The impact of wildfire and droughts on the GHGs exchange in temperate wetland.

We analyzed 10 years (2013-2022) of CO2 and CH4 flux data measured using the eddy covariance method in the Biebrza National Park in north-eastern Poland. The Biebrza wetlands are among the largest in Central Europe, comprising a contiguous area of more than 250 km².  The measurement site (53°35′30.8′′ N, 22°53′32.4′′ E, 109 m a.s.l.) was located in the central basin of Biebrza valley at the area dominated by patches of reeds, high sedges, and rushes, very typical of Biebrza wetlands. In the analyzed period the studied ecosystem was affected by severe droughts in 2015 and 2018-2020. Moreover, on April 20–25, 2020, the Biebrza National Park was touched by huge fires that consumed over 5,500 ha of landscape. The measurement site was located at the north-eastern edge of the burned area and the entire source area of eddy-covariance system was affected by the fire. The system suffered some damage, but flux measurements were re-established about a week after the fire.  The response of the CH4 flux to changes in hydrometeorological conditions is quite simple - the thermally determined annual course of CH4 is strongly modified by the water table level (WTL). The annual emission of CH4 reached 21 gC-CH4·m-2·yr-1 in the wettest year and dropped even below 1 gC-CH4·m-2·yr-1 in dry years. The CO2 exchange response is more complex. In the case of net ecosystem exchange (NEE), a linear relationship is observed between the average WTL and the annual sum of CO2 flux. In wet years the studied peatland was a significant sink of CO2 (down to −250 gC-CO2·m-2·yr-1) whereas in dry years we observed a substantial release of CO2 (up to +300 gC-CO2·m-2·yr-1). A similar linear relation was observed for ecosystem respiration (ER), which ranged from 830 to 1400 gC-CO2·m-2·yr-1 in wet and dry years respectively. In contrast, gross ecosystem production (GEP) followed WTL changes only in the first 3 years of observations. Vegetation then switched to drier conditions and GEP remained on similar level up to 2020, when it increased significantly after the April fire. Excluding 2020, GEP varied in the range of 910-1250 gC-CO2·m-2·yr-1.   Acknowledgements: Funding for this research was provided by the National Science Centre, Poland under project UMO-2020/37/B/ST10/01219 and University of Lodz under project 4/IDUB/DOS/2021. The authors thank the authorities of the Biebrza National Park for allowing the continuous measurements in the area of the Park.