Life at the extreme: Plant-driven hotspots of soil nutrient cycling in the hyper-arid core of the Atacama Desert

The hyperarid core of the Atacama Desert represents one of the most intense environments on Earth, often being used as an analog for Mars regolith. The area is characterized by extremes in climate (e.g., temperature, humidity, UV irradiation) and edaphic factors (e.g., hyper-salinity, high pH, compaction, high perchlorates, and low moisture, phosphorus and organic matter). However, the halophytic C4 plant Distichlis spicata appears to be one of the few species on the planet that can thrive in this environment. Within this habitat it captures windblown sand leading to the formation of unique structures and the generation of above-ground phyllosphere soil. Using a combination of approaches (e.g., X-ray Computed Tomography, TXRF, δ13C/δ15N isotope profiling, microbial PLFAs, 14C turnover, phosphate sorption isotherms) we examined the factors regulating the biogeochemical cycling of nitrogen (N), phosphorus (P) and carbon (C) in both vegetated and unvegetated areas. Our results showed that D. spicata rhizomes with large aerenchyma were able to break through the highly cemented topsoil layer leading to root proliferation in the underlying soil. The presence of roots increased soil water content, P availability and induced a change in microbial community structure and promoted microbial growth and activity. In contrast, soil in the phyllosphere exhibited almost no biological activity. Organic C stocks and recent C4 plant derived input increased as follows: phyllosphere (1941 g C m−2; 85% recent) > soils under plants (575–748 g C m−2; 55–60%) > bare soils (491–642 g C m−2; 9–17%). Due to the high levels of nitrate in soil (>2 t ha−1) and high rates of P sorption/precipitation, our data suggest that the microbial activity is both C and P, but not N limited. Root-mediated salt uptake combined with foliar excretion and dispersal of NaCl into the surrounding area indicated that D. spicata was responsible for actively removing ca. 55% of the salt from the rhizosphere. We also demonstrate that NH3 emissions may represent a major N loss pathway from these soil ecosystems during the processing of organic N. We attribute this to NH3 volatilization to the high pH of the soil and slow rates of nitrification. In conclusion, we demonstrate that the extremophile D. spicata physically, chemically and biologically reengineers the soil to create a highly bioactive hotspot within the climate-extreme of the Atacama Desert

Dust and aerosols in the Atacama Desert

The Atacama Desert is one of the driest and oldest deserts on Earth. The extreme scarcity of rainfall and hence very limited runoff, paired with endorheism, allow sediments and deposited materials to largely remain in the pedosphere for long periods of time, thereby leading to the generation of thick sediment, salt, and soil deposits. Aerosols are the main inputs of exogenous material to this system. The dominant aerosols deposited in the region are from sea spray, soil and salar playa deflation, volcanic emissions, along with secondary aerosols. The whole Atacama region receives particulate matter (minerals, salts, organic compounds, and microorganisms of variable content) from the Pacific Ocean, the coastal desert, and the Andes Cordillera and Altiplano. Some water may reach the western margin of the Atacama hyper arid core due to fog advection via the Coastal Cordillera. However, despite its aridity, large dust outbreaks from the Atacama Desert are rare. Atmospheric deposition is of great relevance for the landscape evolution of the Atacama Desert. This review summarizes current knowledge on the evolution of the landscape and the climatic conditions that led to it, and the salt and soil deposits, along with other geophysical features, in order to identify the frontier of aerosol research in the Atacama Desert

Water-dispersible colloids distribution along an alluvial fan transect in hyper-arid Atacama Desert

As located in one of the oldest and driest deserts on Earth, soils in the Atacama Desert are greatly affected by atmospheric dust deposited on soil surface and the related fate of water-dispersible colloids (WDCs, <300 nm). We hypothesize that formation and content of these WDCs change with topography and age of natural soils. To highlight the processes involved, we investigated a mid-sized and gently (∼5°) sloping alluvial fan system of multi-phase evolution at 1480 m a.s.l. in the Paposo region of the hyper-arid Atacama Desert, which is considered typical for this part of the Coastal Cordillera. Sampling was done along a topographic transect in 11 pits, and assessed the distribution and composition of WDCs by means of asymmetric flow field-flow fractionation (AF4). The younger fan section (optically stimulated luminescence (OSL)-age of ∼13.6 ka) exhibited a pronounced surface roughness and steep slopes. Here, WDCs from the top soils (0–1 cm) free of plants contained nearly 54 ± 7% of medium-sized colloids (MCs, 210–300 nm) with a dominance of Si and Al. The elevated concentrations of fine colloids (FC, 24–210 nm) and particularly nanocolloids (NCs, 0.6–24 nm) was shown in levelled surface soils near shrubs with predominance of organic carbon (OC) and Ca. With higher collodial OC and Ca content in soils near shrubs, more WDC-P was formed concomitantly through increased OC-Ca-P associations. Larger variations in total WDC content were detected in the surface soils of the older fan section, which was dated to ∼56.4 ka. Here, the peaking NC had almost disappeared and thus MC dominated, probably reflecting re-aggregation and wind erosion over longer periods of time across a relatively smooth land surface. The WDCs and WDC-P peaked at 5–10 cm depth in the older fan section, as here a solid mineral/salt layer was present, while in the younger fan section the WDCs were more likely to be translocated from ‘permeable’ surface into deeper layers, likely reflecting leaching with occasional heavy rainfall. Overall, forms and distribution of WDCs depended on both topographic position and sediment age, thus making colloids as unique tracers of soil development processes during myriad or more years