Mg and Ca uptake by roots in relation to depth and allocation to aboveground tissues: results from an isotopic labeling study in a beech forest on base-poor soil

International audienceMany forest stands grow on acid and nutrient poor soils. To better understand how they cope with very low mineral resources, we investigated (1) Mg and Ca uptake in relation to depth, and (2) the allocation of these elements from the roots to the canopy, using a multi-isotopic (Mg-26, Ca-44) tracing experiment in a beech stand on a very poor soil. The distribution of the tracers in the soil was taken from van der Heijden et al. (Plant Soil 369:33-45, 2013a, Geoderma 195-196:12-22, 2013b, For Ecol Manag 293:65-78, 2013c). A model simulating Mg, Ca, Mg-26 and Ca-44 uptake was developed and applied to estimate the vertical distribution of Mg and Ca uptake in the soil profile. The vertical distribution of tracers in aboveground biomass was measured from four felled trees 2 years after the application of the tracers. The modeled distribution of root uptake in relation to depth shows differences between Mg and Ca: the main source of Mg uptake is the litter layer (circa. 43 % of total uptake) and the top mineral soil (0-5 cm) for Ca (circa. 42 %). The deeper soil layers (15-60 cm) also contribute to uptake. The study does not show clear evidence that uptake occurs in the very deep soil layers (> 70 cm). The distribution of tracers in the aboveground biomass shows a vertical gradient from the stump to the canopy with no or very small amounts of tracers being observed in the foliage during the 2 years after the application of tracers. This suggests that Mg and Ca transport from roots to leaves along the xylem sap flow is quite slow. As Ca and Mg supply to the trees from deep soil horizons is not evidenced, and tracer transport from roots to the leaves is slow, we suggest that the tree internal pool of Ca and Mg may be more active than previously thought. This pool may act as a buffer when nutrient availability is in shortage