Background Research on the composition of aggregates and the distribution of organic carbon in soil will be beneficial for understanding the interaction between soil structure and function at the molecular scale.

Methods Three typical artificially restored forests after a 30-year restoration in degraded red soil were selected in this study, which were Pinus massoniana-multiple layer broadleaf forest (PB), Schima superba-Pinus massoniana mixed forest (SP), and broadleaf forest (BF), respectively. Methods of dry sieve and wet sieve were applied to investigate the composition of aggregates and the distribution of organic carbon in different layers (0-60 cm) of soil in each of the three forests, following which the correlations between soil aggregate organic carbon and soil total organic carbon were also determined.

Results For all three forests, particles larger than 2 mm in diameter constituted more than 60% of mechanical-stable aggregates in soil, while particles less than 0.05 mm in diameter made up the majority of water-stable aggregates in soil. The broken rate of aggregate structure in the three forests ranked as:BF (53.38%-84.27%) > SP (52.22%-70.86%) > PB (22.0%-47.83%). The organic carbon mass fractions of both mechanical-and water-stable aggregates were the highest in the soil of PB. Along with the increase of soil depth, organic carbon mass fraction of soil aggregates all showed a decreasing trend in the three forests. Compared with the total organic carbon at all soil layers, macro water-stable aggregates (>0.25 mm in diameter) had higher organic carbon mass fraction, while micro water-stable aggregates ( < 0.25 mm in diameter) showed lower mass fraction, suggesting that organic carbon may play an active role in the formation and the water-stability of macro aggregates. Furthermore, soil aggregate organic carbon showed a positive correlation with soil total organic carbon.

Conculsions PB had the best quantity and quality of soil aggregates among the three forests, which can be attributed to the high density and coverage of shrubs/herbs. Therefore, to accelerate the improvement of soil structure and the recovery of soil functions in degraded lands, we should increase the input of organic carbon by appropriately creating a higher density and coverage of shrubs/herbs at the early stage of forest restoration, which would promote the formation and the stability of soil aggregates. This may provide a scientific basis for the selection and optimal allocation of forest types in the restoration of degraded red soil in the south of China.