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

Abstract

Highlights

Orchards accumulate more soil fluoride than wheat fields.
Orchard fertilization practices increase soil fluoride concentration over time.
Soil fluoride is positively correlated with SOM, P and pH.
Excessive fertilization increases the risk of soil F and P accumulation.

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.

Graphical Abstract

Introduction

Fluoride (F^–) is a naturally occurring anion of fluorine, occurring at an average concentration of 625 mg kg^–¹ in the Earth’s crust [12], [35]. Both natural and anthropogenic sources contribute to fluoride accumulation in the environment [1], [30], [43]. Fluoride is an essential trace element for both humans and animals. Appropriate fluoride levels are beneficial for the healthy growth of bones and teeth [21]; however, chronic exposure to elevated concentrations (>1.5 mg L^–¹) induces severe health consequences [52], [50]. Soil is both a source and sink in the fluoride cycle, and excessive soil fluoride accumulation may elevate fluoride levels in interconnected systems, such as plant and groundwater systems, with negative impacts on human health and plant growth [2], [62].

Phosphate fertilizers typically contain 1.5–4 % fluoride, and phosphate fertilization is recognized as a significant source of fluoride in soils [30]. Commonly used phosphate fertilizers such as compound fertilizers (NPK), diammonium phosphate (DAP) and superphosphate (SSP) in Chinese farmland contain higher fluoride concentrations compared with other fertilizer [23]. Moreover, the fluoride concentration in compound fertilizers increases with the proportion of phosphorus [40], [62]. The fluoride introduced to soil through phosphate fertilizers may subsequently transfer to surface water systems via runoff [40], [41]. Numerous studies have shown that long-term phosphate fertilizer application is closely related to soil fluoride accumulation [29], [30], [37]. However, these studies have mainly focused on the accumulation of fluoride in shallow soil profiles (e.g., a sampling depth of < 1 m in New Zealand pasture sites); the effects of fertilizer on temporal variations in fluoride accumulation and the vertical distribution of fluoride in agricultural systems are still unclear. In intensive agricultural production, chemical phosphorus fertilizers and organic fertilizers are widely applied to enhance crop yields [16], [5]. Global agricultural phosphorus fertilizer consumption increased from 5 Tg P yr^–¹ in 1961–18 Tg P yr^–¹ in 2013, and is projected to reach 22–27 Tg P yr^–¹ by 2050 [66]. Agricultural fertilizer application in China increased from 12.694 million tons in 1980 – 59.959 million tons in 2014, a growth of 4.7 times, with an average annual growth rate of 4.67 % [18]. In particular, the fertilization inputs for vegetables and fruits have rapidly increased, with fruit crops receiving the highest fertilizer input per unit area compared to other agricultural crops [18]. Land-use changes significantly alter biogeochemical cycles [49]. The conversion of crop fields to horticultural fields with frequent fertilizer overuse may elevate soil fluoride accumulation risks. Consequently, paying close attention to the temporal dynamics and vertical distribution of fluoride accumulation under long-term land-use change is crucial for maintaining ecological health.

Since the 1990s, extensive areas of grain-crop farmland on the northern slopes of the Qinling Mountains in Shaanxi Province have been converted into kiwifruit orchards, driven by the high economic value of kiwifruit and favorable climatic conditions [31], [6]. Zhouzhi County and Mei County in Shaanxi Province account for a large proportion of China’s kiwifruit production. To ensure high fruit yields, excessive inputs of phosphate fertilizers, compound fertilizers, and organic fertilizers are routinely applied in these orchards. The fertilizer application rates in kiwifruit orchards significantly exceed those in traditional wheat fields [10], [31], and the application rates in orchards far exceeded the recommended dosage of 80–120 kg ha^–¹ for P [33], and therefore are considered excessive. Research related to excessive fertilization in kiwifruit orchards in this region indicates that land-use conversion from croplands to kiwifruit orchards enhances the accumulation of soil nitrogen (N) and phosphorus (P) [10], [11], [32], [31], [34]. In addition, compared to traditional farmland, kiwifruit orchards require greater amounts of artificial irrigation and higher irrigation intensity. Groundwater is an important source of domestic and irrigation water for the agricultural areas of the Guanzhong Plain of the Weihe River Basin [4]. In the certain areas of Guanzhong Plain of Shaanxi Province, groundwater fluoride exposure risks have been reported [4], [8], and endemic fluorosis has also been discovered in Dali County and Pucheng County within the Guanzhong Plain [64]. However, the risk of soil fluoride accumulation resulting from excessive fertilization in kiwifruit orchards in this region has not yet been reported. Therefore, it is essential to investigate the impact of fertilization on fluoride accumulation in soil, including the vertical distribution of fluoride and the potential environmental risks.

In this study, we aimed to: (1) clarify the contribution of long-term phosphate fertilizer application to fluoride accumulation in kiwifruit orchard topsoil, distinguishing it from natural background levels; (2) evaluate the factors controlling the accumulation and vertical distribution of fluoride in soil; and (3) analyze the co-accumulation of fluoride and phosphorus in soil under different fertilization conditions. Compared to previous studies, this study focuses on the temporal dynamics of fluoride accumulation in soil following a specific land-use change (from wheat to kiwifruit), as well as the vertical distribution characteristics across a substantial soil profile (0–100 cm) in a particular region (Shaanxi, China). The findings provide insights into fluoride accumulation and vertical distribution patterns under different fertilization histories, and could be used to develop fertilization strategies to mitigate soil fluoride over-accumulation, enhance soil environmental quality, and safeguard soil health.

