Rare earth elements in grapes and soil: study of different soil extraction methods

Lanthanides, together with scandium and yttrium, make up the group of Rare Earth Elements (REEs). An official method for analysis of the bioavailable REEs accumulated by plants, depending mainly on soil characteristics, chemical speciation in soil and the specific ability of the plant, is still lacking.
In this study we analysed the content of 13 REEs (yttrium, Y; lanthanum, La; cerium, Ce; praseodymium, Pr; neodymium, Nd; samarium, Sm; europium Eu; gadolinium, Gd; dysprosium, Dy; holmium, Ho; erbium, Er; thulium, Tm; ytterbium, Yb) in Chardonnay grapes in relation to the content in the soil, extracted using different methods in order to assess which of the extractants used could best reflect the amount of elements taken up by the plant.
The vineyard, located in north-eastern Italy, is characterised by a silt loam, calcareous, alkaline soil. Four different extraction methods were tested: (1) aqua regia microwave digestion; (2) with DTPA, CaCl₂ and TEA; (3) with ammonium acetate and (4) with ammonium nitrate.
The amount of REEs extracted followed the order: aqua regia > DTPA > CH₃COONH₄ > NH₄NO₃. Compared to the “so-called total” content in soil, the sum of the REEs extracted with DTPA, ammonium acetate and ammonium nitrate was roughly 0.80%, 0.065% and 0.002% and each individual element was extracted in amounts of <2.9%, <0.5% and <0.2% respectively. Only 7 elements (Y, La, Sm, Eu, Dy, Er, Tm) were found in quantifiable amounts after extraction with ammonium nitrate.
The concentration of individual REEs in berries would seem to correspond best to the concentrations extracted using aqua regia.