Greenhouse gas dynamics in drained peatlands: 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) from peatlands 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. It is necessary to study relationships between stem and soil fluxes, and their chemical, physical and biological drivers to understand the fluxes' origin.

Our ongoing project focuses on measuring GHGs from tree stems and soil in the Agali Birch Forest Research Station in Estonia, representing a drained peatland with Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees. Twelve representative sub-sites were selected in the study area. One half consist of an adjacent set of a Downy Birch and a Norway Spruce tree with manual tree stem chambers, plus one automatic dynamic soil chamber. The remaining sub-sites are set pairs of birch trees and soil chambers. Six birch trees and all six spruce trees have stem chambers installed at 10, 80 and 170 cm above the ground to measure stem fluxes' vertical profile. Chambers on the six remaining birch trees were only installed at the lowest height. During the weekly ongoing sampling campaigns that started in October 2020, we use manual static gas extraction from rigid stem chambers to analyse hourly changes in chamber headspace concentrations of CH₄ and N₂O. The gas samples are analysed in the laboratory within two weeks of collection using gas chromatography. Automated soil chambers collect CH₄ and N₂O flux data every two hours per chamber, and a connected Picarro measuring unit analyses 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. Within a larger research framework, these GHG flux data will be joined with detailed soil biogeochemistry and microbial dynamics to further improve process-based modelling of peatland GHG emissions. We plan to continue our measurements for one full year to understand the seasonal changes in CH₄ and N₂O emissions patterns.