Cryptogamic stem covers may contribute to nitrous oxide consumption by mature beech trees

Area of Science:

• Environmental Science
• Forest Ecology
• Biogeochemistry

Background:

• Nitrous oxide (N2O) is a potent greenhouse gas produced by soil microbes.
• Forest ecosystems' capacity to exchange N2O with the atmosphere requires accurate quantification.
• Trees have been largely overlooked in N2O inventories, with current studies often presenting them as emitters.

Purpose of the Study:

• To investigate the role of mature beech tree stems (Fagus sylvatica) in atmospheric nitrous oxide (N2O) exchange.
• To determine if beech tree stems act as sinks or sources of N2O.
• To understand the contribution of epiphytic organisms on tree bark to N2O fluxes.

Main Methods:

• Field measurements of N2O exchange rates on mature beech tree stems.
• Laboratory experiments quantifying N2O exchange of lichens, mosses, and algae under controlled rehydration and temperature (25°C).

Main Results:

• Beech tree stems consistently acted as N2O sinks, with consumption rates ranging from -2.4 to -3.8 µg N2O m⁻² h⁻¹.
• Epiphytic photoautotrophic organisms (lichens, mosses, algae) on beech bark were also net N2O sinks under laboratory conditions.
• Observed N2O consumption rates by bark organisms were comparable to field measurements of stem N2O consumption.

Conclusions:

• Mature beech tree stems can function as significant sinks for atmospheric N2O, particularly when associated soils are also consuming N2O.
• Cryptogamic stem covers (lichens, mosses, algae) are likely key contributors to the N2O sink capacity of beech tree stems.
• European beech forests may represent a more substantial N2O sink than currently accounted for in inventories.