Dust is a major vehicle for the dispersal of microorganisms across the globe. While much attention has been focused on microbial dispersal in dust plumes from major natural dust sources, very little is known about the fractionation processes that select for the 'dust microbiome'. The recent identification of highly emissive, agricultural land dust sources in South Africa has provided the opportunity to study the displacement of microbial communities through dust generation and transport. In this study we aimed to document the microbial communities that are carried in the dust from one of South Africa's most emissive locations, and to investigate the selective factors that control the partitioning of microbial communities from soil to dust. For this purpose, dust samples were generated at different emission sources using a Portable In-Situ Wind Erosion Lab (PI-SWERL), and the taxonomic composition of the resulting microbiomes were compared with the source soils. Dust emission processes resulted in the clear fractionation of the soil bacterial community, where dust samples were significantly enriched in spore-forming taxa. Conversely, little fractionation was observed in the soil fungal communities, such that the dust fungal fingerprint could be used to identify the source soil. Dust microbiomes were also found to vary according to the emission source, suggesting that land-use significantly affected the structure and fractionation of microbial communities transported in dust plumes. In addition, several potential biological allergens of fungal origin were detected in the dust microbiomes, highlighting the potential detrimental effects of dust plumes emitted in South Africa. This study represents the first description of the fractionation of microbial taxa occurring at the source of dust plumes and provides a direct link between land-use and its impact on the dust microbiome.

The sandy croplands in the Free State have been identified as one of the main dust sources in South Africa. The aim of this study was to investigate the occurrence and strength of physical soil crusts on cropland soils in the Free State, to identify the rainfall required to form a stable crust, and to test their impact on dust emissions. Crust strength was measured using a fall cone penetrometer and a torvane, while laboratory rainfall simulations were used to form experimental crusts. Dust emissions were measured with a Portable In-Situ Wind Erosion Laboratory (PI-SWERL). The laboratory rainfall simulations showed that stable crusts could be formed by 15 mm of rainfall. The PI-SWERL experiments illustrated that the PM10 emission flux of such crusts is between 0.14% and 0.26% of that of a non-crusted Luvisol and Arenosol, respectively. The presence of abraders on the crust can increase the emissions up to 4% and 8% of the non-crusted dust flux. Overall, our study shows that crusts in the field are potentially strong enough to protect the soil surfaces against wind erosion during a phase of the cropping cycle when the soil surface is not protected by plants.

Experiments in large wind tunnels have made vital contributions to our knowledge of aeolian processes. However, the size of these instruments makes them impractical for field application. To facilitate field measurements on the dust emission potential of soils, the Portable In-Situ Wind Erosion Lab (PI-SWERL) was developed. Previous research shows that the PI-SWERL can be used to quantify dust emission potentials and (threshold) friction velocities. Studies that compare the PI-SWERL to traditional wind tunnels mainly focus on the dust emission potential at various friction velocities. In the present study, we quantified the threshold friction velocity for PM 10 emission using a PI-SWERL and compare it to results obtained with a straight-line wind tunnel: the Portable Wind and Rainfall Simulator of the University of Basel (PWRS). Tests were performed on two types of substrate: fine sand (NS1) and loamy sand (DS1). For NS1, a threshold friction velocity of 0.33 m s − 1 was identified from both the PI-SWERL and the PWRS data. For DS1, identified threshold friction velocities showed differences: 0.25 m s − 1 by the PI-SWERL and 0.39 m s − 1 by the PWRS. The position of the DustTrak II monitor's inlet tube and variations of the fan's speed by different operators could explain the difference in identified thresholds. Although different threshold friction velocities were obtained for one of the substrates, we believe that comparable results can be achieved by adjusting the experimental design in future research. Therefore, the PI-SWERL can be successfully used to quantify thresholds, facilitating dust emission studies in more remote regions.

Whirlwinds and visible dust devils occur over semi-arid ecosystems and entrain particles from the ground surface. Fires produce abundant charcoal across savannahs and the resulting blackened surfaces create a large albedo contrast. Whirlwinds have been observed associated with active fires; yet, there are few published observations on post-fire landscapes. Spatiotemporal patterns of whirlwinds have been documented for a limited number of regions and have not been made for the ecosystems of eastern Africa. From field-based sightings in the Serengeti National Park, Tanzania, we report on whirlwinds over burned savannah patches that entrained large quantities of charcoal to produce black coloured charcoal devils that lofted charcoal into the atmosphere. Two occurrences of charcoal devils were sighted and photographed, one each in the Western Corridor (Bunda District) and Lamai (Serengeti District), Mara Region. The observations were compared with regional scale meteorological data and remote sensing satellite imagery and albedo estimates of the land cover conditions. Although direct meteorological or particulate matter measurements were not made, the observations show that both charcoal devils differed in colour, funnel shape, height, and savannah land cover types (different woody to grass fuel canopies), and thus different charcoal morphologies. Charcoal laden whirlwinds require further study and characterization to analyse the contribution to local-scale redistribution of matter and regional-to-global fluxes of terrestrially derived atmospheric particulates. Future research focusing on the spatiotemporal patterns of whirlwinds over burned patches of savannah, the formation, duration and dissipation mechanisms, and characterisation of the entrained material would contribute to our understanding of the phenomena. The redistribution of organic and clastic material would contribute to understanding of detrital fluxes to depositional environments, such as lakes, wetlands, and snow. Keywords:    Atmospheric boundary layer; Convection; Detritus; Dust devils; Fires; Particulate matter.

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