Macrophyte-associated methane oxidation as a key process diminishing methane emissions from agricultural drainage ditches

Methane (CH 4 ), a potent greenhouse gas, plays an important role in climate change, with natural and human-made aquatic ecosystems contributing about half of global annual emissions. Drainage ditches are known hotspots for CH 4 production and emission. These human-made shallow inland waters are heavily managed through dredging and mowing of submerged vegetation. Submerged vegetation impacts CH 4 production, oxidation, and transportation, thereby potentially strongly impacting CH 4 emissions. We investigated how three submerged macrophyte species ( Elodea nuttallii (Planch.) H. St. John, Stuckenia pectinata (L.) Börner, and Ceratophyllum demersum L.) affect CH 4 dynamics in agricultural ditches. At three moments during the growing season, we quantified CH 4 oxidation and production potentials in roots and shoots using incubations. In addition, we measured diffusive and ebullitive emissions from two vegetated ditches. The macrophyte-associated CH 4 oxidation potential ranged from 0.33 to 15.1 and 0.003 to 16.3 μmol CH 4 gWetWeight -1 h -1 for roots and shoots respectively. At the beginning of the growing season, when macrophyte biomass was highest, we estimated that up to 97% of the CH 4 that was produced in the ditch was oxidized through macrophyte-associated CH 4 oxidation, while at the end of the growing season only 6-52% was oxidized. This drop could be explained by lower per-biomass potential oxidation rates, as a result of seasonal changes in e.g. temperature and daylight, as well as a decrease in macrophyte biomass due to dredging. The importance of submerged macrophytes for the natural ‘CH 4 filter’ was further highlighted by the overall effect of dredging: While dredging reduced total CH 4 production, the removal of macrophyte biomass and the associated CH 4 oxidation resulted in higher CH 4 emissions. We argue that to foster the macrophyte-associated CH 4 mitigating effect, the timing of dredging and mowing of ditches is pivotal. Avoiding macrophyte removal until the end of the growing season prolongs the duration of macrophyte-associated CH 4 oxidation, while late-season removal limits subsequent anaerobic decomposition that would otherwise fuel CH 4 production. • Macrophyte-associated CH 4 oxidation consumed up to 95% of total ditch CH 4 production • Dredging reduced the total amount of CH 4 oxidized resulting in higher emissions • Optimization of ditch management can contribute to climate-smart water management