Microtopographic heterogeneity regulates the temperature sensitivity of SOM decomposition in permafrost forests

Permafrost forests store substantial soil organic matter (SOM), which is vulnerable to climate warming. However, predictions of SOM-climate feedback remain highly uncertain because limited understanding of how fine-scale terrain variability regulates SOM decomposition and its temperature sensitivity (Q10), particularly along the gentle, continuous slopes that dominate permafrost landscapes. In this study, we established paired transects on north-facing (N-facing) and south-facing (S-facing) slopes in a permafrost forest in China and classified eight microtopographic positions for each aspect based on subtle variations in elevation and slope. Using high-frequency CO₂ measurements collected at 1-s intervals, we quantified the SOM decomposition rates (RS) and Q10 across microtopographic gradients. The results showed that soils on N-facing slope exhibited significantly higher RS20 (mean RS at 20 °C; 27.95 μg CO2 g-1 soil day-1) than soils on S-facing slope (24.07 μg CO2 g-1 soil day-1). In addition, RS20 declined significantly along the microtopographic gradient on both aspects. In contrast, Q10 did not differ significantly between different aspects at the aggregate level but displayed distinct aspect-dependent microtopographic patterns. Structural equation modeling further revealed that the pathways through which microtopography regulates SOM decomposition and Q10 differed markedly between different aspects. Specifically, microtopography primarily influenced SOM decomposition and Q10 through its effects on the local microenvironment, which subsequently regulated microbial activity and soil nutrient availability. These findings highlight the importance of incorporating microtopography into the representation of soil C processes in complex terrains and provide a critical basis for improving the spatial precision of the regional C cycle under climate warming.