Beyond monocultures: Optimizing soil carbon sequestration with diverse planting strategies on the Loess Plateau

Monoculture plantations, often characterized by low species diversity and simple structure, are significant contributors to limited carbon stocks, forced ecosystem degradation, and reduced long-term resilience. To tackle these issues, mixed-species plantations have gained popularity as a promising strategy. However, current knowledge about the effects and mechanisms of increasing planting diversity on soil carbon storage (SCS) remains limited. Furthermore, there is a lack of scientific guidance on selecting the most appropriate mixed afforestation strategy. In this study, pure Robinia pseudoacacia (RP) plantations and RP plantations mixed with tree species Pinus tabuliformis (RP-PT), shrub species Forsythia suspense (RP-FS) and Caragana korshinskii (RP-CK) were selected to assess differences of vegetation diversity, litter characteristics, soil properties, and SCS across the soil profile (0–60 cm) on the Loess Plateau, China. We used random forest analysis and variance decomposition to examine the relative effects of various environmental factors on SCS. Structural equation models (SEM) were employed to determine how these variables directly or indirectly affect SCS in mixed plantations. Our findings demonstrated that species mixtures notably enhanced SCS in RP-PT by 23.2 % and RP-CK by 27.7 % comparing with RP plantations. Soil properties, particularly soil water content and nitrogen content, emerged as the most influential factors on SCS. Litter characteristics were more impactful on SCS in the upper soil layers (0–30 cm), while vegetation diversity had a greater effect on deeper soils (30–60 cm). SEM analysis showed that the predictor variables collectively explained 67 % to 87 % of the variation in SCS, with soil properties directly influence SCS, while litter biomass, litter carbon, litter nitrogen, and the Shannon-Wiener diversity index (H) primarily indirectly affect SCS by influencing soil factors. H can also have a direct positive impact on deeper SCS. These results suggest that, given the region’s water and nutrient limitations, the optimal afforestation strategy should focus on improving water-use efficiency and nutrient availability. Incorporating a mix of coniferous and broadleaved species, as well as nitrogen-fixing species, could be an effective approach to enhancing ecological health and carbon sequestration on the Loess Plateau.