Soil microaggregates store phytoliths in a sandy loam

Phytoliths are fine silt-sized amorphous silica particles that form in living plant tissues. Once deposited in soils through plant debris, they may dissolve and increase the fluxes of silicon (Si) towards the biosphere and hydrosphere, thus enhancing positive Si impacts on e.g., plant health and carbon fixation by marine diatoms. Here we analyzed the role of soil aggregates in phytolith protection against dissolution. We investigated the distribution of phytoliths in the size fractions of a sandy loam topsoil subjected to two long-term treatments: conventional (CT) and no tillage (NT). The topsoil size fractions were separated through wet sieving and named, respectively: macroaggregates (250–2000 µm), free microaggregates (50–250 µm), and silt + clay (<50 µm). Protected microaggregates held within macroaggregates were extracted using a constant and continuous water flow process. We assessed the pool of phytoliths through heavy liquid separation and alkaline dissolution (Na2CO3), and we assessed the bioavailability of Si using CaCl2 extraction. As expected, NT generates larger amounts of aggregates than CT. Concentrations and stocks of phytoliths determined by heavy liquid separation are ten times higher than those measured by Na2CO3 in bulk soil and size fractions. Soil microaggregates contribute for over 60% to the pool of phytoliths, which control bioavailable Si. The release of bioavailable Si from microaggregates is slower than that from the silt + clay fraction, suggesting that soil microaggregates can trap phytoliths and protect them from rapid dissolution. No-tillage and associated conservation agricultural practices may thus promote the stabilization of phytoliths in soils and decrease their dissolution rate. We propose that the entrapment of phytoliths in soil aggregates may be one of the processes favoring the persistence of phytoliths in soils and sediments. We expect that this persistence should be enhanced in strongly aggregated soils.