Methane rhizospheric oxidation and its ecosystem significance across different aquatic plant species

Abstract

Vegetated inland water habitats are important global emitters of methane (CH₄), but the variability in their CH₄ emission is poorly understood, making estimates and predictions uncertain. Methane is believed to be primarily released into the atmosphere via plant lacunal tissues in dense vegetated zones, although experiments have also shown that a large fraction of CH₄ can be consumed in the rhizosphere before it diffuses into the roots. Methane rhizospheric oxidation measurements have been limited to few species in situ, and it is still unclear how it differs between species. To determine the implications of CH₄ rhizospheric oxidation for plant-CH₄ release, I measured CH₄ rhizospheric oxidation and transport rates for 11 plant species with different gas transport efficiencies using dual isotopes in situ. Overall, the fraction of CH₄ produced that was oxidized was higher for plants that transport gases efficiently (up to 81%), and was negatively correlated to plant CH₄ release. These findings suggest that CH₄ oxidation decreases CH₄ release from plants that efficiently transport gases, although further studies are needed to assess the causality of this relationship. Such mechanistic understanding of plant-CH₄ release is necessary to define aquatic plant functional types with contrasting CH₄ emissions, and to reduce uncertainties in estimates of CH₄ freshwater emissions.