Bedrock outcrops promote rather than inhibit soil carbon sequestration after vegetation restoration

Vegetation restoration can increase soil carbon (C) sequestration in karst regions characterized by highly exposed carbonate rocks; yet, it remains unclear whether and how bedrock outcrops contribute to soil C accumulation after vegetation restoration. Herein, we selected 362 fixed locations to investigate the magnitude and mechanisms underlying soil organic carbon (SOC) sequestration before and after croplands restoration in a karst catchment with varying bedrock exposure ratios and initial soil C pools prior to restoration. Active vegetation restoration (i.e., cropland converted to forage grass, plantation forest, and a combination of grass and forest) and natural regeneration (cropland abandoned) was compared, with cropland maintained with no change as a control. Compared with unrestored croplands 10 years ago, the SOC density substantially increased in the four vegetation restoration types, but remained unchanged in croplands. The SOC accumulation rate was higher for natural regeneration (39 g C m–2 yr–1) than for the three active restoration strategies (18–27 g C m–2 yr–1). SOC accumulation decreased with a higher initial pool size of soil C but increased with nitrogen accumulation and soil exchangeable calcium (Ca2+). Higher bedrock outcrops reduced soil volume but increased SOC content through their indirect effects on the initial pool size of soil C, external nitrogen inputs, and soil Ca2+. This weakly promoted rather than inhibited SOC sequestration rates. Our findings highlight the effectiveness of various restoration techniques in promoting SOC accumulation in karst areas, as well as the need of taking bedrock outcrops and initial soil C pools into consideration when modelling SOC dynamics and maximizing C sinks for vegetation restoration.