High-resolution imaging of labile P & S in coastal sediment: Insight into the kinetics of P mobilization associated with sulfate reduction

Abstract

The internal P cycling in coastal sediments during early diagenesis is of great importance to aquatic ecosystems, but its relationship with sulfate reduction is still unclear and lacks visualized evidence. In this study, diffusive gradients in thin films (Zr-oxide DGT and AgI DGT) techniques at a two-dimensional (2D) submillimeter resolution were utilized to synchronously characterize the in situ labile P and dissolved sulfide in coastal sediments around Xiamen Bay, China. The distributions of dissolved sulfide formed from sulfate reduction showed large vertical and horizontal variations, which are mainly controlled by the reactive organic carbon (OC) and redox conditions at different sampling sites. Multiple correlation analysis revealed that significant and linear correlations exist between dissolved sulfide and DGT-labile P, demonstrating that P mobilization is predominantly regulated by sulfide-dependent reductive dissolution of Fe(III)-bound P and sulfate reduction-associated OC mineralization. Both labile P and dissolved sulfide showed lower values at intertidal sites, due to oxic/suboxic conditions favoring weak sulfate reduction and strong P sequestration. Labile P accumulation during flood tides and removal during ebb tides on tidal timescales indicate that the mobilization and detention of P in surface sediment are regulated by tidal cycles and consequential S and Fe redox cycling. The apparent diffusion flux of P implies that the internal P loading can act as a P source to aquatic ecosystems.