Tree stems are a net source of CH4 and N2O in a hemiboreal drained peatland forest during the winter period

Abstract

Nutrient-rich northern peatlands are often drained to enhance forest productivity, turning peatland soils into sinks of methane (CH _4 ) and sources of nitrous oxide (N _2 O). However, further attention is needed on CH _4 and N _2 O dynamics during the winter period to fully understand the spatio-temporal variability of fluxes. Besides soil, tree stems can also emit CH _4 and N _2 O. However, stem contribution is not considered in most biogeochemical models. We determined the temporal dynamics of winter-time CH _4 and N _2 O fluxes in a drained peatland forest by simultaneously measuring stem and soil fluxes and exploring the relationships between gas fluxes and soil environmental parameters. During sampling (October 2020–May 2021), gas samples from Downy Birch ( Betula pubescens ) and Norway Spruce ( Picea abies ) trees were collected from different tree heights using manual static chambers and analysed using gas chromatography. Soil CH _4 and N _2 O concentrations were measured using an automated dynamic soil chamber system. Tree stems were a net source of CH _4 and N _2 O during the winter period. The origin of stem CH _4 emissions was unclear, as stem and soil CH _4 fluxes had opposite flux directions, and the irregular vertical stem flux profile did not indicate a connection between stem and soil fluxes. Stem N _2 O emissions may have originated from the soil, as emissions decreased with increasing stem height and were driven by soil N _2 O emissions and environmental parameters. Soil was a net sink for CH _4 , largely determined by changes in soil temperature. Soil N _2 O dynamics were characterised by hot moments—short periods of high emissions related to changes in soil water content. Tree stem emissions offset the soil CH _4 sink by 14% and added 2% to forest floor N _2 O emissions. Therefore, CH _4 and N _2 O budgets that do not incorporate stem emissions can overestimate the sink strength or underestimate the total emissions of the ecosystem