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Many 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, Ca) 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, , Geoderma 195-196:12-22, , For Ecol Manag 293:65-78, ). A model simulating Mg, Ca, Mg and Ca 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. [ABSTRACT FROM AUTHOR]

Using nutrient budgets, it has been proven that atmospheric deposition of Mg and Ca sustains the fertility of forest ecosystems on base-poor soils. However the fate of this nutrient input within the ecosystem was presently unknown. Our hypothesis is that the biological cycling of these nutrients is very rapid and conservative to prevent further Mg and Ca losses most especially in ecosystems on base-poor soils. Stable isotopes of magnesium and calcium (Mg and Ca) were used to trace the dynamics of throughfall Mg and Ca in the forest soil of a 35-year-old beech stand. The aim of the present study was to (1) understand the processes and the velocity of the incorporation of tracers in the biogeochemical cycles and (2) compute Mg and Ca budgets for the ecosystem by isotope dilution. Rainfall Mg and Ca were strongly and rapidly retained mainly by ion exchange in the thin OL litter-layer. However, Ca was much more strongly retained in the litter-layer than Mg. As a result, 2 years after the application of tracers (2012), 92 % of Mg and 67 % of Ca was released and transferred to the soil or taken up by trees. The vertical transfer of Mg was very slow only 15 % of Mg was found below 15 cm depth in 2012. Ca was slower than Mg only 9 % of Ca was found below 5 cm depth. Although matrix flow was the main vertical transfer process of Ca and Mg, preferential transfer in macropores occurred. Overall, Mg was more rapidly leached through the soil profile than Ca because the soil CEC was mainly composed of organic charges which affinity for Ca is much higher than for Mg. 27 % of Mg and 20 % of Ca was found in tree biomass and total tracer recovery was close to 100 %. These results suggest that no tracers were lost to drainage over the 2 years. Finally, applying the isotopic dilution theory to the whole-ecosystem enabled us to estimate Mg and Ca budgets −0.9 kg ha year for Mg, which was close to computed input-output budgets −0.8 and 0 kg ha year for Ca, which was very different from input-output budgets (−3.1 kg ha year). Our results suggest that a Ca source is underestimated or not taken into account. Over all, organic matter of the litter-layer and in the soil profile played an essential role in the retention of throughfall Mg and Ca and their cycling within the forest ecosystem. [ABSTRACT FROM AUTHOR]

Background and aims: The sustainability of forest ecosystems may be at stake especially in forests on base-poor soils due to reduced nutrient deposition and intensified silvicultural practices. Understanding nutrient availability and cycling is therefore essential to manage forest soil fertility. This study aims to assess in a beech plot Mg and Ca vertical transfer in soil and root uptake using an isotopic tracing experiment. Methods: A simulated rainfall containing a small amount (960 g Mg.ha; 530 g Ca.ha) of highly enriched Mg and Ca was sprayed on the forest floor of a 35-yr-old beech plot. The isotopic composition of fine roots and of the soil exchangeable Mg and Ca pool was monitored during 1 year. A pool and flux model (IsoMod) was developed to predict the labeling of the soil and vertical transfer of tracers. Results: Tracers (Ca and Mg) were immediately retained in the thin litter layer. During the following year, Mg and to a lesser extent Ca were progressively released. After 1 year, the exchangeable Mg and Ca pools of the upper mineral layer (0-5 cm) were strongly labeled (~660 ‰, representing ~55 % of the tracer input and ~370 ‰, ~41 % of the tracer input respectively). A significant proportion (~8 % Mg, ~2 % Ca) of tracer was leached through the soil, below 10 cm. This amount was much larger than what was predicted using a simple mixing model. The Ca and Mg isotopic composition of fine roots at all depths was close or lower than that of exchangeable Ca and Mg respectively. Conclusions: An in situ ecosystem-scale Mg and Ca isotopic tracing experiment was successfully carried out. Tracers were at first strongly retained in the litter layer, then progressively transferred to soil horizons below. Nutrient cycling of Mg and Ca were proven to be very different. Mg had a higher mobility in the soil than Ca, and nutrient uptake sources were proven to be different. [ABSTRACT FROM AUTHOR]

The mean depth of Sr and water uptake in mixed Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) stands was investigated, using natural variations of 87Sr/86Sr and 18O/16O in soils in relation to depth. Three spruce-pine pairs were studied on a podzol and a peat site in Northern Sweden. Tree leaf and wood, as well as soils, soil solutions and roots below each tree were analysed for Sr and Ca concentrations and 87Sr/86Sr ratio. The 18O/16O ratio was also determined in xylem sap and soil solutions in relation to depth. Soil solution 18O/16O decreased in relation to depth. Comparing with xylem sap 18O/16O data indicated a deeper uptake of soil water by pine than spruce on the podzol site and a superficial uptake by both species on the peat. The 87Sr/86Sr ratio of bioavailable Sr generally increased in soils in relation to depth. Contrastingly, the 87Sr/86Sr ratio in spruce wood was generally higher than in pine wood suggesting a deeper uptake of Sr by spruce. But the 87Sr/86Sr ratio and concentrations of bioavailable Sr were systematically higher below spruce than below pine. In order to explain these unexpected results, we built a simple flux model to investigate the possible effects of interspecific variations in Sr cycling, soil mineral weathering and depth of Sr uptake on soil and tree 87Sr/86Sr ratio. At the study sites, spruce cycled in litterfall up to 12 times more strontium than pine. The use of the model showed that this difference in Sr cycling could alone explain higher isotopic signatures of trees and topsoils below spruce. Besides, high isotopic signatures of roots in the A/E horizons below spruce led us to hypothesise a species-specific weathering process. Finally, the comparison between the 87Sr/86Sr ratios in wood and root or soil solutions below each species suggested that the average depth of Sr and water uptake were close, but irregular variations of the Sr isotopic ratio with depth reduce the accuracy of the results. Tree species strongly influence Sr isotopic ratios in boreal forest soils through differences in Sr cycling, and possibly through specific mineral weathering. [ABSTRACT FROM AUTHOR]