Tree species identity drives the vertical distribution of soil carbon and nutrient concentrations in the Loess Plateau, China

Aims Afforestation is considered an effective strategy to improve soil carbon (C) and fertility in degraded drylands. However, how specific species identities (e.g., conifers or broadleaves) impact C and nutrient concentrations across deep soil layers remains uncertain.Methods Three most important plantation forests, including plantations of native mono-species Pinus tabuliformis (coniferous species), exotic mono-species Robinia pseudoacacia (broadleaf and nitrogen-fixing species), and their mixed, were selected to explore their effects on soil organic C (SOC) and nutrient concentrations throughout soil profile (0-200 cm) in the Loess Plateau.Results The results showed that soil C and nutrient concentrations were strongly affected by species identity. Specifically, R. pseudoacacia contained significantly higher SOC, total nitrogen (N), and ammonia N concentrations than other plantations at the top layer (0-60 cm), but R. pseudoacacia contained lower total phosphorus (P) concentrations than other plantations. In the deep layer (60-200 cm), R. pseudoacacia contained lower total N, nitrate N, and total P concentrations than other plantations. There was no significant difference in SOC concentration among plantation types in the deep layer. We found antagonistic effects of tree mixtures on soil P in the top and deep layers. Furthermore, microbial biomass C and N was the primary driver of SOC and N concentration in the top layer, respectively. Conversely, fine root biomass was the primary factor influencing N and P concentrations in the deep layer. This suggests that planting exotic R. pseudoacacia with higher root biomass has the potential to exacerbate soil N and P depletion in the deep layer.Conclusion Our work emphasizes the key role of species identities in regulating soil nutrient concentrations, especially in the deep layer, and the importance of tree species selection in dryland afforestation.