Temporal dynamics and season-dependent warming responses of reactive soil silicon fractions in a subtropical oak forest

Introduction Seasonal variation in plant activity and soil processes can influence soil silicon (Si) cycling, but the season-dependent warming responses of operationally defined reactive soil silicon fractions remain poorly understood. We therefore tested whether warming effects on each of five reactive soil Si fractions depended on sampling season in a northern subtropical secondary Quercus acutissima forest. Methods Treatments comprised an unwarmed control (CK), +2 °C (T2) and +4 °C (T4). Mineral soil (0–10 cm) was sampled in February, May, August and November in 2021–2022. Sequential extraction quantified calcium chloride (CaCl 2 )-extractable Si (Si dis ), weakly adsorbed Si (Si sorb ), organic matter-associated Si (Si org ), iron (Fe)/aluminum (Al) oxide-associated Si (Si ox ) and amorphous Si (Si amor ). Baseline-adjusted mixed-effects models evaluated warming, sampling season and their interaction for May, August and November; February 2022 observations were analyzed separately using baseline-adjusted ANCOVA. Results Si amor accounted for 88.73–89.36% of the summed concentrations of the five fractions. All fractions differed significantly among the three seasons in the primary analysis, but none showed a significant warming main effect after false discovery rate correction. Warming × sampling season interactions were significant for Si org and Si amor and remained supported under alternative correlation structures and log transformation. In autumn, baseline-adjusted T4-CK contrasts, estimated with equal weighting across 2021 and 2022, were 13.32 mg kg −1 (Tukey-adjusted 95% CI: 4.642–22.000) for Si org and 0.49 g kg −1 (Tukey-adjusted 95% CI: 0.212–0.765) for Si amor . Evidence for an autumn warming response in Si dis remained exploratory, whereas Si sorb and Si ox showed no significant warming responses. Discussion Under the experimental conditions and operational extraction framework examined here, these findings identify season-dependent warming responses in Si org and Si amor and suggest that averaging across seasons may obscure these responses.