Objective The aim was to explore the impact of long-term continuous application of biogas slurry (BS) derived from penicillin residues on soil environmental quality.

Method The grassland soil planted by Trifollium repens L. was taken as the research object in this study. A 5-year field experiment with the indices of soil properties, soil nutrients, soil enzyme activities, soil microbial activities, soil heavy metals and antibiotic residues were determined.

Result The soil fertility significantly improved after applying biogas slurry for 5 consecutive years. The organic matter, ammonia nitrogen, nitrate nitrogen, available phosphorus, and available potassium contents increased by 19.95%, 4.18%, 39.57%, 8.66% and 20.17%, respectively. The soil pH was improved and the electrical conductivity (EC) was increased. Nevertheless, the penicillin residue was not found in soil, and the heavy metals did not exceed the standard in the soil of clover root zone. However, the contents of Pb, Cr and Hg were significantly reduced. In addition, soil microbial communities were affected by BS application, showing it would significantly increase the total number of soil culturable microorganisms. Actinobacteria, proteobacteria and acidobacteria were the dominant bacterial phylum across all soil samples. The BS continuous application resulted in a relative abundance of Actinobacteria increased by more than 10%. The results also indicated that pH and EC were the two main factors affecting soil microbial activity and community.

Conclusion The results suggested that the replacement of chemical fertilizer with BS might be an alternative management practice for improving soil fertility and nutrient balance in grassland soil. It would not lead to the safety problems of antibiotic residues and heavy metals in the soil. These results could provide some data support for the safe return of biogas slurry after anaerobic digestion of antibiotic residue.