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 ( N 2 O ) emissions. We measured rates of N 2 O 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 5.7  K from the lowlands to the upper montane belt in S witzerland. 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, N 2 O emissions were on average 12‐fold larger in Alnus stands than in non‐ N 2 ‐fixing vegetation. The mean emission was 1.3 ± 0.2 kg N 2 O‐N ha −1 under Alnus and 0.1 ± 0.1 kg N 2 O‐N ha −1 under non‐ N 2 ‐fixing vegetation from mid‐ J une to mid‐ O ctober. The largest rates of emission were observed in Alnus viridis ( C haix.) D . C . shrubland at 1680 m a.s.l. (mid‐elevation), with a seasonal emission of 4.2 ± 0.5 kg N 2 O‐N ha −1 . Differences in soil temperature along the elevation gradient did not affect the release of N 2 O in non‐ N 2 ‐fixing vegetation or across the different Alnus species. We conclude that larger N availability through N 2 fixation increases N 2 O emissions in ecosystems even without intense agricultural management. The fast expansion of Alnus viridis across the A lps has been documented mainly in the context of ‘ecological consequences’. Here, we provide evidence that Alnus viridis also adds substantially to greenhouse gas emissions. Highlights We evaluated soil N 2 O emissions under Alnus and non‐fixing species along a temperature gradient of 5.7 K. Alnus stands emitted on average 12‐fold more N 2 O than non‐ N 2 ‐fixing vegetation. Neither short‐ nor long‐term variation in temperature affected N 2 O emissions in Alnus stands. During the growing season, soil under Alnus viridis emitted 1.5% of the annual S wiss N 2 O emission.