Water table depth and plant species determine the direction and magnitude of methane fluxes in floodplain meadow soils.

Methane (CH) is a powerful greenhouse gas with ongoing efforts aiming to quantify and map emissions from natural and managed ecosystems. Wetlands play a significant role in the global CH budget, but uncertainties in their total emissions remain large, due to a combined lack of CH data and fuzzy boundaries between mapped ecosystem categories. European floodplain meadows are anthropogenic ecosystems that originated due to traditional management for hay cropping. These ecosystems are seasonally inundated by river water, and straddle the boundary between grassland and wetland ecosystems; however, an understanding of their CH function is lacking. Here, we established a replicated outdoor floodplain-meadow mesocosm experiment to test how water table depth (45, 30, 15 cm below the soil surface) and plant composition affect CH fluxes over an annual cycle. Water table was a major controller on CH, with significantly higher fluxes (overall mean 9.3 mg m d) from the high (15 cm) water table treatment. Fluxes from high water table mesocosms with bare soil were low (mean 0.4 mg m d), demonstrating that vegetation drove high emissions. Larger emissions came from high water table mesocosms with aerenchymatous plant species (e.g. , mean 12.8 mg m d), suggesting a role for plant-mediated transport. However, at low (45 cm) water tables mesocosms were net CH sinks, suggesting that there is plasticity in CH exchange if aerenchyma are present. Plant cutting to simulate a hay harvest had no effect on CH, further supporting a role for plant-mediated transport. Upscaling our CH fluxes to a UK floodplain meadow using hydrological modelling showed that the meadow was a net CH source because oxic periods of uptake were outweighed by flooding-induced anoxic emissions. Our results show that floodplain meadows can be either small sources or sinks of CH over an annual cycle. Their CH exchange appears to respond to soil temperature, moisture status and community composition, all of which are likely to be modified by climate change, leading to uncertainty around the future net contribution of floodplain meadows to the CH cycle.