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

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.