The annual budget of CH4 and N2O fluxes from a grey alder forest: from the soil to tree level

The estimations of greenhouse gas fluxes in various forest ecosystems under different environmental conditions are essential to understand the effect and role of forests in the Earth's climate. The stands of grey alder (Alnus incana (L.) Moench), which is a fast growing deciduous tree species with high potential for short-rotation forestry in the Northern Europe, are considered as carbon (C) accumulating ecosystems. In addition, due to atmospheric nitrogen (N) fixation by symbiotic bacteria, alder forests have often intensive N cycling.

To estimate CH₄ and N₂O fluxes from hemiboreal alder forest, we conducted a study in a 40-yrs old grey alder stand, which is grown on a former agricultural land in Estonia. From July 2017 to August 2018 we measured CH₄ and N₂O fluxes above the forest canopy using eddy covariance technique: QCL N₂O and CH₄ analyzer and Metek Class A anemometer. As well employing 12 microsites located within the footprint of the eddy covariance tower, we measured CH₄ and N₂O fluxes from soil (dynamic automatic soil chambers with Picarro 2508 analyzer) and tree stems (static tree stem chambers, gas samples taken at 0.1, 0.8 and 1.8 m from the ground analysed by Shimadzu GC). Simultaneously with flux measurements, automatic water level data loggers, soil temperature and moisture sensors (0-10 cm) were established next to each microsite. During seven campaigns, composite soil samples from 0-10 cm were taken for physical and chemical analysis.

Measurements showed that the forest as a whole was a net source of CH₄ and N₂O: annual average ± st.error values were 3.62 ± 2.71 and 8.22 ± 5.52 kg m^-2 yr^-1, respectively. Comparatively, tree stem fluxes showed a similar seasonal trend being the highest in spring and autumn when the soil groundwater level was the highest. Soil flux of N₂O was relatively low in summer and the highest in springtime and winter (most probably due to the freezing-thawing effect), whereas in summer CH₄ was mainly consumed in topsoil. Our results show that stem fluxes of CH₄ and N₂O, which have been quite often left aside, significantly contribute to forest C and N cycles and should always be included in relevant models.