Hot spots and hot moments of methane and nitrous oxide fluxes in forests: from soil to ecosystem

Forests cover about 4 billion ha globally. They are important regulators of carbon dioxide (CO₂) fluxes, whereas the comprehensive understanding of their overall greenhouse gas (GHG) budgets, especially for methane (CH₄) and nitrous oxide (N₂O), are still largely unknown.

Wetland forest soils are commonly recognized as emitters of CH₄, whereas upland forest soils tend to consume CH₄. However, several studies demonstrate that trees can emit a large amount of CH₄ especially from tree stems and substantial amounts also from canopies through poorly studied and partly unidentified aerobic processes. Moreover, tree stems can have substantial concentrations of CH₄ inside, which can originate from soil or be produced by methanogens within the wood, while canopy CH₄ emissions are mostly abiotic and driven by light and temperature. Thus, forest vegetation can be a significant CH₄ source.

Various soil microbiological, chemical and physical properties influence N₂O fluxes in forests. In general, N₂O emissions from tropical wetland forest soils are significantly higher than those from tropical upland forests, temperate and boreal forests. High nitrogen (N) availability, coupled with high moisture content, makes tropical peatland soils especially likely to emit N₂O. Similarly, forests on drained N-rich peatland soils in temperate and boreal areas can be significant N₂O sources. In temperate zone, a considerable part of such emissions appears in winter.

Understanding spatial and temporal dynamics of GHG emissions is crucial for adequate modelling and mitigation of emissions in forests. In comparison with CO₂ fluxes, which are clearly temperature dependent, temporal and spatial variation of soil, tree stem and canopy CH₄ and N₂O emissions is more complex and poorly studied. Soil N₂O emissions in wetland and upland forests are mainly determined by soil moisture (soil oxygen concentration), and N₂O shows bell-shaped (unimodal) dependence on soil water content. In the wet periods, stem flux of CH₄ can be the main source for ecosystem exchange, whereas in the dry periods, emission from canopy adds to the total fluxes from soil and stems. N₂O fluxes from the soil and stems are normally low during the dry periods and peak during the wet periods and the freeze-thaw cycles.

Only a few examples are available on ecosystem-level CH₄ and N₂O budgets (fluxes from the soil, tree stems and shoots + eddy covariance (EC) measurements above the canopy). Nevertheless, estimation of the GHG balance in different forest ecosystems under various environmental conditions is essential for understanding their impact on the Earth’s climate.

In this presentation, we will bring results from ecosystem-level CH₄ and N₂O flux studies in forests growing on both organic and mineral soils in temperate and tropical zones.