Drying-rewetting cycles decrease temperature sensitivity of soil organic matter decomposition

Soil organic carbon (SOC) decomposition is crucial in the global carbon cycle. Its sensitivity to warming significantly impacts climate change. However, the effect of soil drying-rewetting, a consequence of climate change-induced water cycling shifts, on SOC decomposition sensitivity remains poorly understood. This study investigated how drying-rewetting cycles affect the temperature sensitivity (Q10) of SOC decomposition and its underlying mechanisms. We collected soils from two farmlands with 23- and 33-year C3 C4 vegetation switches The soils were incubated at 20 °C or 30 °C for 180 days under alternate drying-rewetting cycles (100 %−20 % water holding capacity, WHC) or constant moisture (60 % WHC). Using 13C natural abundance, we differentiated CO2 sources from recent SOC (C4, <23 or <33 years) and old SOC (C3, >23 or >33 years). Results showed that warming and drying-rewetting enhanced total SOC decomposition. Across moisture conditions, the Q10 of old SOC was 0.25−0.40 units higher than that of recent SOC. Six drying-rewetting cycles decreased the Q10 of total, recent, and old SOC by 0.30−0.44 units compared to constant moisture, as warming became less dominant during the drying-rewetting process. This indicates that the commonly used Q10 might be overestimated under constant moisture, suggesting that the feedback of SOC pools to climate warming might be weaker than previously expected under real soil moisture fluctuations.