Silica alters phosphorus interactions with goethite and hematite: contrasting effects of amorphous silica and monosilicic acid

Introduction Phosphorus (P) fixation on iron (Fe) oxides in soils is a major constraint on crop production and fertilizer efficiency. Silica (Si) has been proposed to influence P dynamics through competitive interaction at mineral surfaces, yet the relative effects of different Si sources (amorphous silica (ASi) vs. monomeric silicic acid (MSi)) and their interaction with dissolved organic carbon (DOC) remain poorly understood at the mineral scale, particularly across pH values relevant to agricultural soils. Methods We evaluated changes in calcium-acetate-lactate (CAL)-extractable P from phosphate-preconditioned hematite and goethite after 30 days of incubation at pH 5 and 7, following the addition of ASi or MSi, with and without DOC co-application. Results In P-only reference treatments, CAL-extractable P remained low over 30 days, ranging from 1.30 to 1.46 and 0.79–0.99 mg P kg -1 for hematite at pH 5 and 7, respectively. Addition of ASi increased CAL-extractable P to 2.14–2.78 mg P kg -1 and 2.28–4.14 mg P kg -1 at pH 5 and 7, respectively. For goethite, CAL-extractable P increased from 0.11 to 0.16 and 0.06–0.37 mg P kg -1 in reference treatments to 0.26–0.45 and 0.20–0.63 mg P kg -1 at pH 5 and 7 following ASi addition. MSi treatment resulted in lower increases in CAL-extractable P and was responsive only for hematite. Co-application of DOC further increased CAL-extractable P (up to 2.7-fold), although the extracted fraction remained small relative to the total mineral-associated P pool. Scanning Transmission X-ray Microscopy revealed localized Si accumulation coinciding with reduced surface-associated P signals on goethite at pH 5, consistent with altered mineral–phosphate interactions following Si addition. Discussion These results suggest that sustained Si supply from ASi dissolution can modify mineral–phosphate interactions, likely through competitive interaction and increased surface coverage by Si, whereas single-dose MSi inputs exert weaker effects. Although the observed increases represent only a small fraction of the mineral-associated P pool, such shifts toward more exchangeable P forms may influence the replenishment of soil solution P. While the experiments were conducted under simplified mineral suspension conditions, the findings indicate that silica inputs may affect labile P pools in Fe-oxide-rich agricultural soils.