Root characteristics and soil microbial entropy can be important indicators of degradation of Populus simonii in sandy areas

Forests play a key role in global carbon sequestration, biodiversity conservation and ecological security, and their degradation weakens essential services such as wind breaking and sand fixation, especially in shelterbelts of arid and semi-arid sandy regions. However, forest degradation is intensifying worldwide, and Populus simonii , a typical sand-fixing species, has undergone large-scale decline in sandy areas. It is therefore urgent to identify sensitive, operational indicators of degradation. In this study, we examined Populus simonii stands along a degradation gradient in sandy soils, focusing on root quantitative traits and the dynamics of soil and microbial biomass carbon, nitrogen and phosphorus contents and stoichiometric imbalance in the 0–100 cm soil layer. The results showed that: (1) With increasing degradation, root vertical distribution contracted and the proportion of fine roots (≤ 2 mm) decreased. (2) Csoil, Nsoil, Csoil:Psoil and Nsoil:Psoil ratios declined significantly, while Psoil and the Csoil:Nsoil ratio fluctuated. Cmic, Nmic, Pmic and the ratios Cimb:Nimb, Cimb:Pimb and Nimb:Pimb generally decreased, whereas q MBC, q MBN and q MBP declined at early stages of degradation and then increased. (3) The Csoil:Nsoil:Psoil and Cimb:Nimb:Pimb ratios were central in regulating q MB, with Csoil:Psoil and Csoil:Nsoil as key drivers; Csoil:Nsoil:Psoil and Cimb:Nimb:Pimb positively affected q MB, whereas Cmic:Nmic:Pmic had negative effects. Overall, root traits and soil microbial entropy provide effective indicators for diagnosing Populus simonii degradation and support targeted restoration and management of degraded shelterbelts in sandy regions.