Redox-dependent surface passivation reduces phytolith solubility

Abstract

Phytoliths are an important silicon (Si) source for plants due to their high solubility, yet some phytoliths are stable for centuries and used in paleosciences to reconstruct past vegetation. Here, we show that this discrepancy can be resolved by considering interactions of phytoliths with soil solution constituents. We tested the hypothesis that recurring alternating redox conditions reduce the solubility of phytoliths due to the formation of protective coatings. Phytoliths from rice plants were repeatedly exposed to solutions from a typical paddy soil under either exclusively oxic conditions or alternately anoxic−oxic conditions. Changes in solution chemistry and surface properties (chemical composition, ζ potential, surface area and porosity) of exposed phytoliths were linked to phytolith dissolution kinetics over 30 days (in 0.01 M CaCl2 at pH 7). We found that phytoliths exposed to alternating redox conditions accumulated iron and carbon at their surfaces, with changes in other properties being little. Dissolution rates decreased after exposure to soil solution regardless of redox conditions, but more pronounced under alternating redox conditions. After six exposure steps, the Si dissolution rate for the alternating redox treatment decreased by factor of 3.2 (oxic treatment: 2.1), with the Si release rate being negatively correlated with total iron contents. We conclude that alternating redox conditions can cause increased phytolith stability due to surface passivation and, in turn, reduced Si availability to plants.