Associations between fine-root traits and soil nutrient profile differentiation during degradation of Populus simonii shelterbelts in sandy areas

Understanding belowground changes during stand degradation is essential for the restoration of degraded shelterbelts in sandy areas. This study investigated Populus simonii shelterbelts across four degradation levels (Healthy, Lightly degraded, Moderately degraded, and Severely degraded) in the Kubuqi Desert. We analyzed the relationships between fine-root traits and soil nutrient variation along the 0-100 cm soil profile to characterize their associations during degradation. The results showed that, as degradation intensified, fine-root activity and biomass decreased significantly, whereas lignin content and the lignin-to-cellulose ratio peaked at the Moderately degraded stage, indicating a shift in fine-root resource-use strategy from an acquisitive strategy to a more conservative one. Soil organic matter, total nitrogen, and alkali-hydrolyzable nitrogen generally declined along the soil profile, with the surface layer showing the strongest response. Total nitrogen, total phosphorus, and alkali-hydrolyzable nitrogen were the key soil factors driving fine-root trait variation. Path analysis further showed that changes in total soil nutrients may contribute to reduced fine-root activity and trait differentiation by affecting the supply of available nutrients and fine-root chemical composition. Overall, fine-root trait changes were significantly associated with soil nutrient profile differentiation, and the Moderately degraded stage exhibited a more pronounced transition in belowground functioning. This study deepens our understanding of the degradation of Populus simonii shelterbelts in sandy areas from a belowground perspective and provides a basis for the ecological diagnosis and restoration of degraded shelterbelts.