Differences in SOM Decomposition and Temperature Sensitivity among Soil Aggregate Size Classes in a Temperate Grasslands

The principle of enzyme kinetics suggests that the temperature sensitivity ( Q 10 ) of soil organic matter (SOM) decomposition is inversely related to organic carbon (C) quality, i.e., the C quality-temperature (CQT) hypothesis. We tested this hypothesis by performing laboratory incubation experiments with bulk soil, macroaggregates (MA, 250–2000 μm), microaggregates (MI, 53–250 μm), and mineral fractions (MF, <53 μm) collected from an Inner Mongolian temperate grassland. The results showed that temperature and aggregate size significantly affected on SOM decomposition, with notable interactive effects ( P <0.0001). For 2 weeks, the decomposition rates of bulk soil and soil aggregates increased with increasing incubation temperature in the following order: MA>MF>bulk soil >MI( P <0.05). The Q 10 values were highest for MA, followed (in decreasing order) by bulk soil, MF, and MI. Similarly, the activation energies ( E a ) for MA, bulk soil, MF, and MI were 48.47, 33.26, 27.01, and 23.18 KJ mol −1 , respectively. The observed significant negative correlations between Q 10 and C quality index in bulk soil and soil aggregates ( P <0.05) suggested that the CQT hypothesis is applicable to soil aggregates. Cumulative C emission differed significantly among aggregate size classes ( P <0.0001), with the largest values occurring in MA (1101 μg g −1 ), followed by MF (976 μg g −1 ) and MI (879 μg g −1 ). These findings suggest that feedback from SOM decomposition in response to changing temperature is closely associated withsoil aggregation and highlights the complex responses of ecosystem C budgets to future warming scenarios.