The temperature sensitivity of stable organic carbon storage rises with increasing soil salinity

Soil salinisation is a key determinant in soil fertility decline, exerting a direct negative impact on soil organic carbon. In the context of global warming, investigating the response mechanisms of soil organic carbon pools with varying salinity levels to climate change is essential for accurately assessing the carbon cycle and emission potential of degraded soils. Based on soil samples (B1-B6) collected along a coastal salinity gradient, indoor incubation experiments were conducted at 15 °C and 25 °C to characterise soil respiration and its temperature sensitivity (Q10). Double-exponential models were used to simulate soil organic carbon (SOC) mineralisation, characterising active and stable organic carbon pools. The results demonstrated that the Q10 value of the stable organic carbon pool (7-8% of SOC mineralisation) was 103% higher than that of the active organic carbon pool (the initial 1% of SOC mineralisation). The Q10 value of the stable organic carbon pool was 32.6% higher at the high-salinity sites (B1, B2) than at the low-salinity sites (B4, B5). Soil organic carbon, total nitrogen (TN), and total salt (TS) were key regulators of Q10. The Q10 of the active organic carbon pool correlated positively with SOC and TN but negatively with TS, whereas the stable pool showed the opposite trends. The stable organic carbon pool exhibits a salinity-amplified Q10, implying that predictive models must account for this mechanism to avoid substantially underestimating carbon losses from degraded saline soils.