Three-year dynamics of N2O fluxes from soil, stem and canopy in a hemiboreal forest: Impacts of floods and droughts

Abstract

<p>Forests are important regulators of carbon dioxide fluxes, whereas overall greenhouse gas (GHG) budgets, in particular, nitrous oxide (N<sub>2</sub>O), are still largely unknown. No studies on ecosystem-level N<sub>2</sub>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<sub>2</sub>O emissions from tree stems. Nevertheless, estimation of the N<sub>2</sub>O and the full GHG balance in different forest ecosystems under various environmental conditions is essential to understand their impact on climate.</p><p>During the period of August 2017 to December 2019, we measured the N<sub>2</sub>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<sub>2</sub>) fixation ability makes alders important for the regulation of nitrogen (N) cycle in forested areas.</p><p>We measured the N<sub>2</sub>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<sub>2</sub> flux in intact soil cores was measured in the lab using the He-O incubation method.</p><p>Average N<sub>2</sub>O fluxes from the soil and tree stems varied from 1.2 to 3.0 and 0.01 to 0.03 kg N<sub>2</sub>O-N ha<sup>–1</sup> yr<sup>–1</sup>, 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<sub>2</sub> flux in the soil was 140 kg N<sub>2</sub> ha<sup>–1</sup> yr<sup>–1</sup> which made the average N<sub>2</sub>:N<sub>2</sub>O-N ratio in the soil about 60. According to the EC measurements, the forest was a net annual source of N<sub>2</sub>O (3.4 kg N<sub>2</sub>O ha<sup>–1</sup>). Thus, the main gaseous nitrogen flux in this forest was N<sub>2</sub> emission. Our carbon (C) budget showed that the forest was a significant net annual C sink.</p><p>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<sub>2</sub>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<sub>2</sub>O production and consumption is another future challenge.</p>