Fog controls biological cycling of soil phosphorus in the Coastal Cordillera of the Atacama Desert

Soils in hyper-arid climates, such as the Chilean Atacama Desert, show indications of past and present forms of life despite extreme water limitations. We hypothesize that fog plays a key role in sustaining life. In particular, we assume that fog water is incorporated into soil nutrient cycles, with the inland limit of fog penetration corresponding to the threshold for biological cycling of soil phosphorus (P). We collected topsoil samples (0‒10 cm) from each of 54 subsites, including sites in direct adjacency (< 10 cm) and in 1 m distance to plants, along an aridity gradient across the Coastal Cordillera. Satellite-based fog detection revealed that Pacific fog penetrates up to 10 km inland, while inland sites at 10‒23 km from the coast rely solely on sporadic rainfall for water supply. To assess biological P cycling we performed sequential P fractionation and determined oxygen isotope of HCl-extractable inorganic P (δ¹⁸O_HCl-Pi). Total P (P_t) concentration exponentially increased from 336 mg kg^-1 to a maximum of 1021 mg kg^-1 in inland areas ≥ 10 km. With increasing distance from the coast, soil δ¹⁸O_HCl-_Pi values declined exponentially from 16.6‰ to a constant 9.9‰ for locations ≥ 10 km inland. Biological cycling of HCl-P_i near the coast reached a maximum of 76‒100%, which could only be explained by the fact that fog water predominately drives biological P cycling. In inland regions, with minimal rainfall (< 5 mm) as single water source, only 24±14% of HCl-P_i was biologically cycled. We conclude that biological P cycling in the hyper-arid Atacama Desert is not exclusively but mainly mediated by fog, which thus controls apatite dissolution rates and related occurrence and spread of microbial life in this extreme environment.