Changing sources of bioavailable silicon during pedogenesis

The importance of silicon (Si) in the global biogeochemical cycles is of the utmost importance. Hence, it acts as an essential nutrient to a number of marine organisms, accounting for up to 50% of oceanic carbon fixation. Furthermore, the chemical weathering of Si-bearing minerals controls long-term sink for atmospheric CO2. In the last twenty years, several studies highlighted the key role of the biological pumping in the global Si cycle. Indeed, plant deposits amorphous silica called “phytoliths” inside the cells of stems and leaves. Once returning to soil via litterfall, phytoliths can be used by paleobotanists to reconstruct past vegetation given specific shapes for different species as well as long resilience in soils. On the other hand, some biogeochemical studies highlighted the fact that biogenic silica can be rapidly dissolved and then recycled by vegetation through Si uptake. In this PhD project, we want to study the sources of bioavailable Si for plants over soil weathering degree. More precisely, we want to test the hypothesis that phytoliths could become the main source of bioavailable Si in highly weathered soils. To do so, we sampled 7 sandy soils over a long-term chronosequence in SW Australia including Holocene (≤ 6.5 ka), Middle Pleistocene (120-500 ka) and early Pleistocene (~ 2000 ka) dunes. Pedogenesis includes decarbonation, iron oxides release, desilification and strong eluviation. For each chronosequence stage, we sampled by pedogenic horizon, to a minimal depth of 1.5 m. We measured the so-called “bioavailable Si” in each horizon, using a 0.01M CaCl2 extractant. We also performed some XRD analysis. Si extracted with CaCl2 strongly increased with the end of decarbonation, from the Holocene to Middle Pleistocene dunes (from 2 to 7.5 mg.kg-1). It then slightly decreases until the most weathered soil, developing on an Early Pleistocene dune (around 3 mg.kg-1). We noticed that the highest concentrations of bioavailable Si have been encountered right after the decarbonation, during the iron release process and the formation of secondary minerals such as kaolinite, detected only in those soils. Henceforth, we suggest that secondary Si-bearing minerals have probably the strongest potential for releasing Si into soil solution. However, the bioavailable Si concentration in the highly weathered soil (E horizon of at least 5 meters) remains higher than in the first stages of the sequence, with an XRD analysis showing 100% of quartz. In this system, it is likely that phytoliths have become the only source releasing bioavailable Si into soil solution. However, it is premature to draw general conclusions and phytoliths physical extraction, total elemental analysis and the use of other extractants (acetic acid, Na2CO3 following the ratios Si/Al, Si/Fe and Si/Mg) will be very useful to go much further in the hypothesis.