Rainfall limit of the N cycle on Earth

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⁻ (Δ¹⁷O = 23.0‰) 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.