Temperature Sensitivity of SOM Decomposition Governed by Physical Protection and Microbial Properties: A Multiple Approach-based Analysis

Temperature sensitivity (Q₁₀) of soil organic matter (SOM) decomposition is a crucial parameter for predicting the fate of soil carbon (C) under global warming, thereby determining the magnitude of C-climate feedback. However, due to the lack of empirical evidence on its controlling mechanisms, the inaccurate parameterization of Q₁₀ for different soil horizons and C pools in Earth System Models (ESMs) constitutes to one of the key sources of model uncertainties. In this study, we used a long-term incubation combined with two-pool model and controlled experiments to provide direct evidence on the mechanisms underlying the Q₁₀ difference in active and slow C pool between soil depths. Our results revealed that low microbial abundance and strong physical protection were two co-existing mechanisms underlying the low Q₁₀ in subsoil. Moreover, microbial properties were the main determinant of Q₁₀ in active pool, whereas physical protection exerted more important control in the slow pool. The different contributions of the two factors between two C pools were further confirmed by the microorganism reciprocal transplant and aggregate crushed experiment. Microorganism reciprocal transplant decreased Q₁₀ difference between soil horizons in the active pool, but had no effect in the slow pool. In contrast, aggregate crushing reduced Q₁₀ difference in the slow pool, but made no sense in the active pool. Taken together, our results demonstrate the fundamental role of microbial properties and physical protection in regulating Q₁₀, highlighting the importance of incorporating these two variables into ESMs for better predicting the long-term soil C dynamics under climate warming.