Nano zinc-oxide coated urea supplemented with Zn solubilizing bacteria improve Zn bio-fortification and nitrogen use efficiency in wheat

Nearly 100% of soils in Pakistan are deficient in nitrogen, while 75% are deficient in zinc, posing a major challenge to food security and contributing to widespread Zn malnutrition. Bulk use of conventional fertilizers results in nutrient wastage, low use efficiency, and environmental pollution. This glasshouse study evaluated the combined use of Zn oxide nanoparticles (ZnONPs)-coated urea and a consortium of zinc-solubilizing bacteria (ZnSB: Stenotrophomonas maltophilia and Pseudomonas aeruginosa) for improving Zn-biofortification, nitrogen use efficiency (NUE), and yield of wheat. The study was conducted during the Rabi season 2021–22. Urea was coated with ZnONPs at rates of 100:0.5 (w/w) (0.5% ZnONPs-coated urea) and 100:1.5 (w/w) (1.5% ZnONPs-coated urea). Treatments included: T1 (control), T2 (uncoated urea; UC), T3 (0.5% ZnONPs-coated urea), T4 (1.5% ZnONPs-coated urea), T5 (UC + 0.5% ZnONPs), T6 (UC + 1.5% ZnONPs), T7 (0.5% ZnONPs-coated urea + ZnSB), T8 (1.5% ZnONPs-coated urea + ZnSB), and T9 (1.5% ZnONPs-coated urea + ZnSB + 10 ppm ZnONPs foliar), following a completely randomized design (CRD) with three replications. T9 significantly improved 1000-seed weight, grains spike⁻¹, grain yield, harvest index, and SPAD values by 33.6%, 50.3%, 74.2%, 27.9%, and 24.0%, respectively, over the control (α < 0.05). However, maximum tillering was observed in T5 and T6. Grain Zn content increased by 30.95%, 38.4% and 63.3% under T7 and T8, T9 respectively, over control. Nitrogen use efficiency metrics of coated-urea were significantly higher (α < 0.05) when applied in combination with ZnSB (T7 and T8), and were further enhanced by foliar ZnONPs supplementation (T9), resulting in maximum agronomic efficiency (25.4 g/g), agro-physiological efficiency (48.8 g/g), nitrogen harvest index (82.8%), partial factor productivity (59.6 g/g), and apparent recovery efficiency (57.9%). These findings highlight that ZnONPs-coated urea, supplemented with ZnSB and foliar ZnONPs, enhances wheat productivity, NUE, and zinc biofortification, offering a sustainable alternative to conventional fertilization and a promising strategy to mitigate Zn malnutrition. However, further field trials are suggested to validate the results.