Influence of summer flood on the net ecosystem exchange of CO2 in a temperate sedge-grass marsh

Characteristics of CO2–C exchange were studied in a temperate herbaceous wetland in 2006 and 2007, years with contrasting hydrological regimes. The wetland Mokré Louky (the “Wet Meadows” near Třeboň), Czech Republic (49°01′30″ N, 14°46′20″ E) is a sedge-grass marsh dominated by Carex acuta L. The site is situated in the inundation area of a large human-made lake (Rožmberk fishpond, 5 km2). The water table is near the soil surface and is fairly stable throughout the year (±0.1 m), except for flood events, when it can reach up to 2 m above the soil surface. Net ecosystem production (NEP) was calculated from the CO2 fluxes measured by the eddy-covariance technique and was compared with the seasonal course of plant live aboveground biomass. The results show that: (1) the annual net uptake of CO2–C was 199 and 220 g C m−1 yr−1 in 2006 and 2007, respectively. The daily net CO2–C uptake reached 12–20 g C m−2 day−1 in sunny summer days. These values are among the highest values reported for wetlands of the northern hemisphere. (2) Although the observed wetland is adapted to floods, the summer flood substantially reduced its net CO2–C uptake. The daily courses of net CO2–C uptake point to reduced rates of CO2–C fixation by the vegetation cover. (3) A recovery growth of plant biomass was associated with a second peak of NEP in September 2006. (4) Lack of knowledge on the dynamics of methane emissions presently limits deeper understanding of the effect of floods on carbon balance in wetlands.