Impact of plant species and intense nutrient loading on CH4 and N2O fluxes from small inland waters: An experimental approach

Severe eutrophication threatens freshwater systems around the world. The application of aquatic buffer zones with plants, for example around agricultural lands, can increase nutrient retention and thereby reduce nutrient loading to downstream systems. However, not much is known about greenhouse gas (GHG) emissions from these buffer zones and how they are affected by nutrient loading and the dominant plant species. Here, using a full factorial mesocosm experiment with different nutrient loadings (20-4000 mg N and 1-200 mg P m(-2) d(-1)) and plant types (e.g. submerged and free-floating species), we show that emissions of methane (CH4) and nitrous oxide (N2O) were strongly related to nutrient loading, leading to total greenhouse gas emissions up to 177.84 g CO2-eq m(-2) d(-1). Overall, total GHG emission (as the sum of CH4 ebullition and diffusive water-atmosphere emission of CH4 and N2O in CO2 equivalents) was not significantly affected by plant species. CH4 ebullition was significantly lower in experimental units with submerged plants that rooted in the sediment as compared to non-rooted plants, possibly related to rhizosphere CH4 oxidation fueled by radial oxygen loss or plant-mediated transport that limits the build-up of gaseous CH4 in the sediment. We conclude that aquatic buffer zones that experience intense nutrient loading (e.g. due to release of sewage or agricultural fertilizer) can be GHG emission hotspots and recommend careful consideration of environmental conditions (e.g. the organic carbon content), expected nutrient loadings, and alternatives, prior to their construction.

Automated summary

Severe eutrophication poses a threat to freshwater systems worldwide. The use of aquatic buffer zones can help reduce nutrient loading to downstream systems. However, the impact of these buffer zones on greenhouse gas emissions is not well understood. This study found that methane and nitrous oxide emissions were strongly correlated with nutrient loading, resulting in significant greenhouse gas emissions. The choice of plant species did not significantly affect total greenhouse gas emissions.