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 millenniaHere, 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.