Matthias Sörgel

HONO Emissions from Soil Bacteria as a Major Source of Atmospheric Reactive Nitrogen

From Soil to Sky Trace gases emitted either through the activity of microbial communities or from abiotic reactions in the soil influence atmospheric chemistry. In laboratory column experiments using several soil types, Oswald et al. (p. 1233) showed that soils from arid regions and farmlands can produce substantial quantities of nitric oxide (NO) and nitrous acid (HONO). Ammonia-oxidizing bacteria are the primary source of HONO at comparable levels to NO, thus serving as an important source of reactive nitrogen to the atmosphere. HONO emissions from soil are comparable to those of NO in arid and arable regions. Abiotic release of nitrous acid (HONO) in equilibrium with soil nitrite (NO2–) was suggested as an important contributor to the missing source of atmospheric HONO and hydroxyl radicals (OH). The role of total soil-derived HONO in the biogeochemical and atmospheric nitrogen cycles, however, has remained unknown. In laboratory experiments, we found that for nonacidic soils from arid and arable areas, reactive nitrogen emitted as HONO is comparable with emissions of nitric oxide (NO). We show that ammonia-oxidizing bacteria can directly release HONO in quantities larger than expected from the acid-base and Henry’s law equilibria of the aqueous phase in soil. This component of the nitrogen cycle constitutes an additional loss term for fixed nitrogen in soils and a source for reactive nitrogen in the atmosphere.

Biological soil crusts accelerate the nitrogen cycle through large NO and HONO emissions in drylands

Significance Biological soil crusts (biocrusts), occurring on ground surfaces in drylands throughout the world, are among the oldest life forms consisting of cyanobacteria, lichens, mosses, and algae plus heterotrophic organisms in varying proportions. They prevent soil erosion and nurture ecosystems by fixing carbon and nitrogen from the atmosphere. Here, we show that the fixed nitrogen is processed within the biocrusts, and during this metabolic activity, nitrogen oxide and nitrous acid are released to the atmosphere. Both of these gases are highly relevant, as they influence the radical formation and oxidizing capacity of the lower atmosphere, also interacting with climate change. In drylands, biocrusts appear to play a key role both in nitrogen fixation and the release of atmospheric reactive nitrogen. 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.

Novel Tracer Method To Measure Isotopic Labeled Gas-Phase Nitrous Acid (HO15NO) in Biogeochemical Studies

Gaseous nitrous acid (HONO), the protonated form of nitrite, contributes up to ∼60% to the primary formation of hydroxyl radical (OH), which is a key oxidant in the degradation of most air pollutants. Field measurements and modeling studies indicate a large unknown source of HONO during daytime. Here, we developed a new tracer method based on gas-phase stripping-derivatization coupled to liquid chromatography-mass spectrometry (LC-MS) to measure the 15N relative exceedance, ψ(15N), of HONO in the gas-phase. Gaseous HONO is quantitatively collected and transferred to an azo dye, purified by solid phase extraction (SPE), and analyzed using high performance liquid chromatography coupled to mass spectrometry (HPLC-MS). In the optimal working range of ψ(15N)=0.2-0.5, the relative standard deviation of ψ(15N) is <4%. The optimum pH and solvents for extraction by SPE and potential interferences are discussed. The method was applied to measure HO15NO emissions from soil in a dynamic chamber with and without spiking 15) labeled urea. The identification of HO15NO from soil with 15N urea addition confirmed biogenic emissions of HONO from soil. The method enables a new approach of studying the formation pathways of HONO and its role for atmospheric chemistry (e.g., ozone formation) and environmental tracer studies on the formation and conversion of gaseous HONO or aqueous NO2- as part of the biogeochemical nitrogen cycle, e.g., in the investigation of fertilization effects on soil HONO emissions and microbiological conversion of NO2- in the hydrosphere.

Investigation of the influence of liquid surface films on O3 and PAN deposition to plant leaves coated with organic/inorganic solution

This study investigates the influence of leaf surface water films on the deposition of ozone (O3) and peroxyacetyl nitrate (PAN) under controlled laboratory conditions. A twin cuvette system was used to simulate environmental variables. We observed a clear correlation between the O3 deposition on plants (Quercus ilex) and the relative humidity (RH) under both, light and dark conditions. During the light period the observed increase of the O3 deposition was mainly attributed to the opening of leaf stomata, while during the absence of light the liquid surface films were the reason for O3 deposition. This finding was supported by experimentally induced stomatal closure by the infiltration of abscisic acid. In the case of PAN, no relationship with RH was found during the dark period, which indicates that the non-stomatal deposition of PAN is not affected by the liquid surface films. Consequently, the ratio of the O3 and PAN deposition velocities is not constant when relative humidity changes, which is in contrast to assumptions made in many models. The flux partitioning ratio between non-stomatal and stomatal deposition as well as between non-stomatal and total deposition was found to be Rnsto/sto = 0.21 – 0.40, Rnsto/tot = 0.18 – 0.30 for O3 and Rnsto/sto = 0.26 – 0.29, Rnsto/tot = 0.21 – 0.23 for PAN. Furthermore, we demonstrate that the formation of the liquid surface film on leaves and the non-stomatal O3 deposition are depending on the chemical composition of the particles deposited on the leaf cuticles as proposed previously.

