Abstract
Naturally produced by microbial processes in soil, nitrous oxide (N(2)O) is an important greenhouse gas contributing to climate change. Accordingly, there is a need to accurately quantify the capability of forest ecosystems to exchange N(2)O with the atmosphere. While N(2)O emissions from soils have been well studied, trees have so far been overlooked in N(2)O inventories. Here, we show that stems of mature beech trees (Fagus sylvatica) may act as a substantial sink of N(2)O from the atmosphere under conditions of soils consuming N(2)O. Consistent consumption of N(2)O by all stems investigated (ranging between -2.4 and -3.8 µg m(-2) h(-1)) is a novel finding in contrast to current studies presenting trees as N(2)O emitters. To understand these fluxes, N(2)O exchange of photoautotrophic organisms associated with beech bark (lichens, mosses and algae) was quantified under laboratory conditions. All these organisms were net N(2)O sinks at full rehydration and temperature of 25 °C. The consumption rates were comparable to stem consumption rates measured under field conditions. Cryptogamic stem covers could be a relevant sink of N(2)O in European beech forests.