Cold-period Greenhouse Gas Dynamics in Drained Peatland Forests: CH4 and N2O Fluxes from Tree Stems and Soil

Peatland soils are considered the dominating source of nitrous oxide (N2O) and methane (CH4) to the atmosphere. However, there are high spatio-temporal uncertainties regarding the budgets of these greenhouse gases (GHG) due to complex dynamics between the chemical, physical and biological variables occurring in the soil. GHG fluxes from peatland soils are relatively well studied, however, tree stems have received far less attention and are often overlooked in GHG models and assessments. In addition, previous studies have reported seasonal variability of CH4 and N2O dynamics from soils and tree stems, but data during the dormant season are rare. It is necessary to study relationships between stem and soil fluxes, and their chemical, physical and biological drivers to understand the fluxes' origin and seasonality. Our project focuses on measuring cold-period GHGs from tree stems and soil in the Agali Birch Forest Research Station in Estonia a drained peatland forest with Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees. The study area comprises 12 sub-sites: 6 sets of a Downy Birch and a Norway Spruce tree with manual tree stem chambers, plus one automatic dynamic soil chamber, and 6 sets of birch trees and soil chambers. 6 birch trees and all 6 spruce trees have stem chambers installed at 10, 80 and 170 cm above the ground to measure fluxes' vertical profile. Chambers on the six remaining birch trees were only installed at the lowest height. During the weekly sampling campaigns (October 2020 May 2021), we used manual static gas extraction from rigid stem chambers to analyse hourly changes in chamber headspace concentrations of CH4 and N2O. The gas samples were analysed using gas chromatography. Automated soil chambers collected CH4 and N2O flux data every two hours per chamber, and a connected Picarro measuring unit analysed the gas samples in-situ. When extrapolated, our results can help understand stem and soil GHG emissions on an ecosystem level and acknowledge the role of tree stems for local and regional GHG budgets. GHG flux data will be joined with detailed soil biogeochemistry and microbial dynamics to further improve process-based modelling of peatland GHG emissions. We will continue measurements for one full year to understand the seasonal changes in CH4 and N2O emissions patterns.