Fig. 4. Biogeochemical activity measurements of photosynthetic and hydrogenotrophic processes in biocrusts and topsoils collected along the aridity gradient.

a Rates of H₂ oxidation under dry and wet conditions based on gas chromatography. Six biologically independent biocrust and topsoil samples were incubated per climatic zone. b Rates of carbon fixation in wet biocrust and topsoil samples based on tracing ¹⁴C-labelled CO₂ incorporation. Three processes are shown: dark carbon assimilation (i.e., basal rate of CO₂ incorporation under dark ambient conditions due to carbon fixation or anaplerotic processes); photosynthetic carbon fixation (i.e., amount of additional CO₂ fixed under light ambient conditions); and hydrogenotrophic CO₂ fixation (amount of additional CO₂ fixed under dark H₂-enriched conditions). For both biocrusts and topsoils, six biologically independent samples were pooled for each climatic zone and experiments were performed in technical triplicates. For both panels, rates are normalized to 16S rRNA gene copy number as a proxy for biomass (unadjusted rates shown in Fig. S9). Boxplots show median, upper and lower quartile, and minimum and maximum values.