Abstract
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 ($N_2O$) emissions. We measured rates of $N_2O$ 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 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, $N_2O$ emissions were on average 12-fold larger in Alnus stands than in non-N2-fixing vegetation. The mean emission was $1.3 \pm 0.2 kg N_2O-N ha^{−1}$ under Alnus and $0.1 \pm 0.1 kg N_2O-N ha^{−1}$ 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 1680 m a.s.l. (mid-elevation), with a seasonal emission of $4.2 \pm 0.5 kg N_2O-N ha{−1}$. Differences in soil temperature along the elevation gradient did not affect the release of $N_2O$ in non-$N_2$-fixing vegetation or across the different Alnus species. We conclude that larger N availability through $N_2$ fixation increases $N_2O$ 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.
Bühlmann, T