Anaerobic methane oxidation can impact phosphorus retention in lake sediments

The high organic content in freshwater sediments is often linked to methane (CH₄) presence and phosphorus (P) release to water column. However, there are few investigations on an impact of the CH₄-oxidation pathway for P retention in freshwater systems. In this study, we investigated an impact of anaerobic methane oxidation (AOM) by iron (Fe) oxyhydroxide on P loading from sediments. We performed a field experimental study and a modelling balance approach. The study site is a typical northern temperate freshwater ecosystem for northern Laurentian Great lakes catchment around Lake Huron. The redox-sensitive P binding form, linked to P bound on Fe- and manganese-(hydro) oxides was the dominant TP fraction from 0 to 30 cm. The CH₄ concentration reached the maximum concentration of 109 μM at 24 cm in the sediment. We suggest one possible explanation for SRP enrichment at 16-20 cm may be attributed to Fe (III) reduction coupled to AOM (Fe-AOM), which promotes the dissolution of Fe-oxyhydroxides and the subsequent release of adsorbed phosphate. This interpretation is supported by concurrent CH₄ consumption and the prevailing geochemical conditions in this depth interval. Fe-AOM may contribute up to 79.5 µmol m⁻² day⁻¹ P release. Vivianite was identified in deeper sediment layers, below the depth interval where Fe-AOM activity is suggested. Our findings underscore the necessary of comprehending the impact of Fe-AOM, which is documented in numerous freshwater sediments, on the P cycle in freshwater system.