Low carbon availability in paleosols nonlinearly attenuates temperature sensitivity of soil organic matter decomposition.

Temperature sensitivity (Q ) of soil organic matter (SOM) decomposition is an important parameter in models of the global carbon (C) cycle. Previous studies have suggested that substrate quality controls the intrinsic Q , whereas environmental factors can impose large constraints. For example, physical protection of SOM and its association with minerals attenuate the apparent Q through reducing substrate availability and accessibility ([S]). The magnitude of this dampening effect, however, has never been quantified. We simulated theoretical Q changes across a wide range of [S] and found that the relationship between Q and the log -transformed [S] followed a logistic rather than a linear function. Based on the unique Holocene paleosol chronosequence (7 soils from ca. 500 to 6900 years old), we demonstrated that the Q decreased nonlinearly with soil age up to 1150 years, beyond which Q remained stable. Hierarchical partitioning analysis indicated that an integrated C availability index, derived from principal component analysis of DOC content and parameters reflecting physical protection and mineral association, was the main explanatory variable for the nonlinear decrease of Q with soil age. Microbial inoculation and C-labelled glucose addition showed that low C availability induced by physical protection and minerals association attenuated Q along the chronosequence. A separate soil incubation experiment indicated that Q increased exponentially with activation energy (E ) in the modern soil, suggesting that SOM chemical complexity regulates Q only when C availability is high. In conclusion, organic matter availability strongly decreased with soil age, whereas Michelis-Menten kinetics defines the Q response depending on C availability, but Arrhenius equation describes the effects of increasing substrate complexity.