Superlinear control of phosphorus recycling in coastal sediments by organic matter availability

Phosphorus (P) recycling from sediments regulates water column P availability. However, the balance between P regeneration and immobilization in coastal sediments under diverse environmental regimes remains poorly quantified, posing challenges for modeling biogeochemical processes and the estimation of regional and global geochemical budgets. We investigated sediment P cycling across the Pearl River Estuary region (9–63 m water depth), observing substantial variability in sediment phosphate effluxes (4.9–1190 $\mu$mol m⁻² d⁻¹). Sediment P recycling efficiency (P recycled: P sedimentation) varies strongly (24–96%), contrasting with the expected consistency under proportional phosphate release from organic matter remineralization. Unlike the classic model of oxygen controlling P recycling, in the organic-rich sediments where oxygen penetration is consistently shallow, nitrate availability becomes the dominant control. High nitrate concentrations upstream preserve P-binding iron oxides in the sediments, inhibiting phosphate release. At the estuary mouth and offshore areas with low nitrate, sediment phosphate efflux increases with sediment oxygen uptake, an indicator of organic matter remineralization rate. However, the effect is disproportionate, following a superlinear (power-law) relationship. This is because high organic matter remineralization not only regenerates more phosphate but also reduces more P-binding iron oxides through iron and sulfate reduction, doubling the promotion of P efflux. This superlinear control of sediment P recycling by organic matter should be considered in estimating sediment-water exchanges in similar coastal systems that are both iron and organic-matter rich.