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Zusammenfassung:
Reactive nitrogen species have a strong influence on atmospheric
chemistry and climate, tightly coupling the Earth's nitrogen cycle with
microbial activity in the biosphere. Their sources, however, are not
well constrained, especially in dryland regions accounting for a major
fraction of the global land surface. Here, we show that biological soil
crusts (biocrusts) are emitters of nitric oxide (NO) and nitrous acid
(HONO). Largest fluxes are obtained by dark cyanobacteria-dominated
biocrusts, being similar to 20 times higher than those of neighboring
uncrusted soils. Based on laboratory, field, and satellite measurement
data, we obtain a best estimate of similar to 1.7 Tg per year for the
global emission of reactive nitrogen from biocrusts (1.1 Tg a(-1) of
NO-N and 0.6 Tg a(-1) of HONO-N), corresponding to similar to 20% of
global nitrogen oxide emissions from soils under natural vegetation. On
continental scales, emissions are highest in Africa and South America
and lowest in Europe. Our results suggest that dryland emissions of
reactive nitrogen are largely driven by biocrusts rather than the
underlying soil. They help to explain enigmatic discrepancies between
measurement and modeling approaches of global reactive nitrogen
emissions. As the emissions of biocrusts strongly depend on
precipitation events, climate change affecting the distribution and
frequency of precipitation may have a strong impact on terrestrial
emissions of reactive nitrogen and related climate feedback effects.
Because biocrusts also account for a large fraction of global
terrestrial biological nitrogen fixation, their impacts should be
further quantified and included in regional and global models of air
chemistry, biogeochemistry, and climate.
