Phosphate pools and oxygen signature in the hyper-arid Atacama Desert

The Atacama Desert is a temperate desert restricted by the Pacific Ocean and the Andeans, which is an ideal place to study the biogeochemical phosphate-water dynamics in the conditions with extreme limited water and biomass. We hypothesized that phosphate pools and oxygen signature change along with the increasing distance to the coast and thus aridity. The surface soils (0-10 cm) were sampled along the transect with distance to coast in Paposo region (~25°S) which is located in the Coastal Cordillera nearby the Pacific Ocean from 2.3 to 22.9 km, including 9 altitude sites (600 m, 900 m, 880 m, 920 m, 1000 m, 1200 m, 1450 m, 1700 m, 2110 m). Each site involved 3 samples surrounding the plant with a distance of 0-10 cm and other 3 samples far from the plant with 1 m. The Ca-bound P (HCl-extracted P followed the Hedley sequential P fractionation) accumulated along the increasing distance to coast within 37.9 km and could be described by a mono-exponential regression mode. However, an initial declining trend was detected for phosphate ¹⁸O of HCl-P_i and it reached a steady-state condition beyond 10 km from the coastline, which was the maximum distance that advective fog could penetrate inland. Only the nearest site at 2.3 km (600 m.a.s.l) to coast showed an isotope value within the range of full isotopic equilibrium with biologically cycled phosphate. Furthermore, the effects of the present plant distribution on surface soil Hedley P stocks and phosphate ¹⁸O signatures were very limited. We concluded that both P stocks and phosphate ¹⁸O signatures followed primarily the aridity gradient but phosphate ¹⁸O signatures could work as a tracer for long-term climate conditions.