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Cryptogamic covers, which comprise some of the oldest forms of terrestrial life on Earth (Lenton & Huntingford, ), have recently been found to fix large amounts of nitrogen and carbon dioxide from the atmosphere (Elbert etal., ). Here we show that they are also greenhouse gas sources with large nitrous oxide (N2O) and small methane (CH4) emissions. Whilst N2O emission rates varied with temperature, humidity, and N deposition, an almost constant ratio with respect to respiratory CO2 emissions was observed for numerous lichens and bryophytes. We employed this ratio together with respiration data to calculate global and regional N2O emissions. If our laboratory measurements are typical for lichens and bryophytes living on ground and plant surfaces and scaled on a global basis, we estimate a N2O source strength of 0.32-0.59 Tg year(-1) for the global N2O emissions from cryptogamic covers. Thus, our emission estimate might account for 4-9% of the global N2O budget from natural terrestrial sources. In a wide range of arid and forested regions, cryptogamic covers appear to be the dominant source of N2O. We suggest that greenhouse gas emissions associated with this source might increase in the course of global change due to higher temperatures and enhanced nitrogen deposition.