Nitrogen fixation by Alnus species boosts soil nitrous oxide emissions

The aim of this study was to assess how higher soil temperatures and increased nitrogen (N) availability, mediated by the N-2 fixation of different Alnus species, affect soil nitrous oxide (N2O) emissions. We measured rates of N2O emission in situ under non-N-2-fixing tree species or grassland (controls) and under Alnus species by the closed chamber technique along a temperature gradient of 5.7K from the lowlands to the upper montane belt in Switzerland. Similar soil conditions and representative tree and bush taxa at a given elevation were a key prerequisite for the site selection. Across the 12 test plots, and irrespective of the elevation, N2O emissions were on average 12-fold larger in Alnus stands than in non-N-2-fixing vegetation. The mean emission was 1.30.2kgN(2)O-Nha(-1) under Alnus and 0.10.1kgN(2)O-Nha(-1) under non-N-2-fixing vegetation from mid-June to mid-October. The largest rates of emission were observed in Alnus viridis (Chaix.) D.C. shrubland at 1680m a.s.l. (mid-elevation), with a seasonal emission of 4.20.5kgN(2)O-Nha(-1). Differences in soil temperature along the elevation gradient did not affect the release of N2O in non-N-2-fixing vegetation or across the different Alnus species. We conclude that larger N availability through N-2 fixation increases N2O emissions in ecosystems even without intense agricultural management. The fast expansion of Alnus viridis across the Alps has been documented mainly in the context of ecological consequences'. Here, we provide evidence that Alnus viridis also adds substantially to greenhouse gas emissions.HighlightsWe evaluated soil N2O emissions under Alnus and non-fixing species along a temperature gradient of 5.7K.Alnus stands emitted on average 12-fold more N2O than non-N-2-fixing vegetation.Neither short- nor long-term variation in temperature affected N2O emissions in Alnus stands.During the growing season, soil under Alnus viridis emitted 1.5% of the annual Swiss N2O emission.