Objective The effects of different proportions of biogas slurry nitrogen on rice yield, quality, soil heavy metal pollution, soil fertility and greenhouse gas emissions were studied, so as to provide a theoretical basis for exploring the optimal proportion of rice biogas slurry returning to the field and realizing the clean and safe utilization of biogas slurry.

Methods From 2021 to 2024, field experiments were carried out for four consecutive years at the Yingshang Experimental Station of Long-term Fixed Observation of National Agricultural Green Development in Fuyang City, Anhui Province, and six treatments were set up, namely, no nitrogen application (PK); total fertilizer (NPK); biogas slurry substituting 25%, 50%, 75%, or 100% nitrogen fertilizer denoted as 25%BS, 50%BS, 75%BS, or 100%BS, respectively. during the rice growing season, greenhouse emission were monitored regularly, and the cumulative emission fluxes and greenhouse potential were calculated. After harvest of rice, the yield, processing quality of rice, the soil physicochemical properties were investigated, the nitrogen fertilizer use efficiency over a four-year period were calculated, and the optimal substitution rate of chemical fertilizer by biogas slurry were fitted using the Gauss curve.

Results Compared with the NPK treatment, 25%BS treatment significantly increased rice yield by an average of 4.48%, primarily achieved through increased panicle number and grain number per panicle, and increased the brown rice rate, polished rice rate, and whole polished rice rate by 1.1% to 4.4%. As the substitution rate of biogas slurry increased, the soil pH showed a downward trend, while the soil total N, available P, available K, and organic matter content all increased significantly, the total N and organic matter increment was as high as 87.8% and 72.1%, respectively, and the soil fertility index was significantly higher than that of the NPK treatment by 4%. Under the 25%BS treatment, the soil heavy metal As and Pb contents were significantly reduced by 26.7% and 10.0%, respectively, compared to the NPK treatment. Regarding greenhouse gas emissions, the biogas slurry treatment significantly reduced the cumulative emissions of N₂O compared to the NPK treatment, but the cumulative emissions of CH ₄ and CO₂ increased significantly, with an upward trend as the nitrogen substitution ratio increased. The nitrogen fertilizer absorption efficiency of the 25%BS treatment was significantly higher than that of the NPK treatment by 21.63%, and the agronomic utilization rate also improved significantly.

Conclusion Substitution 25%−50% of chemical fertilizer nitrogen with biogas slurry demonstrated significant effect on increasing rice yield and processing quality, improving soil fertility, and reducing greenhouse gas emission and heavy metal contents in soil. The optimal nitrogen fertilizer input is 134.5 kg/hm², with the substitution rate of biogas slurry by 25.27%, fitted using the Gause curve in the tested area.