To identify the optimal substitution ratio of biogas slurry to chemical fertilizer, this study determined the cumulative nitrogen use efficiency (CNUE) of wheat and carbon pool in Lime concretion black soil. The following treatments were applied: control (CK), conventional chemical nitrogen fertilizer application (CN), optimized chemical fertilizer application (ON), and biogas slurry replacing 15% (ONL15%), 30% (ONL30%), and 50% (ONL50%) of fertilizer. The results indicated that CNUE was the highest in the ONL30% treatment and 67.26–80.26% higher in the ONL15%, ONL30%, and ONL50% treatments than it was the CN treatment. The soil dissolved organic carbon of 2023–2024 increased by 11.93–22.93% compared to that in the CN treatment, and the highest particulate organic carbon content was observed in ONL30% treatment. In 2024, the carbon pool management index was 22.20, 42.42, and 29.34% higher in ONL15%, ONL30%, and ONL50% treatments than it was in CN treatment, respectively. In summary, biogas slurry replacing 30% of fertilizer regulated the carbon pool in Lime concretion black soil and improved the yield, quality, and cumulative nitrogen use efficiency of wheat, which was the optimal substitution ratio of biogas slurry to chemical fertilizer in the Northern Anhui Plain of China.