Hagar Siebner

Drylands cover nearly half of Earth's terrestrial surface and are expanding under climate change. Biogenic volatile organic compounds (VOCs) significantly influence atmospheric chemistry, yet microbial VOC emissions from dryland soils remain poorly understood. Here, we investigate VOC emissions associated with petrichor—the characteristic scent following rainfall after prolonged drought—across a steep aridity gradient extending into the Negev Desert. Soil rewetting triggered strong VOC fluxes (1–3.5 nmol m ⁻² s ⁻¹ ground area) dominated by sesquiterpenes and benzenoids, with emission patterns and rates linked to climatic regions, soil aridity, and microbial community composition. Petrichor consisted of a complex bouquet of 58 VOCs, and emission fluxes showed a pronounced initial burst. We upscaled the results to Israel and show that petrichor emissions contribute significantly to ozone and secondary organic aerosol formation potentials, comparable in magnitude to those estimated for anthropogenic VOC sources. Soil aridity shaped VOC chemical composition and emission dynamics, indicating strong microbial and environmental controls on the fluxes. As climate change alters precipitation regimes and promotes dryland expansion, these rainfall‐driven bursts of microbial VOCs may represent an overlooked climate‐microbe feedback influencing atmospheric chemistry and radiative forcing at regional and global scales.

Biological dinitrogen (N2) fixation is increasingly recognized as an important source of nitrogen in freshwater ecosystems; however, the magnitude and drivers of this process are poorly quantified at the global scale. This limited understanding stems from a historical focus on cyanobacterial diazotrophs in a limited number of regions, obscuring the true magnitude, community composition, and environmental drivers of freshwater diazotrophy. This study presents the first standardized global assessment quantifying diazotroph abundance, N2 fixation rates, and nifH gene diversity at 206 freshwater sites across 36 countries on five continents. Diazotrophs were ubiquitous across freshwater ecosystems, comprising up to 16% of the total bacterial abundance, with N2 fixation rates spanning more than 5 orders of magnitude (0.001–133 nmol N L-1 d-1). Noncyanobacterial diazotrophs (NCDs) dominated these communities at 85% of the sites, challenging the cyanobacteria-centric paradigm. Multilinear regression models revealed that water temperature is a key regulator of diazotrophy, delineating distinct regimes for N2 fixation rates (21°C) and the relative abundance of NCDs (9°C). These models further indicated that warmer conditions may reduce the relative abundance of NCDs while favoring cyanobacterial diazotrophs. Ultimately, these thermally defined regimes highlight that warming freshwater environments would change the physicochemical and community drivers that currently regulate freshwater N2 fixation patterns, with major implications for nitrogen cycling models.

Biogenic volatile organic compounds (VOCs) significantly influence atmospheric chemistry, yet the importance of microbial VOC emissions remains understudied. We investigate petrichor VOC emissions, the characteristic scent following rainfall after prolonged drought, in an aridity gradient across Israel, by simulating soil rewetting events. Rewetting triggered strong VOC fluxes (1-3.5 nmol m ⁻² s ⁻¹ ground area) dominated by sesquiterpenes and benzenoids, with emission patterns linked to climatic regions, soil aridity, and microbial community composition. Petrichor consisted of a complex bouquet of 58 VOCs, and the initial VOC burst resembled the CO2 pulse of the Birch effect. Petrichor emissions showed ozone and secondary organic aerosol formation potentials comparable in magnitude to those estimated for anthropogenic VOC sources in Israel. Despite compositional differences, emissions were of similar magnitudes across the aridity gradient. Given that drylands cover nearly half of Earth’s terrestrial surface and are expanding, these episodic microbial VOC emissions may represent a significant, previously overlooked source of reactive carbon with potential implications for regional and global atmospheric chemistry.

Rain events in arid environments are highly unpredictable, interspersing extended periods of drought. Therefore, following changes in desert soil bacterial communities during hydration-desiccation cycles in the field, was seldom attempted. Here, we assessed rain-mediated dynamics of active community in the Negev Desert biological soil crust (biocrust), and evaluated the changes in bacterial composition, potential function, and photosynthetic activity. We predicted that increased biocrust moisture would resuscitate the phototrophs, while desiccation would inhibit their activity. Our results show that hydration increased chlorophyll content, resuscitated the biocrust Cyanobacteria, and induced potential phototrophic functions. However, decrease in the soil water content did not immediately decrease the phototrophs activity, though chlorophyll levels decreased. Moreover, while the Cyanobacteria relative abundance significantly increased, Actinobacteria , the former dominant taxa, significantly decreased in abundance. We propose that, following a rain event biocrust moisture significantly decreased, almost to drought levels, yet the response of the active bacterial community lagged, in contrast to topsoil. Possible explanations to the described rain-mediated bacteria dynamics are discussed.

To reduce public health hazards, greywater reuse may involve disinfection, which is often performed through chlorination. The formation of toxic disinfection by-products is a negative side-effect of chlorine’s reaction with organic matter, of which trihalomethanes (THM) are one of the most dominant (though not most toxic) groups. Greywater treatment in vertical flow constructed wetlands leads to a decrease in dissolved organic matter. We hypothesized that these dissolved organic carbon (DOC) changes would be reflected in differences in THM formation. Greywater samples, at different treatment levels (i.e., decreasing organic matter content), were exposed to 5 mg/L of chlorine for 1 h. THM formation in raw greywater samples was significantly lower than in the more treated (recirculated) samples, despite their significantly higher DO concentrations. This trend was verified in six different systems. Furthermore, this was also shown when greywater was exposed to higher chlorine doses (25 and 50 mg/L). It is suggested that the increase in THM formation for longer recirculated water is the result of two factors: competition between a higher number of reactive sites in the raw water’s organic matter, which leads to smaller THM yields, and changes in the abundance of THM formation moieties in the recirculated water’s DOC. The latter was reflected in the SUVA increase in the treated water. Overall, THM formation, following treated greywater chlorination at the lower chlorine concentration studied, is not expected to pose an environmental health risk when the water is reused for irrigation.