Sediment controls the recycling and sequestration of phosphorus (P), an essential nutrient in marine ecosystems. Understanding sediment P cycles helps constrain the P budget and predict how ecosystems respond to environmental variability. This study investigates and compares P cycling in sediments of various systems including typical coastal to deep seas, a lagoon (Pinqing lagoon), a mangrove swamp (Mai Po nature reserve), and deep-sea methane seeps (Haima seep). By characterizing the porewater and solid sediment geochemistry of P and related elements including iron (Fe), sulfur (S), and oxygen (O), the work aims to decipher the mechanistic controls of sediment P cycling under diverse regimes. We find that sediment P recycling is higher in coastal compared to deep seas, because organic...[ Read more ]

Sediment controls the recycling and sequestration of phosphorus (P), an essential nutrient in marine ecosystems. Understanding sediment P cycles helps constrain the P budget and predict how ecosystems respond to environmental variability. This study investigates and compares P cycling in sediments of various systems including typical coastal to deep seas, a lagoon (Pinqing lagoon), a mangrove swamp (Mai Po nature reserve), and deep-sea methane seeps (Haima seep). By characterizing the porewater and solid sediment geochemistry of P and related elements including iron (Fe), sulfur (S), and oxygen (O), the work aims to decipher the mechanistic controls of sediment P cycling under diverse regimes. We find that sediment P recycling is higher in coastal compared to deep seas, because organic matter availability controls Fe and sulfate reduction, affecting P fluxes and sequestration. However, such control is nonlinear, which is demonstrated in the sediments of Pinqing Lagoon, where orders of magnitude differences in P effluxes are observed within a single system despite the similar organic matter supplies. This disproportional control is likely due to the organic matter reactivity. Moreover, our results suggest that the classic organic-matter driven Fe-S-P coupling can be further complicated: we find unexpectedly low P recycling in the organic-rich anoxic sediments of a mangrove pond because organic matter fuels denitrification and affects P sequestration. On the other hand, when organic matter is scarce, alternative electron donors may also control P release: in the sediment at deep-sea methane seeps, Fe-S-P coupling can be promoted by anaerobic oxidation of methane, leading to P fluxes as surprisingly high as coastal sediments. These findings demonstrate the complexity of coupled C-Fe- S cycles in controlling the P cycle and highlight several crucial directions to explore in the future.