Climate impact of constructed wetlands treating nitrate-rich agricultural runoff: The methane problem

Constructed wetlands (CW) treating runoff from agricultural catchments can efficiently reduce nitrate contamination, however, most wetlands are inherently net sources of methane (CH₄), 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 CH₄ emission dynamics and environmental conditions governing CH₄ production and consumption in CWs.

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 Vända, Estonia (0.45 ha, depth 0.1-0.6 m, est. 2015). In Rampillon, during four 2-weeks measurement campaigns throughout all seasons CH₄ 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 Vända in-stream CW, CH₄ fluxes were measured twice a month using manual chambers and gas-chromatographs.

The average annual CH₄ emission in Rampillon for 2014-2015 was 7.7 g CH₄-C m^-2 yr^-1, showing highest values from deeper (0.5-1.0 m) parts in summer and autumn. The highest values reached up to 180 mg CH₄-C m^-2 h^-1, 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 CH₄ fluxes: in 2017 the average emission reached to 12.0 g CH₄-C m^-2 y^-1. Differences between the emission values gathered from authomated chambers were about 10% higher than those measured from manual chambers.

In Vända in-stream CW, a clear increase in average annual emissions was found: from 0.4 in2018 to 10.5 g CH₄-C m^-2 yr^-1 in 2021. It was correlated with increasing Typha latifolia-dominated vegetation cover. Emissions showed strong correlation with air and water temperature while no clear relationship was found with the depth of various parts.

Large CH₄ 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 CH₄. The end of growing season is likely the best time for biomass harvesting while avoiding the excessively high CH₄ emissions that the summer harvest may produce.