Flooding, metal mobility, and microbial controls on greenhouse gas emissions from floodplain soils

Floodplains are integral parts of river systems, often characterised by periodic flooding. Increased river flooding caused by climate change potentially has significant impacts on oxidation-reduction processes in floodplain soils, altering soil oxygen availability, microbial populations, carbon and nitrogen cycling, and greenhouse gas (GHG) emissions. Flooding can also cause the deposition of metal-contaminated sediments on floodplain soils. During flooding, redox-driven changes in metal speciation can alter metal solubility and bioavailability, thereby affecting soil microbial composition and GHG emissions. Although the effects of flooding on redox-sensitive soil processes are well documented, few studies have examined the combined effects of flooding and metal contamination on soil GHG emissions.

To address this gap, we conducted an outdoor mesocosm experiment using intact floodplain soil cores collected from sites with contrasting flood histories (frequently vs rarely flooded), with varying levels of background metal concentrations. Soil cores were inundated for seven weeks to simulate a flood event. GHG fluxes were measured twice weekly before, during, and after the flood event. Two closed-chamber approaches were employed: CO₂ and CH₄ concentrations were measured in real-time for three minutes per chamber using an ultraportable greenhouse gas analyser, while discrete chamber gas samples were collected at four time points over a one-hour enclosure period for N₂O analysis by gas chromatography. A wide range of redox-sensitive soil parameters (oxygen concentration, pH, dissolved organic carbon, metals, and anions) were measured in soil pore water weekly. Soil subsamples collected before and after flooding will be used for metagenomic analysis to assess changes in microbial community composition and the abundance of functional genes associated with key redox processes, including GHG emissions. By integrating redox geochemistry and metagenomic analyses, this study aims to provide mechanistic insight into how flooding and metal mobility regulate microbial functions and soil GHG emissions.