Dynamics of net CO2 exchange in the wetland ecosystem recovering from a fire&#160;
Authors: Fortuniak Krzysztof , Pawlak Włodzimierz , Siedlecki Mariusz 
Published: June 2021

&lt;p&gt;Wetlands, even if cover a relatively small fraction of Earth&amp;#8217;s surface, play an important role in global carbon cycle. They are the main terrestrial source of methane (CH&lt;sub&gt;4&lt;/sub&gt;), but due to anaerobic conditions they accumulate significant part of captured in photosynthesis carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;). Due to the progressive climate change these ecosystems are exposed to different climate-inducted extreme events. One of them are fires that can significantly affect the carbon-storage potential of the wetlands.&lt;/p&gt;&lt;p&gt;In this study we analyze the impact of a great fire on one of the largest mid-European wetlands in Biebrza Valley (northeastern Poland) on the CO&lt;sub&gt;2&lt;/sub&gt; net ecosystem exchange (NEE). Over 5,500 ha of landscape of the Biebrza National Park burned down during this event in April 20-25, 2020. In the north-east edge of the core of this fire, there was an eddy-covariance measurement site, where greenhouse gas fluxes (CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt;, H&lt;sub&gt;2&lt;/sub&gt;O) had been continuously recorded since 2013. The measurement system suffered to some extent, but flux measurements were resumed after repair works in approximately 2 weeks. Almost the entire source area of eddy-covariance system was affected by the fire. Thus, post-fire measurements show the dynamics of NEE for an ecosystem recovering from a fire.&lt;/p&gt;&lt;p&gt;In the flux measurements period (2013-2020) the studied ecosystem was affected not only by the above fire event but also by severe droughts in 2015 and 2018-2020. In consequence in non-fire years the annual totals of CO&lt;sub&gt;2&lt;/sub&gt; flux followed the mean ground water table level (WTL) and spanned from -990&amp;#160;gCO&lt;sub&gt;2&lt;/sub&gt;&amp;#8729;m&lt;sup&gt;-2&lt;/sup&gt;&amp;#8729;yr&lt;sup&gt;-1&lt;/sup&gt; (CO&lt;sub&gt;2&lt;/sub&gt; sink) in the wettest year to +1020&amp;#160;gCO&lt;sub&gt;2&lt;/sub&gt;&amp;#8729;m&lt;sup&gt;-2&lt;/sup&gt;&amp;#8729;yr&lt;sup&gt;-1&lt;/sup&gt; (CO&lt;sub&gt;2&lt;/sub&gt; source) in the driest year 2019. However, even taking into account the influence of WTL and temperature fluctuation we observed clear impact of the spring fire on CO&lt;sub&gt;2&lt;/sub&gt; exchange. Shortly after the fire, in May, the wetland was in average a source of CO&lt;sub&gt;2&lt;/sub&gt; (positive monthly total of NEE), which had not happened before even in the driest years. However, already in the second half of May, the absorption of CO&lt;sub&gt;2&lt;/sub&gt; began to predominate over the emissions. From the mid-June to the end of July we observed very intensive growth of plant cover and exceptionally strong absorption of CO&lt;sub&gt;2&lt;/sub&gt;, much higher than in other years with similar thermo-hydrological conditions. Consequently, the total CO&lt;sub&gt;2&lt;/sub&gt; flux in the post-fire period (May-December) was negative, while in remaining dry years the strong emission of CO&lt;sub&gt;2&lt;/sub&gt; was observed for the same part of year. &amp;#160;&lt;/p&gt;&lt;p&gt;Acknowledgements: Funding for this research was provided by the National Science Centre, Poland under project UMO-2020/37/B/ST10/01219. The authors thank the authorities of the Biebrza National Park for allowing the continuous measurements in the area of the Park.&lt;/p&gt;