Grapevine rootstocks in K- and Ca-rich volcanic soils: The effect of mineral assemblage on nutrient uptake efficiency

This study examines element mobility in volcanic soils to evaluate the relationships between mineral assemblage and the uptake capacity of rootstocks. The Gravesac rootstock, which appears to have a low capacity to selectively absorb K, Ca, Mg, and Na relative to one another, was used for this study. We analysed the chemical composition of rootstocks and the adhering soils. These soils were also mineralogically characterised and subjected to leaching using water over variable time periods and ammonium acetate. Chemical weathering of minerals significantly controls the mobility of base cations in vineyard soils and this should strongly impact root absorption and plant nutrient content. In the scarcely evolved volcanic soils (hereafter pozzolanic soils), where clinopyroxene is not significantly weathered, Ca has low availability in soil solution compared to evolved volcanic soils (hereafter clay-bearing volcanic (CBV) soils), despite higher Ca content in pozzolanic soils. This indicates that in pozzolanic soils, Ca and Mg are less mobile due to the minimal weathering of clinopyroxene. Rootstock absorption of Ca and Mg mirrors these findings, with lower uptake in pozzolanic soils despite higher total element content. Leaching experiments show that the bioavailability of Ca and Mg obtained in H2O and ammonium acetate is inversely correlated with the uptake efficiency of rootstocks, which is instead positively correlated with the removal of Ca and Mg from the soils. In CBV soils, the intense chemical weathering of clinopyroxene increases the mobility and subsequent uptake of Ca and Mg by roots. On the contrary, leaching tests show that K bioavailability in pozzolanic soils, derived from leucite weathering, is higher than in CBV soils, which is consistent with their higher K content. The rapid dissolution of K in pozzolanic soils contrasts with its slower and more sustained release in CBV soils, linked to the weathering of phlogopite. This leads to higher K uptake in CBV soils despite their lower K content. The mineral assemblage in CBV soils, particularly the extent of phlogopite alteration, maintains high K activity in the soil solution, enhancing K bioavailability. The effect of mineral speciation (i.e., the solid solution range and weathered form of a particular mineral) on the uptake efficiency of rootstocks is confirmed by the distinct chemical compositions of musts from Merlot grapes grown in the two soils. Our results highlight the pivotal role of mineral assemblage in shaping nutrient availability and rootstock uptake dynamics in viticultural systems.