptake, nickel uptake, and crude protein content were determined at harvest. The results demonstrated highly significant effects (P < 0.001) of N, Ni, and their interaction on all measured parameters. Dry matter yield and nitrogen uptake increased progressively with increasing N fertilization. Nickel exhibited a biphasic dose response, where moderate concentrations (5-10 mg kg⁻¹) enhanced plant performance, whereas higher concentrations (15-20 mg kg⁻¹) reduced growth due to toxicity. The highest urease activity (78 ± 5.1 µg NH₄⁺-N g⁻¹ soil h⁻¹) and crude protein content (37.06 ± 1.62%) were obtained under the combined application of 120 mg N kg⁻¹ and 10 mg Ni kg⁻¹. Quadratic regression models accurately described maize dry matter responses to Ni application (R² = 0.90-0.99), confirming an optimum Ni range before the onset of toxicity. Excessive Ni concentrations significantly reduced biomass production, urease activity, and crude protein accumulation through physiological disturbances associated with metal toxicity. Overall, the study demonstrates that balanced Ni supplementation can substantially improve nitrogen utilization efficiency and nutritional quality of maize grown in Egyptian calcareous soils, whereas excessive Ni application is detrimental. The combined application of 120 mg N kg⁻¹ soil and 10 mg Ni kg⁻¹ soil appears to be the most effective treatment under the conditions of this pot experiment.