Oxidized bituminous coal-modified urea: Coating versus blending effects on growth characteristics and nitrogen use efficiency in wheat

Nitrogen leaching from conventional urea fertilizers is a major environmental concern and reduces nitrogen use efficiency in calcareous soils. We postulated that modifying urea with oxidized bituminous coal (OBC) would create a slow-release fertilizer, effectively reducing nitrogen leaching and enhancing NUE. This study therefore aimed to produce and evaluate coated and blended OBC-urea formulations. Two OBC:urea ratios (60:40 and 35:65) were tested in coated (C1, C2) and blended (B1, B2) forms under greenhouse conditions, each applied at 100 % (H; Higher rate) and 60 % (L; Lower rate) of the recommended nitrogen rate. Compared to conventional urea, all OBC-based treatments significantly reduced nitrate and ammonium leaching, with the C1H treatment achieving the highest reductions: 46.33 % for nitrate and 53.08 % for ammonium. The OBC-containing fertilizers also significantly enhanced NUE indices. In particular, the C1H treatment led to increases of 33.35 % and 28.53 % in NUE and nitrogen uptake efficiency (NUpE), respectively, relative to commercial urea. Moreover, C1H improved shoot nitrogen uptake (28.42 %), total chlorophyll content (33.88 %), total protein (24.19 %), shoot fresh weight (10.63 %), leaf area index (29.01 %), and spike number (32.15 %). Remarkably, the C1L treatment, applied at only 60 % of the recommended nitrogen rate, maintained comparable growth and nutrient uptake levels to full-rate commercial urea, while providing superior NUE. These results suggest that OBC-coated urea can reduce nitrogen leaching and enable lower application rates without compromising plant growth. Overall, this study provides new insights into plant-nitrogen interactions in calcareous soils and supports the development of sustainable nitrogen management strategies.