Stoichiometry of base cations and silicon during weathering of a deep soil profile derived from granite

The evaluation of the stoichiometry (BCs:Si) of base cations (BCs, including K⁺, Na⁺, Ca²⁺, and Mg²⁺) and silicon (Si) during soil mineral weathering is essential for quantifying accurate soil acidification rates. The purpose of this study was to explore the differences and influencing factors of BCs:Si in a deep soil profile derived from granite with different extents of mineral weathering. Soil was collected from Guangzhou, China, located in a subtropical region. The deep granitic soil profile includes different genetic soil horizons and saprolite. Soil type was Typic Acidi-Udic Argosols. First, soil physical, chemical, and mineralogical properties were determined. To ensure that the BCs and Si originated from the mineral weathering process, the soil exchangeable BCs were washed by elution treatment. Then, the BCs:Si value could be obtained through a leaching experiment of simulated acid rain using the batch method. The results show that soil profile from the surface horizon to saprolite has differences in the physical, chemical, and mineralogical properties among different soil genetic horizons. The BCs:Si values in the granitic soil during weathering were 0.3–3.7. The BCs:Si value was 1.7 in the surface horizon (A), 1.1–3.7 in the argillic horizon (Bt), and 0.3–0.4 in the cambic (Bw) and transition horizons (BC), including horizon C (saprolite). The general pattern of BCs:Si values in different horizons of the granite soil developed into Argosols was Bt > A > Bw, BC, and C. The amount of Si released in the horizons with more clay particles was less, and the BCs:Si value during weathering was higher. The saprolite with weak weathering intensity had a lower release amount of BCs and smaller BCs:Si values during weathering. Although BCs:Si values were influenced by weathering intensity, they did not correlate with the chemical index of alteration (CIA). Comprehensive analysis showed that the BCs:Si values were a combined result of the following factors: clay, feldspar, kaolinite, and soil organic matter content, pH value, and CIA. Soils derived from the same parent material at different genetic horizons had different BCs:Si values. The BCs:Si values of soils with different weathering extents in this study were not significantly affected by individual soil properties. The release of Si and BCs is the result of the synthesized influences of soil mineral composition and physical and chemical properties. The main controlling factors of BCs:Si in different parent material types still require extensive research to accurately estimate the soil acidification rate. The wide variance of BCs:Si values in the deep soil profile indicates that H⁺ consumed by soil mineral weathering is very different in the soil with different weathering intensity derived from the same parent matter. Therefore, the estimation of soil acidification rate based on H⁺ biogeochemistry should take into account the specific BCs:Si value.