Net acidification preserves phytoliths over a century in a bare fallow soil

Acid soils cover approximately 40% of the world's land area and 50% of potential arable land. Net acidification is a natural process occurring wherever annual precipitation exceeds evapotranspiration. It can be accelerated or blocked by the use of fertilizers and amendments. Phytoliths are fine-sized biogenic silica particles that can either dissolve to contribute to the global cycle of silicon (Si) or persist in soils and sediments over millennia

Here, we study the resilience of phytoliths in a loess derived Luvisol maintained as a bare fallow for nearly one century (1929-2019). We compared a control with plots affected by the prolonged use of ammonium sulfate [(NH₄)₂SO₄] or calcium carbonate [CaCO₃]. A diachronic dynamic of the kinetic release of dissolved Si (DSi), aluminum (Al) and germanium (Ge) was carried out using a dilute saline solution. Molar Al/Si and Ge/Si ratios were used to identify the DSi source and pH was measured at each kinetic step. Phytolith content was assessed by heavy liquid separation. Extracted phytoliths were separated as not-, little- and highly-weathered by counting dissolution cavities per 100 µm².

Over the period 1929-2019, pH decreases from 6.0 to 5.7 with no input, from 6.0 to 3.5 under prolonged use of (NH₄)₂SO₄, but increased from 6.0 to 8.5 under systematic liming with CaCO₃. Using the total reserve in bases (TRB) as a proxy for the soil Acid Neutralizing Capacity (ANC), we observed that TRB was 110 cmol₍₊₎kg^-1 in 1929. TRB (cmol₍₊₎kg^-1) decreased to 94 and 84 with no input and (NH₄)₂SO₄, respectively, while it increased to 160 with CaCO₃ indicating, respectively: net acidification, accelerated net acidification and net alkalinization.

Clay minerals and phytoliths were the privileged source of DSi under prolonged use of (NH₄)₂SO₄ and CaCO₃, respectively. Phytolith content was around 14 g kg^-1 in 1929. It slightly decreased over a century with no input and CaCO₃ supply, but not under prolonged use of (NH₄)₂SO₄. In addition, the proportions of not-little-highly weathered phytoliths were 26-36-38 % with no input, 21-37-42% with CaCO₃ and 42-33-25% with (NH₄)₂SO₄ indicating a better resilience of phytoliths under accelerated acidification.