Nighttime doesn’t stop N2O and CH4 exchange from riparian forest tree stems with the atmosphere

Riparian forest ecosystems have been considered to be a natural source of nitrous oxide (N₂O) and a natural sink of methane (CH₄), both of which are important greenhouse gases (GHG) originating from microbiological processes. Wetland trees may also contribute to the GHG exchange by the release of both gases to the atmosphere or uptake therefrom. Recent studies have investigated the role of tree stems, underlining their importance in understanding forest GHG dynamics, focussing on various tree species, soil conditions or seasonal dynamics. However, knowledge about the short-termed day and night-time distributed GHG exchange of tree stems with the atmosphere is still scarce. We studied stem fluxes in a riparian forest ecosystem aiming to investigate the diurnal pattern and predict the potential influence of solar radiation.

The diurnal flux measurements were performed at 40-year-old grey alder (Alnus incana) forest stand in Estonia with 12-hour interval during July-September 2017 and May-September 2018 (n=16). The exchange of N₂O and CH₄ was measured from 12 trees at profile height up to 5 m (0.1, 0.8, 1.7, 2.5, 5.0 m) using non-steady state stem chamber systems and gas chromatography. Simultaneously, soil fluxes were automatically quantified using a dynamic chamber system (Picarro 2508); piezometers, automatic groundwater level wells, soil temperature and moisture sensors were installed to determine coherent soil conditions.

Our preliminary results showed N₂O and CH₄ emissions from alder tree stems during daytime (4.91 ± 0.15 µg m^-2 h^-1 and 66.38 ± 16.02 µg m^-2 h^-1, mean ± s.e.) and lower during nighttime (3.65 ± 0.22 µg m^-2 h^-1 and 51.49 ± 13.83 µg m^-2 h^-1, mean ± s.e.) at 0.1 m stem height, revealing a likely link to solar-driven physiological tree activity. Further, with increasing stem height, the relation of night to daytime fluxes diminished. However, the day-wise variation, including a minor GHG uptake indicates a fast response to changing micro-spatial environmental conditions like water regime in the soil and temperature.

Our study demonstrates the GHG exchange between tree stems and atmosphere occurs both in day- and night-time, showing slightly higher values in day-time, probably due to the trees’ physiological activities. Furthermore, our findings provide the potential to predict reaction kinetics in future modelling of flux pathways in forest ecosystems.

Acknowledgement

This research was supported by the Ministry of Education and Science of Estonia (SF0180127s08 grant), the Estonian Research Council (IUT2-16, PRG-352, and MOBERC20), the Czech Science Foundation (17-18112Y), the Czech National Sustainability Program I (LO1415), and the EcolChange Centre of Excellence, Estonia.