Ecosystem feedbacks from subarctic wetlands: vegetative and atmospheric CO₂ controls on greenhouse gas emissions

Abstract. Wetland vegetation provide strong controls on greenhouse gas fluxes but impacts of elevated atmospheric carbon dioxide (CO₂) levels on greenhouse gas emissions from wetlands are poorly understood. This study aims to investigate if elevated atmospheric CO₂ enhance methane (CH₄) emissions from subarctic wetlands and to determine if responses are comparable or species specific within the Cyperaceae, an important group of artic wetland plants. To achieve this we carried out a combined field and laboratory investigation to measure of CO₂ and CH₄ fluxes. The wetland was a CH₄ source with comparable fluxes from areas with and without vegetation and across the different sedge communities. In contrast, the net ecosystem exchange of CO₂ differed with sedge species. Within the laboratory experiment plants grown at double ambient (800 ppm) CO₂, total biomass of Eriophorum vaginatum and Carex brunnescens increased, whereas the total biomass of E. angustifolium and C. acuta decreased, compared to the control (400 ppm CO₂). These changes in biomass were associated with corresponding changes in CH₄ flux. E. vaginatum and C. brunnescens mesocosms produced more CH₄ when grown in 800 ppm atmospheric CO₂ when compared to 400 ppm CO₂ with E. angustifolium and C. acuta producing less. Additionally, redox potential and carbon substrate availability in the pore water differed among the plant treatments and in response to the elevated CO₂ treatment. Together, this suggests species specific controls of CH₄ emissions in response to elevated CO₂, which facilitate differential plant growth responses and modification of the rhizosphere environments. Our study highlights species composition as an important control of greenhouse gas feedbacks in a CO₂ rich future, which need to be considered in models aiming to predict how ecosystems respond to climate change.