Stronger aggregation reduces temperature sensitivity of soil organic carbon decomposition in croplands

The temperature sensitivity (Q₁₀) of soil organic C (SOC) decomposition is an important parameter to predict C dynamics under climate change. Given that SOC is mainly protected by aggregates and minerals, differentiating the Q₁₀ of the two C fractions helps to explain bulk soil C dynamics. In the present study, we collected agricultural soils from adjacent paddy and upland areas in mid-temperate (Mollisols) and subtropic (Ultisols) regions of China. We employed density fractionation to separate aggregate-protected and free mineral-associated C fractions of soil samples and determined the Q₁₀ of SOC and the two C fractions at 15 and 25°C incubated conditions. Results showed that the Q₁₀ of SOC for Mollisols were lower than that for Ultisols, with an exception of aggregates in upland soils. Aggregate-protected C had lower Q₁₀ than free mineral-associated C, except in the upland Mollisols. The Q₁₀ of SOC was negatively correlated with the proportion of C protected in aggregates, whereas it was positively correlated with the proportions of mass or C of free minerals. Given that the mass and C proportion of aggregates in bulk soils of Mollisols were 271% and 80% higher than of Ultisols, respectively, the SOC of Mollisols exhibited lower Q₁₀ than Ultisols. Therefore, stronger soil aggregation and higher proportion of aggregate-protected C contributed to the lower temperature sensitivity of SOC in Mollisols. Consequently, agricultural practices aimed at promoting soil aggregation will alleviate SOC loss under future global warming scenarios.