Our paper presents the results of simultaneous measurements of carbon dioxide fluxes using non-steady state non-through flow chamber method and the amount of green biomass above the ground surface in the sedge community Carex elatea. The comparison of the results of the research allowed us to estimate the amount of carbon dioxide accumulated in green plant biomass. It was concluded that a maximum of 28% of carbon absorbed in the process of photosynthesis was accumulated in green part biomass. The carbon uptake per unit of carbon of green biomass at the beginning of the vegetative season amounted to 491.2 μg·g^-1·MJ^-1·d^-1, whereas at the end of the vegetative season it dropped to 49.1 μg·g^-1·MJ^-1·d^-1.