Effects of nitrogen fertilizer reduction and topdressing methods on wheat yield, nitrogen use efficiency, and N<sub>2</sub>O emissions in wheat fields

To elucidate the mechanisms underlying yield enhancement, efficiency improvement, and emission mitigation in rice-stubble wheat under the integrated “one basal plus one topdressing” mechanized deep fertilization strategy, a field experiment was conducted from 2021 to 2024 within a rice-wheat rotation system in Nantong, located in the lower Yangtze River Basin. The study employed a slow-release blended fertilizer (SRF, N∶P₂O₅∶K₂O = 26∶12∶12) and conventional urea (U, 46% N), utilizing two self-developed machines: the 2BFGK-12(6)260 full-straw-stubble field clean-zone rotary-till intelligent fertilizer applicator and seeder, and the 3ZF-4(200) cultivator-topdresser. Seven fertilization regimes were established under a 30 cm + 15 cm wide-narrow row planting system. The control (CK) received four split applications of urea (N 240 kg hm^-2; basal : tillering : jointing : booting = 5∶1∶2∶2, with basal application in narrow rows and topdressing broadcast across the field). Six reduced-N treatments (N 204 kg hm^-2, a 15% reduction) were applied: M₁ (100% SRF, narrow-row basal); M₂ (60% SRF, narrow-row basal + 40% U, narrow-row broadcast at jointing); M₃ (60% SRF, narrow-row basal + 40% U, wide-row banding at regreening); M₄ (60% SRF, narrow-row basal + 40% SRF, narrow-row broadcast at regreening); M₅ (60% SRF, narrow-row basal + 40% SRF, wide-row banding at regreening); and M₄₊₅ (60% SRF, narrow-row basal + 20% SRF, wide-row banding + 20% SRF, narrow-row broadcast at regreening). The effects of these fertilization regimes on wheat yield performance, economic benefit, root morphological and physiological traits, nitrogen use efficiency (NUE), and N₂O emissions were systematically evaluated. Compared with CK, treatments M₂-M₅ increased wheat yield by 4.0%-19.0% and economic returns by 13.7%-35.7%, with M₄ and M₅ showing optimal performance (yield increases of 14.1% and 19.0%; economic gains of 34.5% and 35.7%, respectively). These treatments significantly improved root traits, with root weight density increasing by 9.7%-111.8% and enhancements in root activity and oxidation capacity of 6.8%-52.0% and 4.2%-44.2%, respectively. Cumulative N₂O emissions were reduced by 22.6%-34.5%, and soil NO₃^--N content in the 0-20 cm layer increased by 11.2%-40.0%. In terms of nitrogen utilization, M₂-M₅ treatments promoted grain N accumulation, post-anthesis N uptake, and its contribution to grain N content. Indicators of nitrogen use efficiency—including partial factor productivity, agronomic efficiency, and apparent recovery efficiency—increased significantly by 22.4%-40.0%, 29.7%-74.3%, and 9.41-18.77 percentage points, respectively. Notably, M₄ and M₅ achieved the most comprehensive benefits, with the greatest reductions in cumulative N₂O emissions (27.0% and 34.5%) and consistently high apparent N recovery efficiency across two growing seasons (mean values of 43.5% and 46.8%), along with sustained root activity and topsoil inorganic N content during late growth stages. In contrast, M₁ achieved the highest N₂O mitigation (35.9%) but resulted in a 10.4% yield loss and 10.8% reduction in economic benefit, with unstable interannual variation in NUE. The optimized M₄₊₅ treatment further enhanced root morphological and physiological traits while improving apparent N recovery efficiency and grain N accumulation. In summary, under a 15% nitrogen reduction (N 204 kg hm^-2), split-application treatments using slow-release blended fertilizer (M₄ and M₅) achieved synergistic improvements in yield, economic return, nitrogen use efficiency, and N₂O emission reduction, with the deep banding topdressing strategy (M₅) delivering the most pronounced benefits. These findings provide strong theoretical support for the optimized and efficient use of slow-release fertilizers in nitrogen-reduced rice-wheat rotation systems.