Afforestation of Terrace Promotes Soil Organic Carbon Accumulation but Reduces Soil Organic Carbon Stability in Mountainous Regions of Southern China

Afforestation of farmland is considered a potential strategy to mitigate elevated atmospheric CO 2 concentration by sequestering organic carbon (OC) in plant biomass and soil. However, whether the trends of soil OC stock and stability vary with tree species after afforestation of terraces remains unclear in subtropical mountainous regions. To clarify the effects of tree species on soil OC dynamics, we measured the concentrations, chemical compositions, and δ 13 C values of soil OC, particulate organic C (POC), and mineral–associated organic C (MAOC), along with other soil physicochemical properties at depths of 0–10 cm and 10–30 cm in abandoned terraces, Cunninghamia lanceolata plantations, and Liquidambar formosana plantations in the western Wuyi Mountains of southern China. Results showed that afforestation of terraces significantly increased soil OC stock, with a greater increase under C. lanceolata plantations (52.4%) than under L. formosana plantations (26.8%). At 0–10 cm soil depth, both plantations had significantly greater soil POC and MAOC concentrations than abandoned terraces. Afforestation of terraces only significantly increased the δ 13 C values of OC at 10–30 cm soil depth, indicating accelerated OC turnover in subsurface soils. In both POC and MAOC fractions, afforestation of terraces had a lower proportion of aromatic‐C fractions, and thus exhibited a lower OC chemical stability index. Soil OC fractions correlated significantly with soil C:N ratio and minerals at 0–30 cm depths. These findings suggest that, regardless of tree species, afforestation of terrace promotes soil OC accumulation at the cost of reduced OC stability in the mountainous regions of southern China.