Abstract: Plantations play an important role in carbon sequestration, with different effects of tree species mixture on soil organic carbon. In this study, we examined the effects of three typical twospecies mixed plantations, Pinus massoniana + Castanopsis hystrix (PC), Pinus massoniana + Erythrophleum fordii (PE), and Erythrophleum fordii + Mytilaria laosensis (EL), on aggregate organic carbon and its fractions in the south subtropical area, as well as the driving factors of organic carbon at different aggregate levels. The results showed that EL promoted the formation of large macroaggregates. The highest value of mean weight diameter of aggregates was observed in EL (1.71 mm), followed by PE (1.39 mm) and PC (1.26 mm). The macroaggregate organic carbon content (>2 mm) was 1.8 and 1.4 times higher in EL than those of PC and PE, respectively. EL promoted the accumulation of particulate organic carbon within aggregates, which was mainly originated from coarse and fine organic particulate carbon within large macroaggregates, and coarse particulate organic carbon within small macroaggregates. EL promoted the stability of aggregates and the accumulation of aggregate organic carbon by increasing inputs of leaf litter and roots.

Key words: mixed plantation, aggregate stability, aggregate organic carbon, carbon fraction