Soil fluoride fractions and their bioavailability to tea plants (Camellia sinensis L.)

Abstract

Drinking teas containing high fluoride (F) imposes fluorosis risk. The soil F bioavailability is an important factor influencing its uptake and contents in teas. The present work was conducted to investigate F fractions in soil and their bioavailability to tea plants. Tea seedlings were cultivated on 6 typical soils treated with a mixture consisting of dolomite, lime, peat and KCl at variable rates in the pot experiment. Soils and young shoots were collected in pairs from 63 sites of 21 plantations in a field experiment. Soil fluoride was sequentially separated into hot water soluble $[{\text{F}}_{{ ( {\text{h, H}}_{ 2} {\text{O,s)}}}} ]$, exchangeable ${\text{F}}\;[{\text{F}}_{{({\text{MgCl}}_{ 2} , {\text{ s}})}} ]$ (by 1 mol L⁻¹ MgCl₂, pH = 7.0), F bound to Mn and Fe hydroxides [F_(oxides,s)], and organic matter [F_(OM,s)] or extracted independently by water $[{\text{F}}_{{ ( {\text{H}}_{ 2} {\text{O)}}}} ]$ or 0.01 mol L⁻¹ CaCl₂ solution $[{\text{F}}_{{(0.01{\text{ M CaCl}}_{ 2} )}} ]$. Averaged ${\text{F}}_{{ ( {\text{h, H}}_{ 2} {\text{O,s)}}}}$, ${\text{F}}_{{ ( {\text{MgCl}}_{ 2} , {\text{ s)}}}}$, F_(oxides,s) and F_(OM,s) accounted for 51, 14, 5 and 30 % of the total sequential extracts, respectively. There were significant correlations among ${\text{F}}_{{ ( 0. 0 1 {\text{ M CaCl}}_{ 2} )}}$, ${\text{F}}_{{ ( {\text{H}}_{ 2} {\text{O)}}}}$ and F_(OM,s). Fluoride contents in leaves correlated with ${\text{F}}_{{ ( {\text{H}}_{ 2} {\text{O)}}}}$ (r = 0.71, p < 0.001), ${\text{F}}_{{ ( 0. 0 1 {\text{ M CaCl}}_{ 2} )}}$ (r = 0.93, p < 0.001) and F_(OM,s) (r = 0.69, p < 0.01) but not other fractions in the pot experiment and with ${\text{F}}_{{ ( {\text{H}}_{ 2} {\text{O)}}}}$ (r = 0.43–0.57, p < 0.001) and ${\text{F}}_{{ ( 0. 0 1 {\text{ M CaCl}}_{ 2} )}}$ (r = 0.42–0.79, p < 0.001) in the field experiment. It was concluded that 0.01 M CaCl₂ extractable fluoride can be a good indicator of soil F bioavailability to tea plants. The significant correlations among some of the F fractions suggested that F in solution, AlF complexes (AlF₂ ⁺, AlF²⁺) and those bound to organic matter likely represent the available pools to tea plants.