Effect of Hummock-Forming Vegetation on Methane Emissions from a Temperate Sedge-Grass Marsh

Plants affect CH4 emissions from wetlands by providing routes for CH4 ventilation from the soil and its re-oxidation, the outcome depending on the vertical distribution of the plant ventilating structures as related to water level. This study investigated the effect of elevated hummocks on CH4 emissions in a temperate wetland dominated by a hummock-forming sedge, Carex acuta L. Comparative measurements of CH4 fluxes from paired plots with or without hummocks revealed a prevailing positive difference interpreted as plant-mediated fluxes. All types of CH4 fluxes responded positively to water level with a hysteresis related to its recent dynamics. Seasonal medians of CH4 emissions from the ecosystem, based on fluxes from both types of plots, were 15.09 and 0.11 mg m(-2) day(-1) in the wet year 2012 and the dry year 2014, respectively. This relatively low magnitude of CH4 emissions, compared to values from similar habitats within the same range of water levels, is ascribed to the presence of hummocks. At water levels near the soil surface, hummocks extend above the water table and serve as aerobic micro-habitats in which plant structures avoid anaerobic stress and CH4 produced in the bulk soil and vented via deep roots can be re-oxidized.