Afforestation Enhances Soil Organic Matter Complexity and Persistence: The Role of Root Inputs and Calcium‐Mediated Stabilization in a Karst Region

Afforestation is a primary nature‐based strategy for climate change mitigation, yet its efficacy relies on the long‐term persistence of sequestered soil organic carbon (SOC). Nevertheless, the molecular‐level mechanisms governing this persistence remain understudied. Here, we investigated the impacts of afforestation on former maize fields after 20 years on soil organic matter (SOM) molecular composition and complexity in topsoil (0–15 cm) and subsoil (30–45 cm) in a subtropical karst region of southwest China, using pyrolysis–gas chromatography/mass spectrometry (Py‐GC/MS) coupled with network analysis. Afforestation significantly reshaped the topsoil SOM composition, driving a 160% increase in lignin‐derived compounds and a 58% decrease in polysaccharides, reflecting the shift from crop residues to woody inputs. Afforestation significantly increased the molecular network complexity of SOM in both topsoil and subsoil, indicating the formation of a more intricate and interconnected web of organic compounds. Structural equation modeling revealed that afforestation's impact was indirect, primarily mediated by increased fine root biomass and enhanced organo–mineral interactions characteristic of the calcareous soil environment. Our findings provide compelling molecular‐level evidence that afforestation in the karst region promotes SOC persistence not only by increasing carbon inputs but by building a more complex SOM architecture. This increased complexity, arising from the interplay between new plant‐derived precursors and the strong protective capacity of the mineral matrix, represents a key mechanism for enhancing SOC persistence and validates afforestation as a potent ecological restoration tool in the karst regions.