Land‐use conversion changes deep soil organic carbon stock in the Chinese Loess Plateau

Abstract Land‐use change is a key factor driving changes in soil organic carbon (SOC) sequestration worldwide. However, the changes in deep (>100 cm depth) SOC stock following land‐use conversion have not been fully elucidated. In this study, to determine the changes in deep SOC stock (to a depth of 400 cm) resulting from conversion of cropland to woodland, shrubland and grassland on the Chinese Loess Plateau, 469 observations from peer‐reviewed publications and original measured data were synthesised. The results were as follows: (a) SOC stock increased significantly at 0–100 and 100–200 cm layers regardless of land‐use conversion types. (b) Carbon loss occurred in the 200–400 cm layers due to land‐use conversion. (c) Changes in SOC stock varied with restoration age, except for conversion of cropland to grassland. Specifically, SOC stock increased with restoration age in the upper 200 cm layers, whereas that in the 200–400 cm layers first increased and then decreased in the middle to later stages under conversion to woodland and shrubland. (d) Initial SOC stock and rainfall zones had significant effects on the changes of deep SOC stock. (e) Furthermore, an accumulation of 1 Mg ha −1 in the upper 100 cm was associated with an approximately 0.45 Mg ha −1 increase in the 100–400 cm soil layers. These results indicate that land‐use conversion, particularly conversion of cropland to woodland, changes deep (>100 cm) SOC stock, and restoration age should be taken into consideration when assessing deep carbon sequestration.