Isolation of Water–Dispersible Colloids from Volcanic and Arid Bulk Soils

The water–dispersible colloid (WDC) fraction (≤ 450 nm) has the potential to leach into groundwater, leading to a loss of nutrients from the bulk soil. The objective of this study was to determine the concentrations of organic carbon (C_org) and inorganic elements in the WDC fraction of volcanic (WDC–V) and arid (WDC–A) Chilean soils. The WDC fractions were isolated from three arid bulk soil at different meters above sea level (masl) from northern Chile and from five volcanic bulk soil from the southern Chile. The isolation of WDC was carried out according to Stokes’ Law, using Milli–Q water as the extracting agent. The WDC were simultaneously separated and analyzed into natural nanoparticles (NNPs; 0.06–30 nm), fine colloids (FC; 30–220 nm), and medium colloids (MC; 220–450 nm) fractions using flow field–flow fractionation coupled to inductively coupled plasma mass spectrometry or ultraviolet–visible detectors. The WDC–A fractions at 300 and 500 masl had higher concentrations of Fe, Al, Si, Ca, Mg, Mn, and Zn, ranging between 35 and 193,070 µg L⁻¹, compared to WDC–V, except for P. Additionally, C_org in WDC–A at 300 and 500 masl ranged from 2600–3600 µg L⁻¹, which was higher than in WDC–V, except for Carilafquen and Lautaro soils. The concentration of Si, Al, Fe, and P increased with larger particle sizes in the WDC–V fraction, whereas in the WDC–A fractions, the concentrations of Si, Al, and Fe were higher in the FC and MC fractions, while P and Ca were higher in the NNPs and FC size fractions. The concentration of inorganic elements and C_org in the WDC fractions differed between the arid northern soils and the volcanic soils of southern Chile, likely attributed to variations in the amount and stability of organic matter present in each soil type.