Land-use change from wheat fields to kiwifruit orchards increases fluoride accumulation and associated environmental risks

Large areas of grain-crop farmland in Shaanxi have recently been converted to kiwifruit orchards, accompanied by intensive inputs including phosphate fertilizer and high-frequency irrigation. While the link between phosphate fertilizer and fluoride accumulation is known, the long-term impact of land-use change to high-input kiwifruit orchards on fluoride accumulation and vertical distribution is still unclear, especially in this region. We investigated the effects of this land-use change on soil fluoride accumulation and associated environmental risks by collecting soil samples (0–100 cm depth) from wheat fields and 10-, 20-, and 30-year-old kiwifruit orchards in Zhouzhi and Meixian counties—major kiwifruit-producing regions of Shaanxi. Compared to wheat fields, kiwifruit orchards exhibited higher soil electrical conductivity (EC), organic matter (SOM), Mg²⁺, total phosphorus (TP), and available phosphorus (AP) in surface soils (0–10 cm). The average total fluoride (TF) concentration in the 0–10-cm soil layer of the 10-, 20-, and 30-year-old orchards was 688.2, 765.1, and 814.4 mg kg⁻¹, respectively, while water-soluble fluoride (WSF) levels were 7.42, 8.87, and 10.46 mg kg⁻¹. TF and WSF levels were significantly higher in kiwifruit orchards than in wheat fields (TF: 630.71 mg kg⁻¹; WSF: 7.16 mg kg⁻¹), and increased with orchard age. We calculated the TF accumulation rate over a 30-year period using the wheat fields as the baseline (year zero) and found that TF accumulated at 6.1 mg kg⁻¹ per year. Fluoride and phosphorus exhibited similar patterns in terms of vertical accumulation, with the highest enrichment in surface soils and a gradual decline with depth. The average TF levels in orchard soils (603.5–814.4 mg kg⁻¹) exceeded those in wheat field soils (581.3–630.7 mg kg⁻¹) at every depth, but the difference in fluoride concentration between land-use systems diminished with depth. The TF level was more strongly correlated with soil properties in upper soil (0–40 cm) than in deep soil (40–60 cm). These findings provide crucial evidence for the environmental risks of fluoride accumulation associated with phosphate fertilizer over-application. Our results also provide a foundation for the development of optimal fertilization management strategies.