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%0 Thesis
%A Moradi, Ghazal
%T Soil phosphorus in the extremely arid Atacama Desert
%I RWTH Aachen University
%V Dissertation
%C Aachen
%M RWTH-2023-05029
%P 1 Online-Ressource : Illustrationen, Diagramme, Karten
%D 2023
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, RWTH Aachen University, 2023
%X Phosphorus (P) is an essential nutrient for various forms of life that follows different pathways of cycling in soils of extremely arid environments compared to typical soils. Unique characteristics of the Atacama Desert as the dry limit of life on Earth (e.g. hyper-arid core receiving < 2 mm yr-1 of precipitation) set it apart from other similar terrestrial environments, and provide the opportunity to study the prerequisites of life and evolution in such extreme terrestrial environments and extraterrestrial ones like Mars. Biogeochemical processes involved in P cycling in the Atacama soils are poorly understood. Therefore, the aims of this thesis were i) to trace evidence of past biological P cycling using oxygen isotope composition of HCl-extractable phosphate (δ18OHCl-P) that has a long turnover time and compared to other P pools might retain the past signal of biological P cycling; ii) to investigate soil colloids (1nm – 1µm) as key soil constituents for P and organic carbon (OC) storage and transport.To do so, two different elevational gradients were chosen: (1) Aroma transect fed mainly by irregular rainfalls from the Andes (< 26 mm yr-1), where simultaneously with the decrease of aridity, elevation increases from 1340 m.a.s.l. at the hyper-arid core to 2720 m.a.s.l. at rising foothills of the Andes; (2) Paposo transect fed by fog brought from the Pacific during austral winter, where with the increase of aridity, elevation increases from 950 to 2210 m.a.s.l at coastal mountains toward the hyper-arid core. In order to identify evidence of biological cycling of P, sequential P fractionation was performed, and the δ18OHCl-P was analyzed in the surface soils of the Aroma transect. Furthermore, δ18OHCl-P was measured in surface soils and four deep soil profiles of Paposo transect. Along Paposo, δ18OHCl-P of soil samples near an individual plant were compared to that of surrounding soils. In order to characterize colloidal constituents for P and OC, water dispersible colloids (WDCs) were analyzed in two adjacent soil profiles at Paposo transect, located either on the active (named: Fan) or passive (named: Crust) sections of an alluvial fan. Colloidal particles (<500 nm) were fractionated using Asymmetric Flow Field Flow Fractionation (AF4), which was coupled online to an ICP-MS and an OC detector to detect the composition of size-fractionated colloids. The results showed that at the driest site of Aroma transect, the δ18OHCl-P values were constant with depth (0-10 cm) and deviated from biologically-driven isotopic equilibrium. In contrast, a considerable increase of δ18OHCl-P values was observed below the soil surface at less arid sites where some isotope values were even within the range of full isotope equilibrium. For the latter sites, this points to more efficient biological P cycling right below the uppermost surface of the desert. Critically, the absolute concentrations of this biologically cycled P exceeded those of P potentially stored in living microbial cells by at least two orders of magnitude. Therefore, this data provides evidence that δ18OHCl-P values trace not recent but past biological activity, making it a powerful tool for assessing the existence, pathways and evolution of life in such arid ecosystems.At Paposo transect, surface δ18OHCl-P values had a general decreasing trend from more humid sites toward to drier ones. However, there was a significant difference between δ18OHCl-P values of the sites where fog can reach and the sites that are out of fog reach, revealing that δ18OHCl-P values could be an indicator of rain-affected versus fog-affected soils. The only inconsistency in the decreasing trend of δ18OHCl-P along Paposo gradient was observed where a plant oasis was located. The deep profiles of Paposo transect showed a decreasing trend of δ18OHCl-P by depth, and only the δ18OHCl-P of two upper layers of the less arid profile were inside the equilibrium range. These results indicate the prevailing effect of past biological cycling of P rather than the current life. Three size categories of colloidal particles were identified by using AF4: nanoparticles (0.6-24 nm), fine colloids (24-210 nm), and medium colloids (210-500 nm). The two soil profiles differed distinctively in vertical WDC distribution and associated P content. Fractograms of the Crust profile predominantly showed fine colloids, whereas the medium-sized colloids dominated those of the Fan. Furthermore, the highest colloid content in the Crust profile was found at the surface, while in the Fan, colloids accumulated at 10-20 cm depth, thus overall reflecting the different genesis and infiltration capacities of the soils. Despite very low concentration of colloidal P in these hyper-arid soils, a strong correlation between colloidal P and Ca, Si, Al, Fe, and OC content were found. This also indicated Ca-phosphates as the primary P retention form, with the association of P to phyllosilicates and Fe/Al (hydr-) oxides as the main soil colloidal fractions. These results highlight that small local scale differences in topographic-derived distribution of water flow pathways defined the formation of the crust-like surfaces, and ultimately the overall movement and distribution of colloids in soil profiles under hyper-arid conditions. This thesis established basic knowledge about P in the arid to hyper-arid soils of the Atacama Desert which will help us to better understand the evolution of life in the conditions of severe water scarcity.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2023-05029
%U https://publications.rwth-aachen.de/record/957950