Leaves of mature European beech consume nitrous oxide (N2O) from the atmosphere
Authors: Machacova Katerina , Schindler Thomas , Mander Ülo , Soosaar Kaido , Grams Thorsten E. E. 
Published: March 2022

&lt;p&gt;Besides soils, tree stems are known to emit nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) 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 N&lt;sub&gt;2&lt;/sub&gt;O flux measurements, and their role in forest N&lt;sub&gt;2&lt;/sub&gt;O exchange is still unknown.&lt;/p&gt;&lt;p&gt;We aimed to investigate the contribution of leaf fluxes to the forest N&lt;sub&gt;2&lt;/sub&gt;O exchange. We determined N&lt;sub&gt;2&lt;/sub&gt;O exchange of stems and leaves of mature European beech (&lt;em&gt;Fagus sylvatica&lt;/em&gt;), 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 CO&lt;sub&gt;2&lt;/sub&gt; exchange and numerous environmental characteristics (soil N&lt;sub&gt;2&lt;/sub&gt;O and CO&lt;sub&gt;2&lt;/sub&gt; concentrations and water content in vertical soil profiles, soil and air temperature).&lt;/p&gt;&lt;p&gt;We found out that the beech stems and especially the leaves were net sinks of N&lt;sub&gt;2&lt;/sub&gt;O from the atmosphere (&amp;#8211;1.07 &amp;#177; 3.47 and &amp;#8211;249.9 &amp;#177; 84.3 mg N&lt;sub&gt;2&lt;/sub&gt;O ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; ground area h&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;, respectively), whereas the soil was a net N&lt;sub&gt;2&lt;/sub&gt;O source into the atmosphere (24.0 &amp;#177; 10.8 mg N&lt;sub&gt;2&lt;/sub&gt;O ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; h&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;). The never studied tree leaves were identified as a key player in ecosystem N&lt;sub&gt;2&lt;/sub&gt;O exchange, taking up in fact 10 times more N&lt;sub&gt;2&lt;/sub&gt;O than the soil emits at the same time. Therefore, native Central European and widely spread European beech trees seem to contribute to forest N&lt;sub&gt;2&lt;/sub&gt;O uptake markedly.&lt;/p&gt;&lt;p&gt;For the first time, we revealed tree leaves being substantial N&lt;sub&gt;2&lt;/sub&gt;O sinks. Our results clearly show that the current and ongoing exclusion of tree leaves from forest N&lt;sub&gt;2&lt;/sub&gt;O flux measurements can lead to a severe underestimation of the overall tree and forest N&lt;sub&gt;2&lt;/sub&gt;O exchange and, therefore, global forest greenhouse gas flux inventories.&lt;/p&gt;&lt;p&gt;&lt;em&gt;&amp;#160;&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&lt;em&gt;Acknowledgement&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&lt;em&gt;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 Hrdli&amp;#269;ka and Thomas Feuerbach for their technical support.&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;