Pedological Thresholds at the Arid/Hyperarid Boundary in the Atacama Desert.

Across much of the planet, soil properties and biogeochemical processes vary monotonically with rainfall. The studies providing these observations have been performed in landscapes supporting biota. In this study, we examined soils along a rainfall transition from arid (vegetated) to hyperarid (abiotic) in northern Chile, and find that key geophysical and biogeochemical processes exhibit a sharp non-monotonic response across the transition from biotic to nearly abiotic conditions. Here we focus on how the N and S cycles, and rates of soil production on hillslopes, vary with rainfall and life. Atmospheric deposition is the main input of N and S at all locations, but soil retention and in situ biogeochemical alteration is strongly dependent on rainfall and biota. At the arid (vegetated) site, soil N is predominantly organic and exceedingly low (0.03 kg m^-2), while at the extreme hyperarid endmember, soil N is predominately NO₃^- and much higher than at the arid site (5.4 kg m^-2). Sulfate retention increases with aridity,  to 240 kg m^-2.  On gentle convex hillslopes, rates of soil production (conversion of rock to soil) decline by an order of magnitude with decreasing rainfall. However, the thickness and composition of the soil mantle varies non-monotonically, from silicate-rich soil, to bare bedrock surfaces, to a thick salt and dust-rich soil, with decreasing rainfall and changing vegetation. In summary, the removal of water and biota essentially closes the common soil N and S cycles, resulting in the accumulation of these elements as soluble inorganic ions. In terms of soil production and erosion, the production and transport of soil shifts from largely biotically-, to abiotically-, driven processes with declining rainfall. The long-term absence of biota strongly alters the behavior and fate of elements, and the nature of geomorphic processes, highlighting the signature of life on Earth surface geochemistry.