title: Biological soil crusts accelerate the nitrogen cycle through large NO and HONO emissions in drylands.
abstract: 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 ∼20 times higher than those of neighboring uncrusted soils. Based on laboratory, field, and satellite measurement data, we obtain a best estimate of ∼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 ∼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.
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany; b.weber@mpic.de.
authors.forename: Bettina
authors.identifier: http://orcid.org/0000-0002-5453-3967
authors.initials: B
authors.lastname: Weber
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany; Biogeochemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany; Key Laboratory of Agricultural Water Research, Center for Agricultural Resources Research, Institute of Genetic and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang 050021, China;
authors.forename: Dianming
authors.initials: D
authors.lastname: Wu
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Alexandra
authors.initials: A
authors.lastname: Tamm
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Nina
authors.initials: N
authors.lastname: Ruckteschler
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Emilio
authors.initials: E
authors.lastname: Rodríguez-Caballero
authors.affiliation: Senckenberg Biodiversity and Climate Research Centre, 60325 Frankfurt am Main, Germany;
authors.forename: Jörg
authors.identifier: http://orcid.org/0000-0002-7861-8789
authors.initials: J
authors.lastname: Steinkamp
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Hannah
authors.initials: H
authors.lastname: Meusel
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany; Biogeochemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Elbert
authors.affiliation: Biogeochemical Processes Department, Max Planck Institute for Biogeochemistry, 07745 Jena, Germany;
authors.forename: Thomas
authors.initials: T
authors.lastname: Behrendt
authors.affiliation: Biogeochemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Matthias
authors.initials: M
authors.lastname: Sörgel
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Yafang
authors.initials: Y
authors.lastname: Cheng
authors.affiliation: Air Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Crutzen
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Hang
authors.initials: H
authors.lastname: Su
authors.affiliation: Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
authors.forename: Ulrich
authors.initials: U
authors.lastname: Pöschl
mesh_terms: D000458:Cyanobacteria / Q000378:metabolism; D017753:Ecosystem; D009569:Nitric Oxide / Q000032:analysis*; D058458:Nitrogen Cycle; D009608:Nitrous Acid / Q000032:analysis*; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis*
chemical_list: D012987:Soil; D012989:Soil Pollutants; D009569:Nitric Oxide; D009608:Nitrous Acid
keywords: biological soil crusts; nitric oxide; nitrogen; nitrous acid; trace gas emission