Fig. 7.

Model prediction and observation of nitrous acid emission. Nitrous acid (HONO) gaseous emissions during biocrust drying as a function of soil hydration conditions (expressed in percent of water-holding capacity). Typical simulations of different conditions in drying patterns, and atmospheric ammonia (NH₃) levels (low: 5 ppb, high: 20 ppb) are denoted as slow/high drying with low-/high-NH₃ level, S–L (green), S–H (blue), F–L (orange), and F–H (red), respectively. The length of each box indicates ±1 standard deviation and each stick ranges the minimum and maximum emission of HONO from all simulations (n = 8). Colour gradients indicate the averaged spatial variability of local pH values (given as ±1 standard deviation) across simulations. a Simulated HONO emission with fast drying under high NH₃ input was comparable with measurements from cyanobacteria-dominated crust in South Africa³⁴. b Simulated HONO emission with slow drying under low NH₃ input was comparable with measurements from light crust in Cyprus¹⁹