Effects of Reduced Nitrogen Application with Humic Acid Urea on Yield, Nitrogen Utilization, and Carbon Emissions in Double-Cropping Rice

• • HAU-20 maintains double-cropping rice yield equivalent to full-rate conventional urea (U), while U-20 significantly reduces yield, indicating HAU compensates for 20% N reduction without yield penalty. • • Nitrogen fertilizer apparent utilization rate under HAU-20 increased by 9.24 and 7.80 percentage points for early and late rice, respectively, compared to U (P < 0.05), directly addressing low N use efficiency in the region. • • Agronomic efficiency and partial factor productivity under HAU-20 improved by 18.51% and 22.79% (early rice) and 26.69% and 25.58% (late rice) versus U (P < 0.05), demonstrating enhanced economic return per unit N applied. • • Full life-cycle carbon footprint under HAU-20 decreased by 26.25% (early rice) and 40.38% (late rice) compared to U, driven by significant reductions in CH4 (28.92% and 44.46%) and N2O (44.34% and 63.85%) emissions, aligning with climate mitigation goals.