Quantifying CO2 and CH4 fluxes and their seasonal dynamics in a temperate wetland with water table fluctuations

Wetlands are the largest and most uncertain natural contributor of atmospheric methane (CH₄) with water table fluctuations being a key factor controlling spatial and temporal variations. Natural dry summer months are here linked to low water table, increased oxygen diffusing into the soil and corresponding carbon dioxide (CO₂) and CH₄ fluxes in a Danish temperate wetland north of Copenhagen. We used the process-based model LPJ-GUESS to quantify the dynamic changes in CO₂ and CH₄ fluxes in the past 17 years, with an improved methane algorithm considering both methane production and oxygen transport influenced by water table level, CH₄ consumption rates with the vertical distribution of methanotrophs. Our findings show that the model successfully can reproduce the temporal of pattern five-year (2007-2011) oxygen concentration in the soil profile with water fluctuation and corresponding CO₂ and CH₄ fluxes. For 2007-2023, the calibrated model simulates the site as a net CO₂ sink of -77 ± 81 g C-CO₂ m^-2 year^-1 and a CH₄ source of 1.48 ± 0.84 g C-CH₄ m^-2 year^-1. For changes in seasonal pattern, precipitation has a significant declining trend in early- and mid-growing season (March to July) (-9.2 mm per year, p < 0.05), with the largest reduction in June (-4.8 mm per year, p < 0.01), encountering the growth peak of vegetation. Such reduced precipitation mitigates methane emission (-0.04 g C-CH₄ m^-2 per year, p < 0.01) and increases net ecosystem exchange (+5.1 g C-CO₂ m^-2 per year, p < 0.05) in early and mid of the growing season with the interplay of a lower water table. The average budget of radiative balance with a lower annual mean water table (-0.31 m) was enhanced to -58 g C-eq m^-2 year^-1, while -556 g C-eq m^-2 year^-1 with a higher annual mean water table (-0.14 m), which is mainly explained by that lower water table decreased C assimilation and increased soil respiration.