Substrate availability reconciles the contrasting temperature response of SOC mineralization in different soil profiles

Purpose: The purpose of this study was to investigate how changes in substrate availability (stimulating root exudate input) affect the temperature response (Q ₁₀) of soil organic carbon (SOC) mineralization across different soil profiles to increase our ability to predict the response of soil organic matter dynamics to climate change. Materials and methods: We sampled the topsoil and subsoil of two typical mineral soil profiles and one buried soil profile. Soils were incubated at 10–25 °C at 0.75 °C intervals, SOC mineralization rates were continuously measured with and without glucose addition, and Q ₁₀ was calculated. Results and discussion: Our results showed that Q ₁₀ decreased with increasing depth in typical mineral soils, but decreased before increasing with depth in buried soil. As expected, substrate addition significantly increased Q ₁₀ across soil depths; however, the magnitude of this increase (ΔQ ₁₀) differed with soil depth and type. Unexpectedly, in typical mineral soils, ΔQ ₁₀ was higher in topsoil than in subsoils, and vice versa for buried soil. ΔQ ₁₀ was negatively correlated with initial soil substrate availability (CAI) and positively correlated with soil inorganic N. Conclusions: Overall, our results suggested that increased substrate availability under climate change scenarios (i.e., increased root exudates with elevated CO₂ concentrations) could further strengthen the temperature response of SOC mineralization, especially in soils with high inorganic N content or regions with high N deposition rates.