Relationship between methane flux and carbon uptake in three cosmopolitan wetland plant genera

Abstract

Wetlands are the most important natural source of methane (CH₄) to the atmosphere. When present, plant efflux is often the dominant pathway for methane emissions from the anoxic wetland sediments, however, they are rarely considered in chamber measurement campaigns of CH₄ flux in wetlands. While studies have focused on the mechanisms that govern transport through plant structures, none have addressed how CH₄ emissions relate with carbon uptake or how this relationship may vary among plants. Understanding this relationship has important implications for linking carbon flux at the ecosystem level to wetland CH₄ emission predictions, and ultimately, to help improve current carbon budget and greenhouse gas models of wetlands. This study presents simultaneous measurements of CH₄ and CO₂ fluxes in three macrophyte species of different cosmopolitan genera: Typha (emergent), Nelumbo, and Nymphaea (floating attached). These measurements were conducted using the Picarro GasScouter G₄301 fitted with custom-designed leaf chambers. Chamber measurements were performed three times during a day (morning, noon and afternoon), in three separate months. Results indicate significant negative correlations between CH₄ flux and carbon uptake in Nelumbo and Nymphaea species and a non-significant positive correlation in Typha sp. We term the rate of CH₄ flux per unit CO₂ uptake – CH₄ flux efficiency. We found that in Nymphaea CH₄ flux efficiency is almost double that of Nelumbo. These results suggest the dependence of CH₄ efflux in stomatal conductance in the floating attached macrophytes, while in the Typha CH₄ flux is primarily through other pores, probably aerenchyma, not regulated by stomata. Our results emphasize the importance of species-specific knowledge and considering the growth type of different wetland macrophytes (e.g. emergent vs floating attached) in scaling of bottom-up observations and ecosystem models.