An ecological and economic approach to enhancing the agronomic quality of anaerobic digestate: Effects of adding agricultural Jiaosu on metabolism and the microbial community

Anaerobic digestate disposal is challenging, hampering the development of large-scale biogas projects. Currently, digestate is primarily stabilized naturally and subsequently applied to farmland. However, during this process, the number of beneficial microorganisms decreases, free ammonia nitrogen increases, and pH rises, decreasing the benefits of digestate to soil and plants. In this study, digestate was treated by adding agricultural Jiaosu (AJ). The agronomic quality of the digestate was explored by conducting a pot experiment. Subsequently, the mechanisms of the changes in digestate properties were investigated based on chemical and microbial parameters. The results showed that the digestate with a 10% AJ supplement significantly improved the plant characteristics such as plant height, fresh weight, and leaf area. Regarding chemical properties, the addition of 10% AJ decreased the total ammonia nitrogen and free ammonia nitrogen in the digestate to below the toxicity threshold. With respect to microorganisms, the addition of AJ improved the microbial diversity of the digestate and increased the relative abundances of Advenella, Trichococcus, and Comamonas spp., which are beneficial to both soil and plants. Moreover, AJ decreased the abundance of potentially threatening microorganisms such as Thiopseudomonas and Papillibacter spp. Collaborative analysis showed that the improvement in digestate agronomic quality was closely related to its effects on microorganisms. Based on these results, the present study suggests an approach to the post-treatment of anaerobic digestate that can improve its fertilizer value while reducing its phytotoxicity. Furthermore, this study provides a model for a more comprehensive evaluation of the agronomic quality of digestate.