Coupled geochemical profiling and metagenomics reveal controls on phosphine preservation and emission in a eutrophic Estuary.

Matrix-bound phosphine (MBP) represents a critical yet poorly constrained component of aquatic phosphorus cycling, and the controls governing its preservation and emission in eutrophic estuarine systems remain incompletely resolved. The spatial controls on MBP preservation and atmospheric phosphine emission across the Pearl River Estuary (PRE) were investigated by integrating sediment phosphorus fractionation, sub-millimeter diffusive gradients in thin films (DGT) profiling, and metagenomic sequencing. Sedimentary MBP was detected at all sites and varied markedly along the estuarine gradient, ranging from 2.38 to 36.85 ng kg-1 ww, with significant positive correlations with Org-P and TP (p < 0.05). The PRE acted as a net atmospheric source of PH3 during summer, with air-water interface (AWI) fluxes ranging from -5.35 ± 0.63 to 28.90 ± 4.67 ng m-2 h-1 and highest emissions concentrated at inner-estuarine nearshore sites. DGT-derived labile P-Fe-S coupling patterns and systematic shifts in microbial metabolic functional potential (e.g., dsrA, mcrA, and ptxD genes) were broadly consistent with the spatial distribution of MBP, suggesting that microscale redox conditions and microbial community function may collectively contribute to reduced-P preservation. The accumulation of Org-P and OM in nearshore depositional zones, driven by terrestrial inputs and local hydrological conditions, may progressively shift sedimentary phosphorus cycling toward pathways that favor reduced-P preservation and sustained atmospheric PH3 emissions. Collectively, these findings offer new insights into the spatial controls on MBP preservation and atmospheric PH3 emission in eutrophic estuarine systems, which are essential to understanding the complex biogeochemical processes that regulate nutrient cycling in these fragile ecosystems.