Calcium isotope fractionation during plant growth under a limited nutrient supply

Hydroponic experiments were performed on bean plants using a nutrient solution at pH 6 with an initial Ca concentration of 5 ppm to test the effect of Ca deficiency on the Δ44/40Calateral roots/nutritive solution and the δ44/40Ca signatures of the different bean organs. The results of the study suggest that the process of Ca uptake by the roots follows a closed-system equilibrium fractionation with a fractionation factor (αbean plant/nutritive solution) of 0.9988, suggesting that Ca forms exchangeable bonds with the root surfaces and thus confirming 40Ca adsorption onto pectic RCOO– groups in the cell wall structure of the lateral roots. The study further suggests that for a constant pH value (i.e., 6), the average signature of the bean plants depends on the Ca isotope signature of the nutritive medium. Moreover, regardless of the concentration of the nutritive solution, the fractionation mechanism between the roots and shoots remains the same, and only the intensity of fractionation between the different organs is modified. Finally, with a decreasing Ca supply in the solution and the appearance of deficiency effects within the bean plants, the Ca isotopic signature of the leaves ceases to reflect that of free Ca but rather that of Ca oxalate crystals.The study also emphasises that Ca isotopes are important tracers of the Ca nutrient availability in soils and may be used as a tool to identify and quantify Ca recycling in soils.