Leaves of mature European beech consume nitrous oxide (N2O) from the atmosphere

Besides soils, tree stems are known to emit nitrous oxide (N₂O) into the atmosphere. However, it seems, stems of some tree species might also take up this important greenhouse gas from the atmosphere under certain conditions. Even though tree leaves dominate the tree surface area, they are entirely excluded from field N₂O flux measurements, and their role in forest N₂O exchange is still unknown.

We aimed to investigate the contribution of leaf fluxes to the forest N₂O exchange. We determined N₂O exchange of stems and leaves of mature European beech (Fagus sylvatica), and adjacent soil in a typical temperate upland mixed forest in Southern Germany, using non-steady-state chamber methods and a system of scaffold towers reaching the top of tree crowns in 35 m. The measurements were accompanied by a parallel determination of stem, leaf and soil CO₂ exchange and numerous environmental characteristics (soil N₂O and CO₂ concentrations and water content in vertical soil profiles, soil and air temperature).

We found out that the beech stems and especially the leaves were net sinks of N₂O from the atmosphere (–1.07 ± 3.47 and –249.9 ± 84.3 mg N₂O ha⁻¹ ground area h⁻¹, respectively), whereas the soil was a net N₂O source into the atmosphere (24.0 ± 10.8 mg N₂O ha⁻¹ h⁻¹). The never studied tree leaves were identified as a key player in ecosystem N₂O exchange, taking up in fact 10 times more N₂O than the soil emits at the same time. Therefore, native Central European and widely spread European beech trees seem to contribute to forest N₂O uptake markedly.

For the first time, we revealed tree leaves being substantial N₂O sinks. Our results clearly show that the current and ongoing exclusion of tree leaves from forest N₂O flux measurements can lead to a severe underestimation of the overall tree and forest N₂O exchange and, therefore, global forest greenhouse gas flux inventories.

 

Acknowledgement

This research was supported by the Czech Science Foundation (17-18112Y) and project SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797). We thank Jan Hrdlička and Thomas Feuerbach for their technical support.