Impact of ecosystem water balance and soil parent material on silicon dynamics: insights from three long-term chronosequences

Recent studies demonstrate a strong influence of soil age on long-term silicon (Si) dynamics in terrestrial ecosystems, but how variation in ecosystem water balance and soil parent material impact this trajectory is unknown. We addressed this by studying a 2-million-year dune chronosequence in southwestern Australia characterized by a positive water balance (+ 50 mm year⁻¹) and a lower carbonate concentration in the parent sand (5%) compared with two chronosequences already characterized (− 900 and − 750 mm year⁻¹; 88 and 74%). We sampled soils from the progressive and retrogressive phases of ecosystem development to quantify pedogenic reactive Si (extracted in ammonium oxalate and oxalic acid), phytoliths (biogenic Si), and plant-available Si (extracted in dilute CaCl₂). Silicon mobilization was buffered by carbonate in the early stages of the two carbonate-rich drier chronosequences, as previously highlighted, but not in the carbonate-poor wetter chronosequence. Reactive pedogenic Si and plant-available Si did not peak at intermediate stages in the carbonate-poor wetter chronosequence, where almost no clay formation occurred, as it did in the carbonate-rich drier chronosequences during clay formation after carbonate loss. This is probably due to a combination of lower content of weatherable minerals in the soil parent material and higher weathering rates. Phytolith stocks were similar across the three chronosequences, suggesting that a climate-driven increase in biomass and associated phytolith production in wetter sites counterbalance the higher phytolith dissolution rates and physical translocation. Together, these results demonstrate that the initial carbonate concentration in the soil parent material and subsequent mineralogical evolution drive long-term soil Si dynamics, and suggest a significant influence of climate-induced variation in biomass production on the Si biological feedback loop, even in old and highly desilicated environments.