Anthropogenic fluorine accumulation in the Waikato and Bay of Plenty regions of New Zealand: comparison of field data with projections

Application of phosphate (P) fertilizers to farmland is projected to be causing worldwide accumulation of fluorine (F) in agricultural soils and associated ecosystems, but wide-scale field data have been lacking. We report results of sampling across two large regions of New Zealand involving soils of 298 native and farmed properties. This has enabled the first wide-scale retrospective estimates of anthropogenic F enrichment in pastoral and horticultural soils. Results validate earlier projections: F accumulation in farmed soils has been comparatively rapid (mean increase 2.1 % per year) and widespread. Over 50 years, average total F concentrations in surface soils have doubled from ~220 to 440 mg/kg. Thresholds protective against chronic fluorosis in grazing animals are being substantively passed, indicating that land may be rendered unsuitable for pastoral production in the mid-term future: when sampled 44 % of dairy farm soil samples exceeded 500 mg/kg total F, and 10 % exceeded 650 mg/kg. Potential for F toxicity to plants and wildlife appear to be significant and marginal, respectively, at current soil F concentrations. Relationships between F and other elements suggest P fertilizers are also a source of several elements geochemically linked to F, and that anthropogenic F modifies soil aluminium (Al) chemistry, potentially opening a broad-scale Al exposure pathway to pollinating insects. Findings signal a need for systematic examination of the range of possible affects that may be linked to increasing soil F, which include various types of ecotoxicity, altered Al chemistry, reduced P availability, and potential for increased dietary intakes of F, Al and aluminofluoride (AlF_x) species over time.