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.