Aboveground plants influence heterogeneously soil organic carbon (SOC) and its labile fractions after mixed afforestation: Three afforestation types of Masson's pine in the Upper Yangtze River, China.

Mixed forests generally have a higher carbon sequestration potential than pure forests. However, the effects of different types of mixed afforestation on soil organic carbon (SOC) and its labile fractions still remain controversial. We examined the concentrations of each SOC labile fraction at 0-50 cm soil depth, understory plant communities, stand plant biomass and studied their integrated effects on soil carbon stocks in three types of Pinus massoniana afforestation: a monoculture (MPF), a mixed forest with Cunninghamia lanceolata (MCLMF) and a mixed forest with Liquidambar formosana (MLMF). The results showed that the SOC stocks, i.e., concentrations of SOC and its labile fractions, across soil depths in all three afforestation types decreased with soil depth and ranked in the following order: MCLMF > MPF > MLMF. The concentrations of SOC and its labile fractions displayed a significant positive correlation with the diversity and biomass of understory plants and a significant negative correlation with tree biomass. The MCLMF had the largest SOC stocks (83.45 ± 7.59 Mg ha) and the smallest aboveground plant biomass carbon stocks (85.2 ± 4.07 Mg ha), while those of the MLMF were the opposite (SOC stocks, 35.63 ± 4.47 Mg ha; plant biomass carbon stocks, 144.28 ± 1.19 Mg ha). The forest carbon stocks (comprising both SOC and plant biomass carbon pools) were ranked as MLMF > MCLMF > MPF. Our results revealed that the diversity and biomass of understory plants can improve the stocks of SOC and its labile fractions, whereas trees may weaken the role of understory plants. In this sense, the diversity and biomass of understory plants should be emphasized in the process of mixed afforestation, especially mixed broadleaf-conifer afforestation, to increase SOC sequestration.