Vegetation complexity as a multi-dimensional indicator of soil organic carbon storage, but not nitrogen storage, in dryland ecosystems

Soil organic carbon (SOC) and soil total nitrogen (STN) are essential for primary productivity and nutrient cycling in terrestrial ecosystems. Although their drivers have been extensively studied, the role of vegetation complexity in regulating SOC and STN dynamics, along with the ecological processes involved, remains insufficiently understood. Using a multidimensional vegetation complexity index (VCI) and structural equation modeling, this study determines how vegetation complexity influences SOC and STN stocks and their ratio (SCN) in the 0–60 cm soil profile of dryland ecosystems in West Africa. We evaluated three non-mutually exclusive mediation pathways through which vegetation complexity may operate: (i) aboveground biomass (AGB; “vegetation quantity”), (ii) tree functional and phylogenetic diversity (“vegetation quality”), and (iii) their combination. We tested whether these pathways were sustained by plant–soil feedback mechanisms, while accounting for environmental filtering and human disturbance. SOC stock and SCN increased significantly with VCI, but STN stock did not. These VCI effects were statistically supported but modest and were mediated by AGB rather than by diversity attributes. Complementarily, SOC stock and SCN exhibited positive feedback on VCI via AGB, although these reciprocal effects changed with soil depth. Aridity and human disturbance were negatively associated with VCI, SOC and STN stocks, and SCN. Our findings highlight VCI as a multidimensional indirect predictor of SOC and SCN, but not STN, operating through the “vegetation quantity” pathway. These results underscore the importance of adopting VCI as a context-specific benchmark for ecosystem management in drylands, as promoting vegetation complexity can enhance carbon sink capacity and soil health.