<i>In situ</i> high-resolution characterization of phosphorus–iron–sulfur dynamics across the sediment–water interface of a typical cold-arid lake (Ugii Lake, Mongolia)

Phosphorus (P), iron (Fe), and sulfur (S) are essential nutrients for living organisms and have significant environmental impacts on aquatic ecosystems. However, the biogeochemistry of P, Fe, and S across the sediment-water interface (SWI) in cold-arid regions remains poorly understood. Herein, we first applied a combination of high-resolution <i>in situ</i> techniques, namely diffusive gradients in thin films (DGT) and a home-made two-dimensional miniature-DGT (2D-MDGT), to simultaneously analyze the dynamic distributions of P, Fe, and S across the SWI in Ugii Lake, Mongolia. The concentrations of labile P, Fe, and S in the sediment profiles range from 0.01 to 0.15 mg L^-1, 0.12 to 1.10 mg L^-1, and 0.15 to 0.4 mg L^-1, respectively, with a considerable number of hotspots. Spatially, labile P and Fe were higher in the near-shore region more vulnerable to exogenous pollution than the central region, while labile S showed the opposite trend. In vertical profiles, the distributions of labile P and Fe showed a significant positive correlation (<i>P</i> < 0.01), indicating that Fe redox cycling dominated P mobility. In contrast, a weak relationship between labile P and S as well as labile Fe and S indicated limited contributions of S to P mobilization. The <i>in situ</i> measurements of diffusion fluxes of P, Fe, and S across the SWI showed values of 0.015-0.031 mg m^-2 d^-1, 0.067-0.288 mg m^-2 d^-1, and 1.087-1.801 mg m^-2 d^-1, respectively, indicating strong upward mobility of these elements from the sediment to the overlying water. Overall, the study first captured 2D fine-scale co-distributions of P, Fe and S across the SWI in Ugii Lake and filled the gap on P-Fe-S redox cycling processes and mechanisms at the fine scale, which provided a reference and theoretical basis for water quality control in cold-arid lakes.