Soil carbon dynamics during grass restoration on abandoned sloping cropland in the hilly area of the Loess Plateau, China

- SOC and SIC increase during grass restoration after cropland abandonment.
- SOC and SIC storages increase during grass restoration after cropland abandonment.
- Grass restoration leads to higher SOCs/STCs ratio and lower SICs/STCs ratio.

Secondary vegetation succession on abandoned cropland significantly influences soil C-pool dynamics in arid and semi-arid region. Much work on soil C-pool dynamics has been focused on soil organic carbon (SOC), whereas considerably less attention has been given on soil inorganic carbon (SIC). Moreover, the relationship between SOC and SIC during natural succession is unclear. The objective of this research was to investigate SOC and SIC storage dynamics in natural restored grassland following cropland abandonment. We measured SOC and SIC content in paired grassland and cropland sites in the Gaoqiao watershed on the Loess Plateau, China. The grassland sites were established on cropland abandoned for 5, 13, 24, and 32 years. Top soil OC was significantly higher after 10-year restoration, especially in the 0-5 cm soil depths; deeper soil OC was lower in younger grasslands (< 20 yr), and significantly higher in older grasslands (> 20 yr). SIC was significantly higher after abandonment, but varied largely among successional grasslands. SOC storage (SOCs) decreased in the younger grassland (5 yr), and SOCs (0-100 cm) was significantly higher after 10-year restoration. SIC storage (SICs) increased after abandonment in the 0-100 cm soil depth, but showed various magnitudes among restored grasslands. SIC storage (SICs) originally contained most part of STC storage (STCs), and grass restoration after cropland abandonment led to higher SOCs/STCs ratio and lower SICs/STCs ratio. Our results indicate that grass restoration enhanced the accumulation of SOC and SIC storages, but with inverse proportional contributions to STC storage. We suggest that SIC should be as much focused as SOC during grass restoration in the arid Loess Plateau.