Lakes as nitrous oxide sources in the boreal landscape

Estimates of regional and global freshwater N₂ O emissions have remained inaccurate due to scarce data and complexity of the multiple processes driving N₂ O fluxes the focus predominantly being on summer time measurements from emission hot spots, agricultural streams. Here, we present four-season data of N₂ O concentrations in the water columns of randomly selected boreal lakes covering a large variation in latitude, lake type, area, depth, water chemistry, and land use cover. Nitrate was the key driver for N₂ O dynamics, explaining as much as 78% of the variation of the seasonal mean N₂ O concentrations across all lakes. Nitrate concentrations varied among seasons being highest in winter and lowest in summer. Of the surface water samples, 71% were oversaturated with N₂ O relative to the atmosphere. Largest oversaturation was measured in winter and lowest in summer stressing the importance to include full year N₂ O measurements in annual emission estimates. Including winter data resulted in fourfold annual N₂ O emission estimates compared to summer only measurements. Nutrient-rich calcareous and large humic lakes had the highest annual N₂ O emissions. Our emission estimates for Finnish and boreal lakes are 0.6 and 29 Gg N₂ O-N/year, respectively. The global warming potential of N₂ O from lakes cannot be neglected in the boreal landscape, being 35% of that of diffusive CH₄ emission in Finnish lakes.