Fig. 3: Time series of biogeochemical proxies at 135 cm depth.

Proxies for mineral weathering intensity and weathering congruency display a degree of temporal coupling with saturation indices of secondary minerals. These proxies show generally consistent trends with organic matter decomposition rate. In the deep soil, fluxes of SO₄²⁻ and Cl⁻ are suppressed by precipitation of secondary minerals. Fluorescence index (FI) shows an opposite trend to the parameter that defines dissolved organic matter bioavailability. Fluxes of organic matter decomposition products are similar to the FI time series, suggesting that “primed” microbial activity accelerates organic matter decomposition. The periods with specific biogeochemical characteristics are highlighted with background colors. For example, light purple, light green, pink, and blue bands represent the periods characterized by low, high, low to high, and high to low weathering congruency, respectively, during which specific ultraviolet absorbance at 254 nm (SUVA₂₅₄) and fluorescence index were relatively stable. Small red arrows indicate changing trends (increasing or decreasing) in biogeochemical proxies. The light yellow band defines the period with FI increase, SUVA₂₅₄ decrease and little variation in weathering congruency, which can serve as a control group for evaluating the influence of weathering congruency on the priming effect. Note that the time-series scale is not uniform. Abbreviations: corrected dissolved inorganic carbon (DIC) fluxes—f_DIC-corr, corrected dissolved organic carbon (DOC) fluxes—f_DOC-corr, SO₄²⁻ fluxes—f_SO4, saturation index—SI, phosphorous fluxes—f_P, normalized Si concentration/Na concentration—[Si]/[Na]_norm.