The effect of slow-release phosphate fertilizers from digestates on maize rhizosphere soil microbial community and nutrient cycling: Response and activation mechanism

Anaerobic digestion (AD) constitutes an efficient technological approach for reducing waste and generating nutrient-rich digestates, which can effectively facilitate the recycling of phosphorus (P). P compounds such as Ca - P and Mg - P undergo mineralization during AD and then transform into insoluble P, which subsequently precipitates within the solid phase of the digestate. However, the effectiveness of these insoluble phosphates, especially Ca - P, and the response of plants and organisms to insoluble P remain unclear. Thus, this study aimed to investigate the effects of struvite (MAP) and hydroxyapatite (HAP), two typical insoluble phosphates from digestates, on maize growth and rhizospheric microorganisms in a maize -soil system. Results showed that the maize yields in the MAP and HAP groups were 13.00 % and 19.26 % higher, respectively, than that in the control group and 13.65 % and 8.87 % lower, respectively, than that in the superphosphate (SSP) group. Similar results were obtained for the shoot nutrients. The fertilizer utilization efficiencies were approximately 18.9 % in the SSP group, 9.6 % in the MAP group, and 12.1 % in the HAP group. Maize in the MAP and HAP groups absorbed more P from deeper soil as a supplement, displaying adaptability to low P environments. Moreover, P-solubilizing microorganisms (PSMs), such as Bacillus sp., Gaiellales sp., and Fusarium sp., were enriched in the black control (CK), MAP, and HAP groups and played important roles in mineralizing and dissolving P. In conclusion, MAP and HAP can be used as potential P fertilizer supplements and can be further developed by strengthening PSMs in fertilizers. This study provides theoretical support for utilizing non-water soluble P in digestates and novel insights into the development of new P fertilizers.