Abstract

The Atacama Desert is the oldest and driest desert on Earth, nevertheless traces of life have been observed in several places, accumulating residues of organic matter (OM) in the desert soil. We evaluated to which degree the distribution of soil organic carbon (SOC) stocks depends on aridity. We questioned that these OM traces of life preferentially accumulate in topsoil and investigated whether there was also an enrichment of OM in deeper subsoil. We sampled four west-east directed transects with increasing distance to the coast, spanning the Atacama Desert from north to south, plus a hyperarid site at Yungay in the centre of the desert. With a nested sampling design we addressed topsoil heterogeneity at each sampling site (n <= 18). For 12 of these sites soil profiles were dug to 0.6-2.0 m depth. The SOC concentrations were determined for each sample by temperature-dependent differentiation of total carbon. We found that with increasing distance to the coast (2.3 to 86.8 km) and thus aridity, SOC stocks decreased from 53.91 +/- 53.90 to 4.11 +/- 1.46 t SOC ha(-1) in the topsoil. Hence, traces of life were ubiquitously preserved in both, arid and hyperarid desert areas, whereas a local hyperarid minimum of 1.02 +/- 0.18 t SOC ha(-1) at 49.2 km distance, right in between the Andes and the coast, was found. Intriguingly, relatively large amounts of OM were found at depth below 15 cm, adding 74.0 to 94.3 % of subsoil SOC to the topsoil SOC stock. Furthermore, the subsoil SOC concentrations peaked between 40 and 150 cm depth in the soil, irrespectively of the presence or absence of visible vegetation. We conclude that carbon accrual in this arid to hyperarid system shifts from preferential carbon enrichment in topsoil to subsoil, therewith providing the potential for deep(er) biosphere food-webs and demonstrating the future need to dig into soil for discovering traces of life in comparable environments, including other planets such as Mars.