Three-year dynamics of N2O fluxes from soil, stem and canopy in a hemiboreal forest: Impacts of floods and droughts
Authors: Mander Ülo , Schindler Thomas , Macháčová Kateřina , Krasnova Alisa , Escuer-Gatius Jordi , Maddison Martin , Pärn Jaan , Veber Gert , Krasnov Dmitrii , Soosaar Kaido 
Published: March 2020

&lt;p&gt;Forests are important regulators of carbon dioxide fluxes, whereas overall greenhouse gas (GHG) budgets, in particular, nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O), are still largely unknown. No studies on ecosystem-level N&lt;sub&gt;2&lt;/sub&gt;O budgets (soil and tree stem fluxes with eddy covariance (EC) measurements above the canopy) are found. Only a few examples are available on N&lt;sub&gt;2&lt;/sub&gt;O emissions from tree stems. Nevertheless, estimation of the N&lt;sub&gt;2&lt;/sub&gt;O and the full GHG balance in different forest ecosystems under various environmental conditions is essential to understand their impact on climate.&lt;/p&gt;&lt;p&gt;During the period of August 2017 to December 2019, we measured the N&lt;sub&gt;2&lt;/sub&gt;O budget of a 40-yr old hemiboreal grey alder (Alnus incana) forest stand on former agricultural land in Estonia considering fluxes from the soil, tree stems and whole ecosystem. Grey alder (Alnus incana) is a fast-growing tree species typically found in riparian zones, with great potential for short-rotation forestry. Their symbiotic dinitrogen (N&lt;sub&gt;2&lt;/sub&gt;) fixation ability makes alders important for the regulation of nitrogen (N) cycle in forested areas.&lt;/p&gt;&lt;p&gt;We measured the N&lt;sub&gt;2&lt;/sub&gt;O budget considering fluxes from the soil surface (12 automated chambers; Picarro 2508), tree stems (60 manual sampling campaigns from 12 model trees with chambers at 0.1, 0.8 and 1.7 m; gas chromatographic analysis in lab) and whole ecosystem (EC technique: Aerodyne TILDAS). Simultaneously, soil water level, temperature and moisture were measured automatically, and composite soil samples were taken for physico-chemical analysis. Potential N&lt;sub&gt;2&lt;/sub&gt; flux in intact soil cores was measured in the lab using the He-O incubation method.&lt;/p&gt;&lt;p&gt;Average N&lt;sub&gt;2&lt;/sub&gt;O fluxes from the soil and tree stems varied from 1.2 to 3.0 and 0.01 to 0.03 kg N&lt;sub&gt;2&lt;/sub&gt;O-N ha&lt;sup&gt;&amp;#8211;1&lt;/sup&gt; yr&lt;sup&gt;&amp;#8211;1&lt;/sup&gt;, respectively, being the highest during the wet periods, peaking during the freezing-thawing, and being the lowest in dry periods. The average annual potential N&lt;sub&gt;2&lt;/sub&gt; flux in the soil was 140 kg N&lt;sub&gt;2&lt;/sub&gt; ha&lt;sup&gt;&amp;#8211;1&lt;/sup&gt; yr&lt;sup&gt;&amp;#8211;1&lt;/sup&gt; which made the average N&lt;sub&gt;2&lt;/sub&gt;:N&lt;sub&gt;2&lt;/sub&gt;O-N ratio in the soil about 60. According to the EC measurements, the forest was a net annual source of N&lt;sub&gt;2&lt;/sub&gt;O (3.4 kg N&lt;sub&gt;2&lt;/sub&gt;O ha&lt;sup&gt;&amp;#8211;1&lt;/sup&gt;). Thus, the main gaseous nitrogen flux in this forest was N&lt;sub&gt;2&lt;/sub&gt; emission. Our carbon (C) budget showed that the forest was a significant net annual C sink.&lt;/p&gt;&lt;p&gt;Results of our long-term study underline the high N and C buffering capacity of riparian alder forests. For better understanding of C and nutrient budgets of riparian forests, we need long-term, high-frequency measurements of N&lt;sub&gt;2&lt;/sub&gt;O fluxes from the soil and tree stems in combination with ecosystem-level EC measurements. The identification of microorganisms and biogeochemical pathways associated with N&lt;sub&gt;2&lt;/sub&gt;O production and consumption is another future challenge.&lt;/p&gt;