Section snippets

Study area and sampling

The study areas were located in Zhouzhi County (33°42′ N–34°14′ N, 107°39′ E–108°37′ E) and Meixian County (33°59′ N–34°19′ N, 107°39′ E–108°00′ E) of Shaanxi Province (Fig. 1). Both areas are situated in the Guanzhong Plain and located in the narrow strip of land south of the Weihe River and northern slopes of the Qinling Mountains, and contains typical intensive agricultural. The groundwater levels are relatively shallow in the plains (<20 m) [4]. The region is characterized by a warm

Soil properties

Table 1 presents the basic physicochemical properties of surface soil (0–10 cm) in wheat fields and kiwifruit orchards. The mean values of soil parameters (including SOM, AP, pH, EC, Ex-Mg, and TP) in the 10-, 20-, and 30-year-old kiwifruit orchards all exceeded those in wheat fields, and showed a progressive increase with orchard age. The mean pH values in the wheat fields and 10-, 20-, and 30-year-old kiwifruit orchards were 6.54, 6.71, 7.22, and 7.11, respectively…

Soil fluoride accumulation

Soil TF concentrations were…

Fluoride accumulation and environmental risks in kiwifruit orchards

The mean soil TF concentration in the 0–10-cm soil layer of wheat fields and kiwifruit orchards exceeded China’s average background soil fluoride level (453 mg kg^–¹) [61]. Notably, the TF concentration in several 20- and 30-year-old orchards surpassed the endemic fluorosis concentration (800 mg kg^–¹) reported by Zhang et al. [63]; however, the concentrations were lower than those in farmlands near phosphate mining zones [51]. The surface soil WSF concentrations in several plots exceeded China’s …

Conclusion

Unlike previous studies on the effects of fertilization on soil fluorine accumulation, this study provides key information on the temporal dynamics following a specific land-use change (from wheat to kiwifruit) and the vertical distribution across a substantial soil profile (0–100 cm) in a specific region (Shaanxi, China). Concentrations of TF and WSF increased with orchard age, indicating that the land-use conversion from wheat fields to kiwifruit orchards has led to enhanced accumulation of..

Environmental implication

This study elucidated the accumulation pattern of fluoride in orchard environments under long-term and intensive phosphorus fertilizer application following land-use change from wheat fields to kiwifruit orchards. In addition, this study clarified the relationship between fertilization history and fluoride content in orchard soil. These findings provide crucial evidence for the environmental risks associated with phosphate fertilizer over-application, while offering a theoretical foundation to..

CRediT authorship contribution statement

Chenxi Wang: Methodology, Investigation. Pengcheng Gao: Writing – review & editing, Validation, Supervision, Resources, Project administration. Xunrong Huang: Writing – original draft, Visualization, Methodology, Investigation. Kun Chen: Methodology, Investigation. Hongbing Gao: Supervision, Resources, Methodology, Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by Key Research and Development Program of Shaanxi (Program No. 2023-ZDLNY-57); China Agriculture Research System of MOF and MARA (CARS-27); Construction Project of Northwest A&F University Experimental Demonstration Station of Weinan Municipal Science and Technology Bureau (2024WNXNZX-3).

References (66)

L.N. Affonso et al. Removal of fluoride from fertilizer industry effluent using carbon nanotubes stabilized in chitosan sponge. J Hazard Mater (2020)

M.N. Ahmad et al. Effects of soil fluoride pollution on wheat growth and biomass production, leaf injury index, powdery mildew infestation and trace metal uptake. Environ Pollut (2022)

B. Ambade et al. Assessing variability and hydrochemical characteristics of groundwater fluoride contamination and its associated health risks in east singhbhum district of jharkhand, India. J Hazard Mater (2024)

J. Chen et al. Hydrogeochemical evidence for fluoride behavior in groundwater and the associated risk to human health for a large irrigation plain in the Yellow River basin. Sci Total Environ (2021)

Q. Chen et al. Shallow groundwater table fluctuations promote the accumulation and loss of phosphorus from surface soil to deeper soil in croplands around plateau lakes in southwest China. J Environ Manag (2024)

Z. Chen et al. Land-use change from arable lands to orchards reduced soil erosion and increased nutrient loss in a small catchment. Sci Total Environ (2019)

Sf Cui et al. Transfer characteristic of fluorine from atmospheric dry deposition, fertilizers, pesticides, and phosphogypsum into soil. Chemosphere (2021)

C. Gago et al. Fluorine sorption by soils developed from various parent materials in galicia (NW Spain). J Colloid Interface Sci (2012)

X. Gao et al. Impact of microbial activity on fluoride release from sediments in areas with high fluoride groundwater: mechanisms, sources and the lithology diversity. Sci Total Environ (2024)

Y. Gao et al. Assessing natural background levels in shallow groundwater in a large semiarid drainage basin. J Hydrol (2020)

There are more references available in the full text version of this article.