Contrasting depth distribution of colloid-associated phosphorus in the active and abandoned sections of an alluvial fan in a hyper-arid region of the Atacama Desert

Abstract

Colloids and their subset nanoparticles are key soil constituents for nutrient and Organic Carbon (OC) storage and transport, yet little is known about their specific role in overall transfer of elements under hyper-arid conditions. We analyzed the Water Dispersible Colloids (WDCs) of two adjacent soil profiles, located either on the active (named: Fan) or passive (named: Crust) sections of an alluvial fan. Colloidal particles (<500 nm) were fractionated using Asymmetric Field-Flow-Field Fractionation (AF4), which was coupled online to an Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) and an Organic Carbon Detector (OCD) to detect the composition of size-fractionated colloids. Three size categories of particles were identified: nanoparticles (0.6–24 nm), fine colloids (24–210 nm), and medium colloids (210–500 nm). The two profiles differed distinctively in vertical WDC distribution and associated phosphorus (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 calcium (Ca), Silica (Si), aluminum (Al), iron (Fe), and OC content were found. This also revealed Ca-phosphates as the primary P retention from, with the association of P to phyllosilicates and Fe/Al (hydr-) oxides as the main soil colloidal fractions. Overall, our results did 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 nanoparticles and colloids in soil profiles under hyper-arid conditions.