Climate impact of constructed wetlands treating nitrate-rich agricultural runoff: The methane problem
Authors: Mander Ülo , Soosaar Kaido , Tournebize Julien , Kill Keit , Chaumont Cedric , Maddison Martin , Kasak Kuno 
Published: March 2022

&lt;p&gt;Constructed wetlands (CW) treating runoff from agricultural catchments can efficiently reduce nitrate contamination, however, most wetlands are inherently net sources of methane (CH&lt;sub&gt;4&lt;/sub&gt;), which is of environmental concern due to its potent global warming capacity. For the mitigation of this negative aspect of CWs we need investigations on the CH&lt;sub&gt;4&lt;/sub&gt; emission dynamics and environmental conditions governing CH&lt;sub&gt;4&lt;/sub&gt; production and consumption in CWs.&lt;/p&gt;&lt;p&gt;This study integrates results from 4-years (2014-2017) investigations in an off-stream CW in Rampillon, France (0.53 ha, depth 0.3-0.8 m, est. 2010) and from 4-years (2018-2021) studies in an in-stream CW in V&amp;#228;nda, Estonia (0.45 ha, depth 0.1-0.6 m, est. 2015). In Rampillon, during four 2-weeks measurement campaigns throughout all seasons CH&lt;sub&gt;4&lt;/sub&gt; fluxes were measured using floating automated chambers connected to the QCLAS laser system. In addition, gas was sampled twice day from manual floating chambers for further analysis in lab. In V&amp;#228;nda in-stream CW, CH&lt;sub&gt;4&lt;/sub&gt; fluxes were measured twice a month using manual chambers and gas-chromatographs.&lt;/p&gt;&lt;p&gt;The average annual CH&lt;sub&gt;4&lt;/sub&gt; emission in Rampillon for 2014-2015 was 7.7 g CH&lt;sub&gt;4&lt;/sub&gt;-C m&lt;sup&gt;-2&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;, showing highest values from deeper (0.5-1.0 m) parts in summer and autumn. The highest values reached up to 180 mg CH&lt;sub&gt;4&lt;/sub&gt;-C m&lt;sup&gt;-2&lt;/sup&gt; h&lt;sup&gt;-1&lt;/sup&gt;, mainly due to ebullition. The emissions in winter and spring were up to 10 times lower, however no negative values were observed. There was an increasing trend in CH&lt;sub&gt;4&lt;/sub&gt; fluxes: in 2017 the average emission reached to 12.0 g CH&lt;sub&gt;4&lt;/sub&gt;-C m&lt;sup&gt;-2&lt;/sup&gt; y&lt;sup&gt;-1&lt;/sup&gt;. Differences between the emission values gathered from authomated chambers were about 10% higher than those measured from manual chambers.&lt;/p&gt;&lt;p&gt;In V&amp;#228;nda in-stream CW, a clear increase in average annual emissions was found: from 0.4 in&lt;sup&gt;&lt;/sup&gt;2018 to 10.5 g CH&lt;sub&gt;4&lt;/sub&gt;-C m&lt;sup&gt;-2&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt; in 2021. It was correlated with increasing &lt;em&gt;Typha latifolia&lt;/em&gt;-dominated vegetation cover. Emissions showed strong correlation with air and water temperature while no clear relationship was found with the depth of various parts.&lt;/p&gt;&lt;p&gt;Large CH&lt;sub&gt;4&lt;/sub&gt; emission from CWs is a major concern and therefore a smart management is needed. Our previous studies in surface flow CWs treating nitrate-contaminated runoff demonstrate that above-ground biomass harvesting of plants can decrease the CH&lt;sub&gt;4&lt;/sub&gt;. The end of growing season is likely the best time for biomass harvesting while avoiding the excessively high CH&lt;sub&gt;4&lt;/sub&gt; emissions that the summer harvest may produce.&lt;/p&gt;