TRANSFORMATION OF THE SOIL N CYCLE AT THE ARID-HYPERARID TRANSITION ON EARTH

In most climates on Earth, biological processes control the soil N cycle. In the Atacama Desert of Chile, aridity severely limits biology, yet the driest soils are rich in N. To understand this seeming paradox, and to place the soils of this region in a global context, we determined how soil N processes change across a series of ancient Atacama Desert soils (>2My) that vary only in rainfall (21 to <2 mm y^-1). With decreasing rainfall, soil NO₃^- increases from 0.011 to 3.6 kg N m^-2, and soil organic N increases from 0.016 to 1.4 kg N m^-2, while soil organic C decreases from 1.5 to 0.3 kg C m^-2 (0.05 to 0.01%), and biological activity declines to negligible levels. Triple O isotope analysis of atmospheric deposition and soils indicates that NO ₃^- produced via tropospheric photochemical oxidation processes increases from 39% to 80% of the total soil NO₃^- as rainfall decreases, reflecting a sharp increase in preservation of atmospheric N. N transformation in the driest soil is predominantly abiotic, with NH₃ volatization (~400 µmol m^-2y^-1) exceeding the slow average N oxidation rates (26 µmol m^-2y^-1). Our results show that the driest soils have crossed a fundamental biogeochemical threshold from a steady state, biologically mediated soil N cycle to slow, passive accumulation of atmospheric NO₃^- and organic N over millions of years. The stores of atmospheric solutes in these soils provide a useful analog for surficial geochemistry on Mars.