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•Nanocolloids dominated soil colloids in Acrisols.•Fine and medium-sized colloids dominated soil colloids in Terra Preta soils (TP).•Elevated pH and Ca2+ conc. correlated with SOM involvement in colloid formation in TP.•Enriched water-extractable and colloidal P in TP.
Amazonian Dark Earths (or terra preta de índico) are known as highly fertile soils that can maintain elevated crop yields for centuries. While this fertility was frequently ascribed to the presence of black carbon, the availability and colloidal binding of major nutrients received limited attention. We examined the size distribution and the elemental compositions of water-dispersible colloids (WDC) in both forested and cultivated Terra Preta topsoils (0–10 cm, Anthrosols), as well as in their adjacent non-Terra Preta controls (Acrisols) via asymmetric flow field-flow fractionation (FFF). Liquid-state 31P-nuclear magnetic resonance (NMR) spectra, black carbon content, and scanning electron microscope (SEM) images were also obtained. We found that WDC in Terra Preta soils contained a significant proportion of organo-mineral associations in the size range 30–300 nm, whereas, in contrast, water-dispersible nanoparticles with a diameter < 30 nm were dominant in the adjacent Acrisols. The shifts to larger WDC sizes in the Terra Preta soils went along with elevated pH values, as well as with elevated contents of Si, Al, Fe, Ca and organic matter-containing particles. Also P concentrations were enriched in both the water-extractable phase (WEP) and WDC extracts of Terra Preta soils relative to the adjacent Acrisols. We assume that the higher pH values and Ca ion concentrations promoted the involvement of soil organic matter (SOM) into the formation of larger-sized colloids consisting of kaolinite-like clay minerals, iron oxides and Ca ions in the Terra Preta soils. The elevated content of Ca in Terra Preta soil colloids may also contribute to the retention of P, likely via bridging of anionic P like orthophosphate to SOM. Preventing soil acidification is thus not only to be recommended for Acrisols, but also for maintaining colloidal structures and related fertility in Terra Preta soils.