Excessive chemical fertilizer and low N use efficiency constrain mulched drip irrigated maize production in China’s Hexi Oasis, while livestock manure is underutilized. Recycling biogas residue and slurry offers a solution, but optimal substitution ratios under reduced N remain unknown. We conducted a two year field experiment with two reduced N rates (15% and 30% relative to the conventional local rate of 360 kg N ha⁻¹) combined with biogas residue replacing basal N at rates of 25%, 50%, or 75% and biogas slurry replacing 50% of topdressed N. The best performing treatment (N₁25: 15% N reduction + 25% residue substitution) significantly increased LAI (11.77%), SPAD value (10.98%), dry matter (2.51%), leaf and stem N translocation (13.81% and 10.00%, respectively), leading to a 2.14% yield gain. NUE, ANUE, and PFPN improved by 27.4%, 19.5%, and 18.5%. Path analysis revealed that N₁25 enhanced canopy photosynthesis and dry matter translocation, increasing grain weight. These findings demonstrate that integrated N reduction with biogas substitutes enhances productivity and N efficiency while recycling organic waste. Therefore, under the present experimental conditions, N₁25 represents a promising strategy for simultaneously improving grain yield and nitrogen use efficiency; however, multisite and long term validation is warranted to substantiate its broader applicability.