Aggregation reduces the release of bioavailable silicon from allophane and phytolith

Abstract

Phytoliths form in plant tissues as fine, silt-sized amorphous silica particles. Once deposited in soils by plant debris, they can dissolve and feed silicon (Si) fluxes to the biosphere and hydrosphere, enhancing the positive effects of Si on plant health and carbon fixation by marine diatoms. In soils, microaggregates (<250 μm) may entrap phytoliths and protect them from dissolution. Here we aimed at analyzing the role of aggregation in protecting phytoliths and delaying the release of bioavailable Si. Synthetic 32-days aged aggregates were composed of (gkg−1): organic matter (50), aluminosilicate mineral (370), iron (Fe) oxide (60), quartz (500) and rice phytolith (20). Models with either amorphous (Am) or crystalline (Cryst) aluminosilicates and Fe-oxides were tested. The Am model included allophane and ferrihydrite while the Cryst model included kaolinite and goethite. Aggregates were visualized by Scanning and Transmission Electron Microscopy. They were compared to individual compounds and unaggregated mixtures through kinetic Na2CO3 and CaCl2 extraction assessing the pools of phytoliths and bioavailable Si, respectively. Aluminum (Al) and germanium (Ge) concentrations, and pH were measured in the CaCl2 extracts. CaCl2 extractable Si decreased in the order (gkg−1): Am mixture (3.72) > Am aggregate (0.95) > Cryst mixture (0.48) > Cryst aggregate (0.39). Aggregation thus reduced Si release by 3.9- and 1.2-fold in the amorphous and crystalline model, respectively. The Si/Al and Ge/Si atomic ratios showed that allophane and phytolith were the main sources of bioavailable Si in the amorphous and crystalline model, respectively. In contrast to the crystalline model (pH5.0–7.8), the acidic medium in the amorphous model (pH3.7–4.9) enhanced allophane dissolution. Aggregation thus protected both allophane and phytolith from dissolution, and reduced the release of bioavailable Si, the source of which depended on the component stability and pH.