Effects of vegetation and nutrients on methanotroph abundance and methane emissions from constructed wetlands

Abstract: Agriculture is the largest anthropogenic source of nitrogen and phosphorous (P) leakage to Swedish coastal seas, causing eutrophication which threatens marine ecosystems. Constructing wetlands is one way to improve nutrient retention in the modern agricultural landscape. However, constructed wetlands can emit substantial amounts of methane (CH4). Methanotrophs facilitate aerobic oxidation of CH4, which transforms the CH4 into less potent carbon dioxide. However, the factors controlling the methanotrophs and their effect on CH4 emissions are poorly understood. The aim of this thesis was to investigate the effect of vegetation and nutrient status on the CH4 emissions and abundance of methanotrophs from constructed wetlands in agricultural areas in Sweden. Water samples for DNA extraction and quantification by qPCR and gas samples for concentrations of dissolved CH4 was taken at 34 wetlands in southwest and middle Sweden. At 5 of the wetlands CH4 fluxes were measured using floating chamber technique. At each wetland samples were taken at one point with and one without vegetation. In addition, water chemistry and nutrient status were measured. The results show that vegetation had no effect on the abundance of methanotrophs or on the dissolved CH4 and CH4 fluxes. This contradicts previous studies and may be explained by even conditions throughout the wetland and the off-vegetative season. Of the nutrients and hydrochemical factors, P and the increased fraction of P in relation to carbon and nitrogen was significantly correlated to the abundance of methanotrophs, which is in line with previous studies. None of the studied variables were correlated to the dissolved CH4. The total fluxes corresponded to fluxes reported in previous studies and indicated that ebullitive fluxes can be large even during winter season. Except the relationship between increased abundance of methanotrophs and increased P concentrations, the lack of significant results in this study may indicate that the abundance of methanotrophs, dissolved CH4 and CH4 fluxes depend on multiple direct and indirect variables that interactively control CH4 emissions and the bacterial community involved in CH4 oxidation.