In meso crystal structure of a novel membrane‐associated octaheme cytochrome c from the Crenarchaeon Ignicoccus hospitalis

The Crenarchaeon Ignicoccus hospitalis lives in symbiosis with Nanoarchaeum equitans providing essential cell components and nutrients to its symbiont. Ignicoccus hospitalis shows an intriguing morphology that points toward an evolutionary role in driving compartmentalization. Therefore, the bioenergetics of this archaeal host–symbiont system remains a pressing question. To date, the only electron acceptor described for I. hospitalis is elemental sulfur, but the organism comprises genes that encode for enzymes involved in nitrogen metabolism, e.g., one nitrate reductase and two octaheme cytochrome c, Igni_0955 (IhOCC) and Igni_1359. Herein, we detail functional and structural studies of the highly abundant IhOCC, including an X‐ray crystal structure at 1.7 Å resolution, the first three‐dimensional structure of an archaeal OCC. The trimeric IhOCC is membrane associated and exhibits significant structural and functional differences to previously characterized homologs within the hydroxylamine oxidoreductases (HAOs) and octaheme cytochrome c nitrite reductases (ONRs). The positions and spatial arrangement of the eight hemes are highly conserved, but the axial ligands of the individual hemes 3, 6 and 7 and the protein environment of the active site show significant differences. Most notably, the active site heme 4 lacks porphyrin‐tyrosine cross‐links present in the HAO family. We show that IhOCC efficiently reduces nitrite and hydroxylamine, with possible relevance to detoxification or energy conservation.

Hydroxylamine released by nitrifying microorganisms is a precursor for HONO emission from drying soils

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH), the atmosphere´s primary oxidant. An unknown strong daytime source of HONO is required to explain measurements in ambient air. Emissions from soils are one of the potential sources. Ammonia-oxidizing bacteria (AOB) have been identified as possible producers of these HONO soil emissions. However, the mechanisms for production and release of HONO in soils are not fully understood. In this study, we used a dynamic soil-chamber system to provide direct evidence that gaseous emissions from nitrifying pure cultures contain hydroxylamine (NH2OH), which is subsequently converted to HONO in a heterogeneous reaction with water vapor on glass bead surfaces. In addition to different AOB species, we found release of HONO also in ammonia-oxidizing archaea (AOA), suggesting that these globally abundant microbes may also contribute to the formation of atmospheric HONO and consequently OH. Since biogenic NH2OH is formed by diverse organisms, such as AOB, AOA, methane-oxidizing bacteria, heterotrophic nitrifiers, and fungi, we argue that HONO emission from soil is not restricted to the nitrifying bacteria, but is also promoted by nitrifying members of the domains Archaea and Eukarya.

Reactive Trace Gas and Aerosol Fluxes

Quantifying the atmosphere-surface exchange of reactive trace gases and aerosols is extremely important for a full understanding of biogeochemical cycles and their implications for air quality and climate. However, turbulent fluxes of reactive gases such as ozone and volatile organic compounds (VOC) as well as aerosol particles are still difficult to measure. Chemical reactions contribute to changes in trace gas or aerosol concentrations, and production or loss processes have to be carefully separated from turbulent transport. Also, for many trace gas measurements and for size-resolved and chemically speciated aerosol measurements, instruments are limited with respect to time resolution, sensitivity, and accuracy, which restricts their application in micrometeorological techniques. Here, we present flux measurements of reactive trace gases and aerosols above tall vegetation. We focus on ozone deposition and its implications for the NO/NO2/O3 triad, biogenic emissions of volatile organic compounds and their subsequent oxidation reactions, and finally, turbulent aerosol fluxes in a spruce forest ecosystem.

Trace Gas Exchange at the Forest Floor

Exchange conditions at the forest floor are complex due to the heterogeneity of sources and sinks and the inhomogeneous radiation but are important for linking soil respiration to measurements in the trunk space or above canopy. Far more attention has therefore been paid to above and within canopy flows, but even studies that addressed forest floor exchange do not present measurements below 1 m or 2 m. We used a multilayer model that explicitly resolves the laminar layer, the buffer layer, and the turbulent layer to calculate fluxes from the measured profiles in the lowest meter above ground and to calculate effective surface concentrations from given fluxes. The calculated fluxes were compared to measured eddy covariance fluxes of sensible heat and O3 and to chamber derived soil fluxes of CO2 and 222Rn. Sensible heat fluxes agreed surprisingly well given the heterogeneity of radiative heating and the generally low fluxes (max. 25 W m−2). The chamber fluxes turned out to be not comparable as the chamber fluxes were too low, probably due to one of the well-known problems of enclosures such as pressure differences, disturbed gradients and exclusion of naturally occurring turbulence events and surface cooling. The O3 fluxes agreed well for high O3 values reaching down to the forest floor during full coupling of the canopy by coherent structures. During most of the time, the model overestimated the fluxes as chemical reactions were dominating within the profile. One new approach was to calculate the effective surface concentration from a given flux and compare this to measured surface concentrations. This allowed the identification of situations with a coupled and decoupled forest floor layer, which has important consequences for respiration measurements in the trunk space or above canopy and should be considered in upcoming studies.