Soil C:N:P:K Stoichiometric Imbalance Regulates the Effects of Karst Rocky Desertification Gradients on Rosa roxburghii Fruit Quality

Karst rocky desertification (KRD) is a critical abiotic stressor that compromises Rosa roxburghii Tratt fruit quality, yet its underlying physiological and ecological pathways remain poorly understood. This study aims to bridge this knowledge gap by elucidating how KRD-driven soil C:N:P:K stoichiometric shifts indirectly affect fruit quality (vitamin C and fructose) by disrupting fruit internal elemental balance. We characterized soil nutrient dynamics and evaluated fruit quality along the natural rocky desertification gradient (RDG) in Guizhou, China. Mantel tests and structural equation modeling (SEM) were applied to identify primary predictors and quantify integrated pathways. With increasing desertification, total organic carbon (TOC), total nitrogen (TN), and soil C:P ratio (C:P) increased monotonically, while total phosphorus (TP) remained at a low concentration without significant change. Available phosphorus (AP) and available potassium (AK) followed a U-shaped trend, reaching their minimum under moderate desertification, whereas total potassium (TK) peaked at this stage before declining. Correspondingly, fruit vitamin C (FVC) and fruit fructose (FF) exhibited a non-linear pattern, first increasing under light stress, then decreasing under moderate stress, and finally rebounding under severe desertification, which is consistent with the nonlinear response pattern of fruit C:P ratio (FC: FP) reported for the same desertification stages. The SEM revealed a dual-pathway mechanism: KRD directly enhanced FVC (β = 0.69, p < 0.001), while KRD and soil properties enhanced FF (β = 0.39 and 0.64, respectively, p < 0.01). KRD also indirectly lowered FVC and FF through a sequential nutrient cascade that promoted soil nutrients (Soil, β = 0.81–0.82, p < 0.001), which triggered negative effects particularly via soil stoichiometry (SS, |β| = 0.76–0.91, p < 0.001), with a modest additional contribution from fruit stoichiometry (FS, |β| ≈ 0.27, p < 0.05). This suggests that KRD influences fruit quality via a soil–plant network mediated by soil P-limitation and K-dynamics. Balanced P and K management may, therefore, be critical for maintaining fruit quality and ecological restoration in karst ecosystems.