Measurement report: Moisture and light driven Volatile Organic Compound emissions of biocrusts from the Succulent Karoo, South Africa

Abstract. Biological soil crusts (biocrusts) cover around 12 % of Earth’s land surface and play critical roles in dryland ecosystems, yet their contribution to volatile organic compound (VOC) emissions remains poorly understood. Despite their extensive distribution, biocrusts are not currently included in global VOC emission inventories, possibly creating a significant gap in the understanding of atmospheric chemistry in arid regions. Here, we quantify VOC emissions and CO₂ fluxes from soil samples dominated by four different biocrust types, namely light cyanobacteria-, dark cyanobacteria-, chlorolichen-, and moss-dominated crusts and bare soil samples, collected from the Succulent Karoo, South Africa. Using a climate-controlled chamber, we measured biocrusts at high air humidity, during and after wetting with artificial rainwater, and during subsequent drying. Interestingly, VOC emissions were strongest in the absence of photosynthesis before wetting, when samples were exposed to humid air in the dark and in the light with strong emissions of ethanol, methanol, and acetone. After wetting most VOC emissions decreased, but some of them, like geosmin and 2-methylisoborneol, compounds associated with petrichor, increased. In general, moss-dominated biocrusts exhibited the highest isoprene and sulfur compound emissions. Dimethyl sulfide emission peaked during darkness, suggesting a light-dependent source or sink in moss dominated biocrusts. The emission of up to 75±48 nmol m^–2 s^–1 of total VOCs across the studied biocrust types highlights them as a significant, overlooked source of reactive VOCs to the atmosphere, potentially influencing atmospheric oxidation capacity and secondary organic aerosol formation, particularly in regions with low vascular vegetation cover.