Objectives The effects of substituting chemical nitrogen fertilizer with different proportions of biogas slurry 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 substitution ratio for biogas slurry application in paddy fields and to provide a theoretical basis for its clean and safe utilization.

Methods A four-year field experiment (2021–2024) was conducted at the Yingshang Experimental Station of the National Agricultural Green Development Long-term Fixed Observation Network in Fuyang City, Anhui Province, China. Six treatments were established: no nitrogen application (PK), full chemical fertilizer application (NPK), and biogas slurry substituting 25%, 50%, 75%, or 100% of chemical nitrogen fertilizer (denoted as 25%BS, 50%BS, 75%BS, and 100%BS, respectively). During the rice growing seasons, greenhouse gas emissions were monitored regularly, and cumulative emissions and global warming potential (GWP) were calculated. After harvest each year, rice yield, grain processing quality, soil physicochemical properties, and nitrogen use efficiency were determined. The optimal substitution ratio was estimated using a Gaussian curve model.

Results Compared with the NPK treatment, the 25%BS treatment increased rice yield, primarily by increasing panicle number per unit area and grain number per panicle. It also improved processing quality, including brown rice rate, milled rice rate, and head rice rate. As the substitution rate of slurry increased, soil pH showed a downward trend, while soil total N, available P, available K, and organic matter contents increased. The soil fertility index under biogas slurry treatments was higher than that under the NPK treatment. Under the 25%BS treatment, the soil As content decreased significantly by 26.7% compared with the NPK treatment. Regarding greenhouse gas emissions, the biogas slurry treatments reduced cumulative N₂O emissions compared to the NPK treatment, but significantly increased cumulative CH ₄ and CO₂ emissions, with emissions rising as the nitrogen substitution ratio increased. The nitrogen fertilizer uptake efficiency of the 25%BS treatment was higher than that of the NPK treatment, and the agronomic utilization rate also improved.

Conclusions Substitution for 25%−50% of chemical fertilizer nitrogen with biogas slurry demonstrates a 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 of 25.27%, fitted using the Gauss curve in the tested area.