Phosphorus in water dispersible-colloids of forest soil profiles

Nanoparticles and colloids affect the mobilisation and availability of phosphorus for plants and microorganisms in soils. We aimed to give a description of colloid sizes and composition from forest soil profiles and to evaluate the size-related quality of colloids for P fixation. We investigated the size-dependent elemental composition and the P content of water-dispersible colloids (WDC) isolated from five German (beech-dominated) forest soil profiles of varying bulk soil P content by field-flow fractionation (FFF) coupled to various detectors. Three size fractions of WDC were separated: (i) nanoparticles < 25 nm (NP) rich in C-org, (ii) fine colloids (25 nm-240 nm; FC) composed mainly of C-org, Fe and Al, probably as associations of Fe- and Al- (hydr)oxides and organic matter, and (iii) medium-sized colloids (240 nm-500 nm; MC), rich in Fe, Al and Si, indicating the presence of phyllosilicates. The P concentration in the overall WDC was up to 16 times higher compared to the bulk soil. The NP content decreased with increasing soil depth while the FC and MC showed a local maximum in the mineral topsoil due to soil acidification, although variant distributions in the subsoil were observed. NP were of great relevance for P binding in the organic surface layers, whereas FC- and MC-associated P dominated in the Ah horizon. The nanoparticles and colloids appeared to be of high relevance as P carriers in the forest surface soils studied, regardless of the bulk soil P content.