Kinetic Exchange of Remobilized Phosphorus Related to Phosphorus‐Iron‐Sulfur Biogeochemical Coupling in Coastal Sediment

Abstract

AbstractPhosphorus (P) cycling in coastal sediment can be strongly coupled to iron (Fe) and sulfur (S) geochemical transformations. In this study, the high‐resolution dialysis (HR‐Peeper) and diffusive gradients in thin films (DGT) techniques were employed to synchronously characterize the in situ P‐Fe‐S biogeochemistry in coastal sediments around Xiamen Bay, China. The results show that P remobilization is mainly controlled by dissimilatory Fe reduction in the Jiulong River Estuary (JE) and the eastern coast of Xiamen Bay (EX), while it is dominated by sulfate reduction in the Jiuxi River Estuary. The confined oxidizing depth and the higher DGT‐labile S and soluble Fe (II) concentrations in October indicate that the hypoxia induced by tides and river runoff has a greater effect on sulfate reduction and Fe reduction than high temperatures in July, which accelerate P remobilization and exchange flux. Model simulations of the P resupply and adsorption/desorption kinetics demonstrate that the enhancement of Fe reduction and sulfate reduction triggered by hypoxia promotes the continuous release of P. Both DGT‐labile P and S presented distinct distribution patterns in crab burrows during flood tide and ebb tide. A negative P exchange flux to the overlying water controlled by Fe (III) retention at JE and a positive P exchange flux controlled by intense sulfate reduction at EX were observed, both of which were significantly higher than the molecular diffusion flux. We suggest that internal P loading related to Fe and S redox geochemistry should receive more attention, especially in the context of expanding global marine hypoxia.