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@PHDTHESIS{Moradi:957950,
author = {Moradi, Ghazal},
othercontributors = {Klumpp, Erwin and Schäffer, Andreas},
title = {{S}oil phosphorus in the extremely arid {A}tacama {D}esert},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2023-05029},
pages = {1 Online-Ressource : Illustrationen, Diagramme, Karten},
year = {2023},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2023},
abstract = {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.},
cin = {162710 / 160000},
ddc = {570},
cid = {$I:(DE-82)162710_20140620$ / $I:(DE-82)160000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2023-05029},
url = {https://publications.rwth-aachen.de/record/957950},
}
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