Water-driven soil phosphorus dynamics in the hyper-arid Atacama Desert

HAL (Le Centre pour la Communication Scientifique Directe)

Abstract

Unravelling how soil functions, such as biogeochemical phosphorus (P) cycling, adapt to extreme water scarcity in hyper-arid regions provides insights to address the ecological challenges of global desertification. The Atacama Desert (Chile) is one of the oldest and driest deserts on Earth. Water exists in the Atacama Desert in the form of fog penetration and occasional rainfall, but their role in the biogeochemical processes of soil P remains unclear. Therefore, the aims of the thesis were (1) to investigate the role of water, e. g. fog vs. rainfall, on soil biological P cycling by analyzing the soil oxygen isotope composition of HCl-extractable phosphate (δ18OHCl-Pi). It can trace past biological activity because only water-driven enzymatic activity can alter the signal during a long turn-over time under natural soil conditions; (2) to explore the effect of topographic development induced by occasional rainfall events on the distribution and composition of water-dispersible colloids (WDCs, <300 nm). WDCs are the most physiochemically active size fraction in soils and usually act as carriers for storing and mobilizing P; and (3) to identify the response of WDCs and P associated with colloids (WDC-P) to water addition through an artificial experiment. The work was mainly conducted in the Paposo region located at the Coastal Cordillera, which is the geologic unit near the Pacific Sea in the Atacama Desert. It exhibits a transect along the distance away from the Pacific towards inland with increasing aridity. Satellite-based fog detection had revealed that the diminishing gradient of Pacific fog penetrates up to 10 km, while the inland region >10 km from the coast relies solely on sporadic rainfall. At a site 13.4 km (1470 m. a.s.l.) away from the coast, there is an alluvial fan system with two distinct sections that formed during different wetter periods. The younger section (~13.6 ka) exhibits past water pathways, while the older section (~56.4 ka) is characterized by long-term morphodynamic inactivity. The surface soil P pool was detected by the sequential Hedley extraction method and δ18OHCl-Pi was analyzed for samples along the distance gradient. Results showed soil δ18OHCl-Pi values declined exponentially from 16.6‰ to a constant 9.9‰ for locations ≥10 km inland. Biological cycling of HCl-Pi near the coast reached a maximum of 76‒100%, which could only be explained by predominately fog water driven biological P cycling. In inland regions, with minimal rainfall (<5 mm) as the single water source, only 24±14% of HCl-Pi was biologically cycled. Therefore, biological P cycling was not exclusively but predominantly mediated by fog water in the region, which thus controls apatite dissolution rates and related occurrence, spread and activity of microbial life in this extreme environment. WDCs from soils collected in the inland alluvial fan system (13.4 km, 1470 m. a.s.l.) were fractionated by asymmetric flow field-flow fractionation (AF4). By coupling online to an ICP-MS and an organic carbon detector (OCD), the composition of size-fractionated colloids was identified. Results showed the concentrations of fine colloids (FCs, 24‒210 nm) and nano colloids (NCs, 0.6–24 nm) was elevated in surface soils (0–1 cm) near shrubs with predominance of organic carbon (OC) and Ca, and thus more WDC-P was formed. In the older fan section, in surface soils greater variations in total WDC content were observed compared to the younger section. Here, the NC had almost disappeared, and medium colloids (MCs, >210 nm) dominated the WDC, probably reflecting re-aggregation over a longer time across this relatively smooth land surface. Furthermore, 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, reflecting leaching during occasional heavy rainfall.An artificial water irrigation experiment was performed at the same alluvial fan to stimulate past heavy rainfall events. The soil aggregate was separated by dry sieving and the depth distribution of WDCs analysis was conducted by AF4-ICP-MS/OCD before and after irrigation. No surface run-off occurred as both investigated soil sections had ~100% infiltration. The mechanical breakdown of soil aggregates from raindrop impact significantly contributed to the increase of WDCs in the younger soil surface layer (0‒1 cm) with 1% clay content. In the layer below 1 cm, about 30% of the WDCs was washed away due to subsurface water discharge along the slope. In contrast, the WDCs in the older surface layer with a higher clay content (7%) decreased due to limited soil aggregate disintegration following irrigation. In the layer below 1 cm, however, WDCs increased due to soil aggregate breakdown resulting from swelling and clay dispersion. The abundance of WDC-P was primarily influenced by the associated presence of the predominant elements of Si, Fe, and Al. Therefore, as response of colloid dynamics and the associated fate of WDC-P were driven by the induced water flow leading to aggregate formation or disintegration processes at different soil depths. Overall, this was shaped by the varied sediment age of the fan sections and their different initial soil structure and clay contents. In conclusion, the thesis emphasized the specific role of water on the biogeochemical processes of soil P in the hyper-arid region of the Atacama Desert. These findings enhance the understanding of the mechanisms underlying life's adaptation and survival in hyper-arid environments, including potential comparable habitats on Mars.

Authors 1

1. Xiaolei Sun corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen