Revegetation re-carbonizes soil: Patterns, mechanisms, and challenges.

Revegetation has been widely implemented throughout the world for controlling soil loss, conserving biodiversity, increasing ecosystem productivity, mitigating climate change, and contributing to soil carbon (C) sequestration. However, systematic knowledge is still lacking regarding the variations, mechanisms, and challenges in soil C sequestration after revegetation despite its crucial role in resolving the ongoing C sink/source debate and achieving the C neutrality targets. In this review, we summarize the spatiotemporal patterns and drivers of soil organic carbon (SOC) sequestration in restored ecosystems based on existing studies at multiple spatial and temporal scales and the mechanisms associated with C stabilization in soils after revegetation, and suggest future research into soil C in restored ecosystems. Revegetation, i.e. grassland restoration or afforestation/reforestations, significantly increases the SOC sequestration by 21.4% with rapid SOC accumulation in the initial few years (generally within 30 years), followed by a relatively stable stage. We also clarified the three key mechanisms (including physical protection within soil aggregates, chemical protection by interacting with organo-mineral associations, and inherent biological recalcitrance protection) associated with SOC stabilization in soils after revegetation. Revegetation re-carbonizes soil by increasing new inputs of C and decreasing the ratio of soil C outputs to new inputs of C (i.e., increasing the decomposition of SOC and reducing erosional SOC loss), both of which promote SOC sequestration. Based on the key issues identified in this review, future research should focus on the fate of C sequestrated in soils after revegetation, and feedback to environmental changes and human activities in order to achieve C neutrality by around 2050.