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

Abstract

Phosphorus (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.Special Section: Coastal hydrology and oceanography Key Points: • Redox conditions affected by hydrodynamic forces (tide and/or runoff) dominated iron/sulfate reduction and accompanied P remobilization • Enhanced sulfate reduction fueled by abundant organic matter and hypoxia in crab burrows significantly promoted P remobilization • The P flux via hydrological exchange in burrows was much higher than the conventional molecular diffusion fluxSupporting Information:• Supporting Information S1

Molybdenum background and pollution levels in the Taipu River, China

“…Previous studies have shown that background values of an element in sediments are often location-specific due to the differences in parental material and accumulation rate of fine sediment particles which are more enriched in elements (Bindler et al 2010;Lin et al 2018). In Taipu river, because of its river network and shipping perturbation, the hydrodynamic is very complicated and lead to variability in background value of Mo in sediment (Mao et al 2019;Pan et al 2019).…”

Coupling mechanisms of S–Fe–P in surface sediments under the stresses of high salinity and heavy metals in coastal rivers

“…Hence, the fixation of Fe-P was directly affected by FeS x formation via OM decomposition because the transformation of P was mainly caused by the oxidation of Fe(III) in sediments (Li et al 2013). The supply of FeOOH might transiently exceed the supply of sulfide in Fe-rich sediments (Pan et al 2019). Moreover, the organic colloids that formed from the aerobic breakdown of OM were covered on an inorganic solid surface (e.g., clay minerals, alumina, Fe oxide and calcium carbonate) (Leng and Lewis 2017).…”

“…Sulfur (S) is an important driving factor in coastal environmental evolution due to its roles as an electron acceptor, carrier, and donor (Sun et al 2016;Latour et al 2018;Liu et al 2019). Sulfate reduction can significantly affect the cycling of phosphorus (P) and iron (Fe) in sediments and produce extensive subtoxic or anoxic zones and toxic sulfides (Wu et al 2019;Pan et al 2019). Especially at the sediment-water interface with frequent substance exchange, subtoxic or anoxic conditions may accelerate the pollutant or nutrient release from the sediment into the water column, thereby causing deterioration of the water quality (Pandey and Singh 2017;Jiang et al 2020).…”

“…Insoluble sulfide minerals (e.g., pyrite and mackinawite) were formed via the reaction of sulfides and Fe-containing particles, which are referred to as sinks for trace metals due to their affinity for metal ions (Audry et al 2010;Oueslati et al 2018). Additionally, sulfides can be reoxidized into elemental S (ES) by Fe(III) (oxyhydr) oxides (FeOOH) under anoxic conditions, thereby inducing chemical Fe reduction and remobilization of previously bound P (FeOOH-P) (Pan et al 2019). After being released into porewater, the reduced Fe(II) can precipitate with the dissolved sulfide as acid volatile sulfur (AVS, FeS) and react with ES to form chromium reducible sulfur (CRS, FeS 2 ) (Sheng et al 2015).…”

“…The accumulated reductive S and Fe minerals have minimal affinity for P sequestration (Thouvenot-Korppoo et al 2012). Consequently, Fe sulfides and Fe oxides, via reductive dissolution, not only release the adsorbed and co-precipitated contaminants but also accelerate the release of P (e.g., Fe-P) from sediments (Sun et al 2016;Pan et al 2019).…”