Mechanisms of phosphorus conversion in chicken manure and straw composting systems regulated by flax-retting wastewater and a combination of flax-retting wastewater and biochar

Phosphorus (P), a critical non-renewable nutrient element, is essential for the growth and development of plants. The composting process of agricultural solid wastes, such as chicken manure and straw, can efficiently transform them into P-rich organic fertilizers. To develop efficient organic P fertilizer products, improve the P utilization efficiency, and reduce P losses, this study investigated the effects of adding flax-retting wastewater (FRW) and biochar to chicken manure and straw composting systems on P conversion using microbiome and metrology methods. The results showed that the total phosphorus (TP), inorganic phosphorus (IP), and organic phosphorus (OP) content increased throughout the composting process. In contrast, the microbial biomass phosphorus (MBP) content initially increased and then decreased. FRW significantly increased P content, while biochar inhibited the increasing effect of FRW on the P fraction content. This phenomenon was attributed to FRW promoting the expression of OP mineralization genes (phnW and phnX) by increasing the abundance of Bacillales, which increased IP content. In contrast, biochar enhanced the expression of P regulatory gene phoU by increasing the abundance of Bacillales and Rhodothermales, which inhibited the P conversion. This study provides technical support for effective management and resource utilization of P in agricultural solid wastes and industrial wastewater.