The effects of afforestation on soil organic and inorganic carbon: A case study of the Loess Plateau of China

The determination of the changes in soil organic carbon (SOC) and inorganic carbon (SIC) in the subsoil following afforestation is meaningful and necessary for assessing carbon sequestration, but such knowledge is limited. In this case study, a paired-site approach was used to determine the differences in the SOC and SIC stock in the topsoil and subsoil, respectively, between a black locust (Robinia pseudoacacia L.) forest site (converted from cropland 30 years ago) and a cropland site in the middle of the Loess Plateau of China. Compared with the cropland, the SOC stock was significantly greater under the forest in both the top 20 cm and the subsoil (30–60 cm layer). The annual litter input under the forest was more than twice that of the cropland, and the fine root biomass was significantly higher in the forest. We conclude that the higher litter input and fine root biomass may partly contribute to the greater SOC in the forest. In addition, the soil nitrogen (N) content changed in synchronicity with SOC during afforestation, which indicates that SOC accumulation in the top/subsoil may be determined by the increase in soil N in these layers. In contrast, the SIC stock in the top 20 cm of the forest was significantly lower than that of the cropland. However, this decrease in the SIC level in the topsoil of the forest was offset by an increase in SIC in the subsoil (60–100 cm). The change in SIC along the soil profile following afforestation could be explained by the dissolution and leaching of SIC from the topsoil and subsequent precipitation in the subsoil. The dissolution and leaching of SIC in the forest topsoil were due to the high biological activity associated with the high aboveground litter input, fine root biomass and SOC stock as well as the high soil water content, whereas the precipitation of the leached SIC in the subsoil was a result of the dramatic decrease in the soil water content and fine root biomass in the subsoil. These findings suggest that soil can accumulate organic carbon in the topsoil and subsoil following black locust plantation establishment on cropland (as in the Loess Plateau study area) and that this type of cropland to forest plantation conversion in this area can redistribute SIC along the soil profile without affecting the net SIC accumulation.

► SOC and SIC were examined by a paired-site approach following afforestation.
► SOC pool increased in both plow layer and subsoil during afforestation.
► Afforestation redistributed SIC along soil depth without affecting SIC pool.