Floodplain forests outperform croplands as methane sinks while exhibiting greater microbial methane-production potential

Abstract

Floodplains represent major sources of methane, a greenhouse gas with significant warming potential, as high soil humidity favors microbial methane production (methanogenesis) over consumption (methanotrophy). Land use also influences methane dynamics by modulating soil properties that regulate methanogenesis and methanotrophy, with forest soils generally showing the greatest methanotrophic potential. However, how land use shapes microbial functions regulating soil methane fluxes remains largely uncharacterized in floodplains. This study aims to better understand the role of forests relative to grasslands and croplands in floodplain methane budgets by assessing soil methane fluxes and microbial functional potential and activity. We used metagenomics and metatranscriptomics to investigate carbon- and methane-cycling gene abundance and expression across land uses, and how these were linked to variations in soil methane fluxes in the Lake St-Pierre floodplain (Quebec). We demonstrated that forests were long-term methane sinks, exhibiting significantly higher uptake rates than croplands, underscoring the importance to protect forests in floodplain ecosystems. Despite greater methane uptake over years, forest soils exhibited a higher potential for anaerobic metabolisms and could switch to net sources at specific times and sites under high water table conditions. The expression of genes regulating acetate conversion, CO₂ reduction, and H₂ oxidation explained flux variations, indicating that substrate conversion and electron generation are key steps driving methane production and spatiotemporal variation in soil methane fluxes in floodplains. Overall, our study demonstrates how methane cycling varies between floodplain forests and croplands across temporal scales in response to environmental controls, clarifying their respective roles in climate regulation.