Phosphorus adsorption in subtropical Histosol and Inceptisol with contrasting organic matter contents and clay mineralogy

• The Histosol had a higher maximum P adsorption capacity (MPAC) compared to the Inceptisol. • MPAC is associated with the higher soil organic carbon matter (SOM), low crystallinity oxides and kaolinite . • MPAC is associated with ternary complex (SOM-Ca-H 2 PO4 - ). • Removal of SOM decreases P adsorption. • Selective extractions promoted partial or total rupture of the soil aggregates exposing P adsorption sites. Soil phosphorus adsorption depends on organic and mineral components and their interactions. The objective was to evaluate the effect on P adsorption of the colloidal components (organic matter and minerals) in soil aggregates < 0.2 mm. Two soil classes (Histosol and Inceptisol) from Southern Brazil under different hydric regimen were selected. Selective extractions of organic and mineral colloids were carried out: hydroxylamine (HL), sodium dithionite (SD), sodium hypochlorite (SH), sodium hydroxide (NaOH), hydrogen peroxide (H 2 O 2 ), and control treatment (CT). To evaluate the formation of cation bridges (colloid-cation-H 2 PO 4 - ), the pH of both soils was adjusted with Ca(OH) 2 or KOH. The higher total organic carbon (TOC), kaolinite and low crystallinity Fe and Al oxides contents of the Histosol are coherent with its higher maximum P adsorption capacity (3862 mg kg −1 ) compared to Inceptisol (2537 mg kg −1 ). The addition of Ca 2+ to Histosol increased P adsorption on 5.2 %, which suggests the formation of the ternary complex (SOM-COO – )-Ca 2+ -H 2 PO 4 - . NaOH was efficient in extracting TOC, but also extracted Al and Si from the minerals, resulting in lower P adsorption. The highest P adsorption was found in the residue of the SD extraction. SD was the most efficient to remove pedogenetic Fe oxides, which favors the greater “opening” of the sample (partial or total rupture of the soil aggregates < 2 mm) and consequently the exposition of carboxylic groups of the SOM, favoring the formation of ternary complexes SOM(COO – )-Fe 2+/3+ -H 2 PO 4 - . Soil P adsorption is not a result of isolated effect of each colloidal component (organic matter and minerals) of the solid phase, but rather through their interactions.