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

Forests are important regulators of carbon dioxide fluxes, whereas overall greenhouse gas (GHG) budgets, in particular, nitrous oxide (N₂O), are still largely unknown. No studies on ecosystem-level N₂O budgets (soil and tree stem fluxes with eddy covariance (EC) measurements above the canopy) are found. Only a few examples are available on N₂O emissions from tree stems. Nevertheless, estimation of the N₂O and the full GHG balance in different forest ecosystems under various environmental conditions is essential to understand their impact on climate.

During the period of August 2017 to December 2019, we measured the N₂O 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 (N₂) fixation ability makes alders important for the regulation of nitrogen (N) cycle in forested areas.

We measured the N₂O 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 N₂ flux in intact soil cores was measured in the lab using the He-O incubation method.

Average N₂O fluxes from the soil and tree stems varied from 1.2 to 3.0 and 0.01 to 0.03 kg N₂O-N ha^–1 yr^–1, respectively, being the highest during the wet periods, peaking during the freezing-thawing, and being the lowest in dry periods. The average annual potential N₂ flux in the soil was 140 kg N₂ ha^–1 yr^–1 which made the average N₂:N₂O-N ratio in the soil about 60. According to the EC measurements, the forest was a net annual source of N₂O (3.4 kg N₂O ha^–1). Thus, the main gaseous nitrogen flux in this forest was N₂ emission. Our carbon (C) budget showed that the forest was a significant net annual C sink.

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 N₂O fluxes from the soil and tree stems in combination with ecosystem-level EC measurements. The identification of microorganisms and biogeochemical pathways associated with N₂O production and consumption is another future challenge.