Nitrous oxide emissions from stems of ash (Fraxinus angustifolia Vahl) and European beech (Fagus sylvatica L.)

Aims: We investigated N₂O emissions from stems of Fraxinus angustifolia and Fagus sylvatica, hypothesizing that trees emit N₂O through the stem via diffusion out of the transpiration stream. Methods: We used static chambers fixed at different heights of the stem to estimate N₂O stem effluxes. Chambers were also used for monitoring soil N₂O emissions. To stimulate soil N₂O production and stem N₂O emissions we fertilized the soil. Results: Before soil fertilization, stem N₂O emissions were at most 2 μg N₂O-N m⁻² bark h⁻¹. After fertilization, stem and soil emissions were linearly correlated; stem emissions decreased linearly with increasing height. Stems of Fagus sylvatica emitted up to 80 μg N₂O-N m⁻² bark h⁻¹ at 20 cm above soil level; at 200 cm, stem N₂O emissions were below detection limit. Fraxinus angustifolia stem N₂O emissions reached 35 μg N₂O-N m⁻² bark h⁻¹ after soil fertilization. Conclusions: Stem N₂O emissions in upland trees occur even without aerenchyma, associated with xylem water transport. However, stem N₂O emissions represented only 1–3 % of total (soil + stem) N₂O emissions at the forest level. If this holds for other forest ecosystems, stem N₂O emissions would be a minor pathway of N₂O loss from terrestrial ecosystems into the atmosphere.