Background and aims Leaching of nitrogen (N) from natural ecosystems poses serious environmental problems. Extreme events such as winter warm spells could exacerbate N leaching by disrupting biogeochemical cycles and will become more frequent with the projected climate change.Methods We used lysimeters to investigate N leaching in response to a 12-day winter warm spell at two field sites with contrasting winter climate and in seven different vegetation covers in 140 well established mesocosms. Results Mean rates of N leaching reached 3.4 ± 0.4 mg N m −2 d −1 over the 49 days of observations in late winter/ early spring. The winter warm spell resulted in an 82 % increase of N leaching after the warm spell (up to 18 mg N l −1 ). Stronger leaching occurred at the colder site which can be explained by plants becoming photosynthetically active with subsequent frost damaging their above-ground tissue due to missing insulation by snow and/or dehardening, resulting in reduced N uptake. N leaching differed by >600 % among contrasting vegetation composition with almost no leaching from grassland types and strongest leaching from shrubland types that even surpassed leaching from bare ground controls. Conclusions Winter warm spells can affect the biogeochemistry of temperate ecosystems with plant performance and vegetation composition controlling the amount of N leaching.

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