Global NO and HONO emissions of biological soil crusts estimated by a process-based non-vascular vegetation model

Philipp Porada,Axel Kleidon,Yafang Cheng,Jose Raggio,Alexandra Tamm,Bettina Weber,Ulrich Pöschl

doi:10.5194/bg-16-2003-2019

Abstract

Abstract. The reactive trace gases nitric oxide (NO) and nitrous acid (HONO) are crucial for chemical processes in the atmosphere, including the formation of ozone and OH radicals, oxidation of pollutants, and atmospheric self-cleaning. Recently, empirical studies have shown that biological soil crusts are able to emit large amounts of NO and HONO, and they may therefore play an important role in the global budget of these trace gases. However, the upscaling of local estimates to the global scale is subject to large uncertainties, due to unknown spatial distribution of crust types and their dynamic metabolic activity. Here, we perform an alternative estimate of global NO and HONO emissions by biological soil crusts, using a process-based modelling approach to these organisms, combined with global data sets of climate and land cover. We thereby consider that NO and HONO are emitted in strongly different proportions, depending on the type of crust and their dynamic activity, and we provide a first estimate of the global distribution of four different crust types. Based on this, we estimate global total values of 1.04 Tg yr−1 NO–N and 0.69 Tg yr−1 HONO–N released by biological soil crusts. This corresponds to around 20 % of global emissions of these trace gases from natural ecosystems. Due to the low number of observations on NO and HONO emissions suitable to validate the model, our estimates are still relatively uncertain. However, they are consistent with the amount estimated by the empirical approach, which confirms that biological soil crusts are likely to have a strong impact on global atmospheric chemistry via emissions of NO and HONO.

Highlights

Biological soil crusts are complex communities of organisms which cover large areas around the globe, mainly in arid and semiarid regions (Sancho et al, 2016)
Emissions of nitric oxide (NO) and HONO by biocrusts have been shown to strongly depend on the type of biocrust. To take this into account, we introduced a new scheme into the Lichen and Bryophyte model (LiBry) model which allows for a representation of different biocrust types and their associated NO and HONO emissions
Spatial patterns of relative cover of the four types light cyanobacteria-dominated, dark cyanobacteria-dominated, chlorolichen-dominated, and moss-dominated biocrust simulated by the LiBry model at the global scale are shown in Fig. 4a to d

Summary

Introduction

Biological soil crusts (hereafter “biocrusts”) are complex communities of organisms which cover large areas around the globe, mainly in arid and semiarid regions (Sancho et al, 2016). They consist of various different species of free-living cyanobacteria, green algae, lichens, bryophytes, fungi, and bacteria. Today’s biocrusts have been suggested to affect biogeochemical cycles, both at the regional and the global scale (Elbert et al, 2012; Sancho et al, 2016) In drylands, they provide several essential ecosystem services, such as protection of the soil surface against erosion (Rodríguez-Caballero et al, 2012, 2018) and input of carbon, nitrogen, and phos-

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