Post-immobilization phosphorus fluctuations drive selective microbial hydrolysis of sedimentary organic phosphorus and long-term P release.

In-situ immobilization has been widely utilized as an efficient and cost-effective phosphorus (P) treatment technology for lake eutrophication. However, post-immobilization P fluctuations in the overlying water may trigger the compensation of endogenous P from sediments over the long term. Microbially mediated hydrolysis of sedimentary organic phosphorus (SOP) is widely recognized as a pivotal process driving this P replenishment. However, the specific SOP hydrolysis pathways and the underlying microbial interactions remain to be systematically elucidated. Herein, this study explored the post-immobilization P release and SOP fraction change over the long term. Sediment fractionation, enzymatic results, FT-ICR-MS, 31P NMR spectroscopy, and Zipi analysis collectively revealed two distinct SOP utilization patterns, explained by molecular composition shift and substrate selectivity to post-immobilization P fluctuations. During major fluctuations, r-strategists emphasized rapid growth of keystone species (Betaproteobacteria and Firmicutes) carrying phoX genes, potentially boosting orthophosphate esters (C-O-P) hydrolysis to increase phosphate flux rapidly (1.49 mg·m-2·d-1) while impeding the complete mineralization of SOP (47.11%). By contrast, the minor P fluctuation showed that K-strategists exhibited more diverse Proteobacteria populations carrying the phnJ gene, which possibly preferred hydrolysis for methylphosphonate (C-P) and achieved higher SOP resource utilization (61.22%). Structural equation modeling also indicated that microbial r-K strategies attained DIP compensation by regulating dissolved IP flux in porewaters through SOP substrate selectivity. This work provides novel insights into microbial-mediated SOP hydrolysis in the sediment after immobilization, highlighting long-term post-remediation lake eutrophication risk and developing progressive multi-stage immobilization application strategies.