Reforestation makes a minor contribution to soil carbon accumulation in the short term: Evidence from four subtropical plantations

•Effects of reforestation on soil carbon storage were evaluated in five stand ages.•Plantation types did not show a significant effect on soil carbon storage.•Soil carbon showed a similar change pattern for plantations and natural recovery.•Reforestation makes a negligible contribution to soil carbon accumulation.•Plant growth and soil carbon accumulation is asynchronous.

Reforestation increases substantial carbon stock in plant biomass. However, reforestation's effect on soil carbon accumulation remains unclear, which hampers our understanding of carbon cycling in forest ecosystems. The change patterns of soil carbon storage in four young plantations, Eucalyptus urophylla monoculture (EU), Acacia crassicarpa monoculture (AC), Castanopsis hystrix monoculture (CH), a mixed plantation of 10 native tree species (MX), and a naturally recovered shrubland (NS), were compared at five stand ages during development in subtropical China. We observed that plant biomass was higher in plantations with fast-growing species (i.e. EU and AC) than with slow-growing species (i.e. CH and MX). However, no significant differences in soil carbon storage were observed among the four plantations with the same stand ages. Meanwhile, there were no significant differences in soil carbon storage among the four plantations and NS. Furthermore, soil carbon storage exhibited a similar change pattern for the four plantations and naturally recovered shrubland during the 10-year period of early vegetation development. Specifically, soil carbon storage decreased slightly and non-significantly during the first 4 years (from 23.84 Mg ha−1 to 20.79 Mg ha−1) and increased thereafter (35.85 Mg ha−1 in 10-year-old plantations). These results suggest that plant biomass increment and soil carbon accumulation were unsynchronized, and early reforestation had no significant effect on soil carbon accumulation. We conclude that plantations did not accelerate carbon sequestration in soils at early developmental stages compared with natural recovery and plant biomass may not be an appropriate index for evaluating soil carbon sequestration in young plantation.