Characterizing nutrient pathways in Quebec (Canada) vineyards: Insight from stable and radiogenic strontium isotopes

Abstract

Soils are a crucial interface of Earth's Critical Zone, responsible for the biogeochemical cycling of mineral nutrients. While the sources of essential nutrients such as Ca (and Sr) in plants have been widely studied, the processes by which these nutrients are transferred from the soil to the plants is less clear. We present new evidence for systematic nutrient fractionation between soils, grapes and wines from five vineyards from the Quebec terroir (Canada) using the novel stable Sr (δ88/86Sr) isotope system. We coupled this approach with traditional radiogenic 87Sr/86Sr measurements on the same samples (bulk soils, labile fraction of these soils, whole grapes, and wines) from five vineyards from the Quebec terroir (Canada) to ensure that variations in δ88/86Sr values were provoked by processes involved in Sr transfer instead of mixing between different sources. The δ88/86Sr values generally decrease from the bulk to the labile fractions of the soils to the grapes. A decrease in the difference between the soil fractions (Δ88/86Srbulk-labile) with increasing δ88/86Sr values in the labile fraction is interpreted to reflect increasing pedogenic mineral precipitation and higher soil fertility. The depleted δ88/86Sr values of the grapes are consistent with a preferential uptake of lighter isotopes by plants. This effect is magnified in soils showing higher secondary mineral precipitation. An increase in the δ88/86Sr values from the grapes to the wines reflects the separation of the skin/seeds and the juice (pulp enriched in 88Sr) in the winemaking process. This mixing relationship is supported by a correlation between the Sr concentrations and the δ88/86Sr values of the wines and grapes. These findings suggest promising new applications of stable Sr isotopes values for studying soil fertility and the impact of nutrient availability for plant growth in agricultural ecosystems as well as the impact of the maceration processes in the production of wine.The overall similarity of 87Sr/86Sr ratios of the labile soil fractions, grapes and wines from the same vineyard support the hypothesis of a same Sr source, from the soil solution to its transfer to the grape and wine and confirm the potential of the 87Sr/86Sr ratios as a proxy for geographic provenance of wine.However, slightly lower 87Sr/86Sr ratios in the grape pulps (by 0.00040 on average compared to the skins and seeds) suggest that the sources of nutrients available for plant uptake vary during the vegetative cycle. The grape seeds and skins formed in the early growth cycle are characterized by rapid growth due to enhanced rhizosphere activity (enhanced mineral dissolution) providing higher nutrient levels. The enhanced mineral solubility results in higher 87Sr/86Sr ratios in the skin and seeds derived from enhanced dissolution of 87Sr-enriched silicate minerals. In contrast, the pulp develops later in the growth cycle when rhizosphere activity and mineral solubility is reduced, resulting in a lesser contribution from silicate minerals and thus reflect a higher proportion of dissolved carbonate contributing to the lower 87Sr/86Sr ratios of the pulps.