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In most climates on Earth, biological processes control soil N. In the Atacama Desert of Chile, aridity severely limits biology, and soils accumulate atmospheric NO3-. We examined this apparent transformation of the soil N cycle using a series of ancient Atacama Desert soils (> 2 My) that vary in rainfall (21 to < 2 mm yr(-1)). With decreasing rainfall, soil organic C decreases to 0.3 kg C m(-2) and biological activity becomes minimal, while soil NO3- and organic N increase to 4 kg N m(-2) and 1.4 kg N m(-2), respectively. Atmospheric NO3- (Delta O-17 = 23.0 parts per thousand) increases from 39% to 80% of total soil NO3- as rainfall decreases. These soils capture the transition from a steady state, biologically mediated soil N cycle to a dominantly abiotic, transient state of slowly accumulating atmospheric N. This transition suggests that oxidized soil N may be present in an even more arid and abiotic environment: Mars.