Distinguishing sources of base cations in irrigated and natural soils: evidence from strontium isotopes

Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of soil solids, soil cation extracts, irrigation water and plant material are used to determine strontium and therefore cation sources and fluxes in irrigated and natural soil–plant systems. Strontium isotopes of soil solids from four soil profiles (two irrigated vineyard soils and two 'natural' profiles from nearby reserves) show large differences between soil horizons with depth. These differences are not reflected in ⁸⁷Sr/⁸⁶Sr ratios of soil labile cations, which show both little variation down-profile and much lower ratios than soil solids. In the undisturbed, natural soil profiles, labile cation ⁸⁷Sr/⁸⁶Sr ratios are close to the ratio of modern seawater, indicating that solutes of marine origin from precipitation are the primary input of strontium (and calcium) to the labile cation pool. In the irrigated soil profiles, ⁸⁷Sr/⁸⁶Sr ratios of labile cations are consistently shifted towards that of the irrigation water. Mass-balance calculations using ⁸⁷Sr/⁸⁶Sr ratios of the different inputs to the labile cation pool indicate more than 90% of labile strontium is derived from precipitation solutes in unirrigated soils, and up to 44% from irrigation water solutes as an additional source in irrigated soils. The ⁸⁷Sr/⁸⁶Sr ratios of grapes grown in irrigated soils match precisely with those of the labile soil cations, demonstrating that cation nutrients are drawn wholly from the labile cation pool and have the same mix of precipitation, irrigation, and soil solid sources. The ⁸⁷Sr/⁸⁶Sr ratios of grapes grown in the irrigated soils may therefore vary over time depending on (1) the changing mix of irrigation water and local precipitation and (2) potential change to irrigation water. These findings suggest limitations to the use of strontium isotopes in the tracing of grapes and wines to their soil of origin.     

Bone strontium in pregnant and lactating females from archaeological samples

Because plants and animals consume or absorb different amounts of strontium and calcium, anthropologists are able to use strontium/calcium (Sr/Ca) ratios from archaeologically recovered human bone to estimate the relative contributions of meat and plants to paleodiets. Often females exhibit higher Sr/Ca ratios than males, a fact usually attributed to lower meat intake among women. However, in vivo and in vitro experiments with laboratory animals show that pregnancy and lactation elevate maternal bone strontium and depress maternal bone calcium because 1) strontium is discriminated against in favor of calcium in the transport of ions to the placenta and mammary glands and 2) pregnancy and lactation facilitate absorption of alkaline earth metals from the gut. In this study, bone Sr/Ca ratios and strontium concentrations were compared between reproductive-age females, postmenopausal females, and adult males from two late prehistoric Native American sites in Georgia: the King site (N = 43) and the Etowah site (N = 51). At the King site, the mean Sr/Ca ratio of females was over 14% greater than that of males. At Etowah, the mean strontium level of reproductive-age females exceeded that of postmenopausal females by almost 25%. Most of the difference, it is argued, is due to pregnancy and lactation. A dietary preference among pregnant and lactating women for foods high in alkaline earths, particularly nuts and corn, may also be partially responsible. Until we assess the influence variables other than nutrition exert on trace element concentrations, our reconstructions of paleodiets will be suspect.     

Transfer factors for Sr as influenced by species Ca uptake and soil Ca availability

Strontium (Sr) and calcium (Ca) concentrations were studied in different plant species grown in five soil treatments. For either shoots or roots, a positive linear relationship was found between Sr and Ca concentrations in different plant species grown in the same soil treatment. Strontium and calcium concentrations of different species were related to the soil selectivity coefficient for Sr and Ca, defined as the ratio of CH₃COONH₄-extractable Sr and Ca to the ratio of Sr and Ca in the soil solution. For the species used in all soils, transfer factors (TF) for Sr, defined as the ratios of the Sr amount per g of dry plant material and the Sr amount per g of dry soil, were negatively correlated with extractable Ca of the soil. Transfer factors for Sr varied greatly among species or between roots and shoots. This variation of transfer factor was reduced when transfer factor values were divided by the shoot or root Ca concentration of each species. The proposed index TF for Sr per Ca concentration could be used to compare various soils according to their ability to supply plants with Sr when different plant species are grown in these soils.     

Étude de la discrimination strontium-calcium chez le Haricot en fonction du rapport et des teneurs respectives en Sr et Ca de la solution nutritive

The discrimination between strontium and calcium is not only conditioned by the Sr/Ca ratio of the nutrient solution but also by the total dose (Ca + Sr) of the medium. The variation of the observed ratio in relation to the total dose is closely related with the Sr/Ca ratio of the solution. Sr is preferentially fixed by the roots when the Sr/Ca ratio of the solution is low; such a result is observed in the shoots for higher ratios. For all the treatments, the OR of the leaves is lower than unity (discrimination against strontium). When the stable strontium concentration of the solution is not detectable, an increase in the calcium concentration reduces the radiostrontium retention by the roots.     

