Soil Carbon Flux Responses to Warming and Drought Are Mediated by Soil Moisture and Vary Among Quercus Species

Climate change intensifies temperature extremes and drought frequency. However, the interactive effects of warming and drought on soil carbon fluxes remain poorly understood, particularly during extreme temperature events and across plant species. We conducted a factorial experiment examining warming (+3 and +5 °C) and drought effects on soil CO2 emissions and CH4 uptake in one-year-old Quercus variabilis Blume and Quercus acutissima Carruth seedlings during three successive warming periods (period 1, 2–12 July; period 2, 19–30 July; and period 3, 7–18 August 2024). In both species, warming initially increased CO2 emissions by 23%–26% and subsequently reduced them by 26%–37% during period 2 (coinciding with the rainy season), highlighting critical temperature-moisture interactions. Drought reduced CO2 emissions by 12%–36%. CO2 emissions were positively correlated with soil moisture (r = 0.45–0.56). In Q. variabilis, warming initially reduced CH4 uptake at +5 °C; however, during period 3, uptake increased by 44.5% and 24.8% under +3 and +5 °C treatments, respectively, while the drought treatment reduced CH4 uptake by 11.7%. Contrarily, Q. acutissima showed no warming or drought treatment effects. These findings demonstrate that soil carbon flux responses to extreme climate conditions exhibit nonlinearity and are affected by soil moisture and species type.