Greenhouse Gas Dynamics in a Drained Peatland Forest: Annual CH4 and N2O Fluxes from Tree Stems and Soil

Abstract

<p>Peatland soils are considered the dominating source of methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) to the atmosphere. However, there are high spatio-temporal uncertainties regarding the soil greenhouse gas (GHG) fluxes due to complex dynamics between the soil chemical, physical and biological variables. Although GHG fluxes from peatland soils are relatively well studied, tree stem fluxes have received far less attention and are often overlooked in GHG models and assessments. Moreover, simultaneous year-long measurements of soil and tree stem CH<sub>4</sub> and N<sub>2</sub>O fluxes in peatland forests are missing, as previous studies have primarily focused on the growing season. We aim to determine the seasonal dynamics of CH<sub>4</sub> and N<sub>2</sub>O fluxes in drained peatland forests, as drainage can lead to release of the large amounts of carbon and nitrogen stored in peat into the atmosphere as GHGs.</p><p>Our research focuses on tree stems and soil GHG fluxes in the Agali Drained Peatland Forest Research Station in Estonia, dominated by Downy Birch (<em>Betula pubescens</em>) and Norway Spruce (<em>Picea abies</em>) trees. During the weekly sampling campaigns (November 2020–December 2021), we used manual static stem chambers to collect gas samples, which were later analysed for CH<sub>4</sub> and N<sub>2</sub>O in the laboratory using Shimadzu GC-2014 gas chromatography. We measured soil CH<sub>4</sub> and N<sub>2</sub>O fluxes using an automated dynamic soil chamber system connected to a Picarro G2508 analyser.</p><p>Preliminary results show that on average, birch stem GHG fluxes were greater than spruce stem fluxes. Birch trees were a net annual source of both CH<sub>4 </sub>(0.38 ± 0.09 µg C m<sup>-2</sup> stem area h<sup>-1</sup>, mean ± SE) and N<sub>2</sub>O (0.94 ± 0.32 µg N m<sup>-2</sup> h<sup>-1</sup>). Spruce trees were a net source of CH<sub>4</sub> (0.08 ± 0.05 µg C m<sup>-2</sup> h<sup>-1</sup>) but a net sink of N<sub>2</sub>O (–0.08 ± 0.02 µg N m<sup>-2</sup> h<sup>-1</sup>). Temporal dynamics of birch stem CH<sub>4</sub> emissions were characterised by significant emission peaks in November and June. During the rest of the year smaller fluxes with fluctuations between emissions and uptake were observed. Spruce stem CH<sub>4</sub> fluxes followed a roughly similar pattern as birch fluxes. However, during the birch emission peak in June, spruce stems showed uptake of CH<sub>4</sub>. Birch stem N<sub>2</sub>O emissions remained very small for most of the year, with increased emissions in autumn months and March. Spruce stem N<sub>2</sub>O fluxes remained very low throughout the year.</p><p>Soils were a net annual sink of CH<sub>4</sub> (–6.44 ± 0.76 µg C m<sup>-2</sup> ground area h<sup>-1</sup>) and source of N<sub>2</sub>O (41.68 ± 3.15 µg N m<sup>-2</sup> h<sup>-1</sup>). CH<sub>4</sub> was taken up by the soil most of the year, however occasional emissions occurred. A substantial increase in CH<sub>4</sub> uptake was observed in June, peaking at –49.53 µg C m<sup>-2</sup> h<sup>-1</sup> at the end of July, and diminishing towards the end of summer. Hot moments – notably higher daily average emissions compared to the period average – characterised the temporal dynamics of soil N<sub>2</sub>O emissions.</p><p>Further results on soil meteorological and biogeochemical properties will help determine the possible drivers of stem and soil fluxes’ dynamics and their origin.</p>