Rhizosphere oxygenation decreases greenhouse gas emissions from thawed permafrost soil

With thawing, permafrost soils can shift from dry, oxic conditions to wetlands, driving significant changes in soil biogeochemistry and plant community composition, ultimately altering climate-relevant greenhouse gas (GHG) dynamics. Graminoid species, such as Carex spp. and Eriophorum spp., thrive in anoxic soils, exhibit high primary productivity, release substantial amounts of organic root exudates fueling CO₂ and CH₄ emissions, and possess adaptive traits for anoxia. Among these traits is the formation of aerenchyma tissues, which enable oxygen release into the rhizosphere. Rhizosphere oxygenation promotes aerobic metabolisms increasing CO₂ emissions, yet the effect on CH₄ can be variable: it may enhance soil organic matter breakdown into small molecules such as acetate - potentially fueling CH₄ production - or suppressing methanogenesis. Currently, it is uncertain whether the combination of rhizosphere oxygenation and organic exudation in anoxic soils contributes to more production of climate-relevant GHGs or whether it has a suppressing effect.

To tease apart the individual and combined effects of rhizosphere oxygenation and organic exudation on thawed permafrost soil biogeochemistry and GHG fluxes, we incubated soil obtained from a fully thawed site in Stordalen mire, Sweden, under anoxic conditions. The soil was subjected to one of four treatments via an artificial root: a non-spiked control; organic exudate-mix alone (added three times a week); continuous ambient air addition; and a combination of both organic exudate mix and air. Concentrations were chosen to mimic plant release amounts under field conditions. Porewater geochemistry analysis is combined with extractions of organic carbon and iron precipitated on the artificial root and discussed along headspace GHG data and microbial functional gene profiling.

Organic exudate-mix alone strongly increased both CO₂ and CH₄ emissions, accompanied by substantial mobilization of iron and organic carbon. In contrast, the addition of air - either alone or combined with organic exudation - decreased CH₄ and CO₂ emissions. Decreased CH₄ may potentially be caused by thermodynamic suppression of methanogenesis by less reducing soil conditions as indicated by less mobilized and more oxidized iron. Based on the used amounts of organic carbon and air inputs and the chosen lab incubation parameters, the combination of organic exudation and oxygenation could lead to less stimulation of GHG production as anticipated from classic priming studies.