Fruit Yield and Quality of Zanthoxylum planispinum var. dingtanensis and Their Response to Different Fertilization Regimes

Zanthoxylum planispinum var. dingtanensis (hereafter Z. planispinum) is a pioneer plant for the ecological restoration of karst rocky desertification, offering both ecological rehabilitation and economic benefits. Due to the combined pressure from soil degradation and fruit harvesting, nutrient depletion and quality decline have intensified. Therefore, investigating the effects of fertilization on pericarp yield and quality provides the scientific basis for its precise fertilization. This study examined a Z. planispinum plantation subjected to five treatments in 2021: no fertilization (CK); organic fertilizer + chemical fertilizer + sprinkler irrigation (T1); chemical fertilizer + sprinkler irrigation (T2); chemical fertilizer alone (T3); and legume (soybean) intercropping + chemical fertilizer + sprinkler irrigation (T4). It explored the intrinsic relationships between fruit quality and soil physicochemical properties, identified key soil factors, and conducted a comprehensive quality evaluation, providing a scientific basis for precise fertilization in karst regions. The results indicated that (1) whole-fruit water content ranged from 61.30% to 64.37%, showing no significant differences, while variations were observed in the other phenotypic traits. T1 exhibited the highest values for Hydroxy-β-sanshool (2.42 mg·g−1), Hydroxy-ε-sanshool (0.80 mg·g−1), essential oil content (8.57%), and fresh weight per plant (9.9 kg). After long-term soybean intercropping, pericarp thickness reached its maximum (0.45 mm), but the content of aroma compounds decreased significantly. Compared to the other four treatments, the unfertilized control (CK) showed significantly higher values for the pericarp dry weight ratio, proportion of closed-eye peppercorns, and relative content of d-limonene, with increases of 5.5–13.94%, 130.91–568.42%, and 8.74–14.46%, respectively. (2) An inhibitory effect was observed between pericarp numbing compounds and soil calcium/phosphorus levels, while the synthesis of aroma compounds was constrained by the soil C/N ratio. Soil P/K ratio was identified as the dominant factor affecting overall quality. (3) The comprehensive fruit quality index ranked as follows: T1 (1.2933) > T3 (0.666) > T2 (0.5285) > CK (−1.1555) > T4 (−1.2098). Therefore, the T1 treatment is recommended for promotion as the fertilization management practice for Z. planispinum plantations in karst regions.