Identifying calcium sources at an acid deposition-impacted spruce forest: a strontium isotope,alkaline earth element multi-tracer approach

Depletion of calcium from forest soils has important implications for forest productivity and health. Ca is available to fine feeder roots from a number of soil organic and mineral sources, but identifying the primary source or changes of sources in response to environmental change is problematic. We used strontium isotope and alkaline earth element concentration ratios of trees and soils to discern the record of Ca sources for red spruce at a base-poor, acid deposition-impacted watershed. We measured ⁸⁷Sr/⁸⁶Sr and chemical compositions of cross-sectional stemwood cores of red spruce, other spruce tissues and sequential extracts of co-located soil samples. ⁸⁷Sr/⁸⁶Sr and Sr/Ba ratios together provide a tracer of alkaline earth element sources that distinguishes the plant-available fraction of the shallow organic soils from those of deeper organic and mineral soils. Ca/Sr ratios proved less diagnostic, due to within-tree processes that fractionate these elements from each other. Over the growth period from 1870 to 1960, ⁸⁷Sr/⁸⁶Sr and Sr/Ba ratios of stemwood samples became progressively more variable and on average trended toward values that considered together are characteristic of the uppermost forest floor. In detail the stemwood chemistry revealed an episode of simultaneous enhanced uptake of all alkaline earth elements during the growth period from 1930 to 1960, coincident with reported local and regional increases in atmospheric inputs of inorganic acidity. We attribute the temporal trends in stemwood chemistry to progressive shallowing of the effective depth of alkaline earth element uptake by fine roots over this growth period, due to preferential concentration of fine roots in the upper forest floor coupled with reduced nutrient uptake by roots in the lower organic and upper mineral soils in response to acid-induced aluminum toxicity. Although both increased atmospheric deposition and selective weathering of Ca-rich minerals such as apatite provide possible alternative explanations of aspects of the observed trends, the chemical buffering capacity of the forest floor-biomass pool limits their effectiveness as causal mechanisms.     

Use of foliar Ca/Sr discrimination and <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr ratios to determine soil Ca sources to sugar maple foliage in a northern hardwood forest

Calcium/strontium and ⁸⁷Sr/⁸⁶Sr ratios in foliage can be used to determine the relative importance of different soil sources of Ca to vegetation, if the discrimination of Ca/Sr by the plant between nutrient sources and foliage is known. We compared these tracers in the foliage of sugar maple (Acer saccharum) to the exchange fraction and acid leaches of soil horizons at six study sites in the White Mountains of New Hampshire, USA. In a previous study, sugar maple was shown to discriminate for Ca compared to Sr in foliage formation by a factor of 1.14 ± 0.12. After accounting for the predicted 14% shift in Ca/Sr, foliar Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios closely match the values in the Oie horizon at each study site across a 3.6-fold variation in foliar Ca/Sr ratios. Newly weathered cations, for which the Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios are estimated from acid leaches of soils, can be ruled out as a major Ca source to current foliage. Within sites, the ⁸⁷Sr/⁸⁶Sr ratio of the soil exchange pool in the Oa horizon and in the 0–10 cm and 10–20 cm increments of the mineral soil are similar to the Oie horizon and sugar maple foliar values, suggesting a common source of Sr in all of the actively cycling pools, but providing no help in distinguishing among them as sources to foliage. The Ca/Sr ratio in the soil exchange pool, however, decreases significantly with depth, and based on this variation, the exchange pool below the forest floor can be excluded as a major Ca source to the current sugar maple foliage. This study confirms that internal recycling of Ca between litter, organic soil horizons and vegetation dominate annual uptake of Ca in northern hardwood ecosystems. Refinement of our understanding of Ca and Sr uptake and allocation in trees allows improvement in the use of Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios to trace Ca sources to plants.     

Comparative study on the uptake of strontium-85 from nutrient solutions and potted soils by lettuce

The relation between strontium-85 uptake by young lettuce plants and soil solution composition is discussed. Uptake from soils is furthermore compared to the uptake from nutrient solutions. A close relationship is shown to exist between the concentration of Sr in the plant and the Sr/Ca ratio in the solution, either in the nutrient medium or in the soil solution. The activity of the other ions in solution is shown to have only minor effects on the uptake of Sr. Results are discussed in the context of the hypothesis that the soil liquid phase is the environment from which plants primarily withdraw their nutrients.     

Mechanisms of neurotransmitter release facilitation in strontium solutions

Mechanisms of neurotransmitter release facilitation were studied using electrophysiological recording of end-plate currents (EPC) and nerve ending (NE) responses after substitution of extracellular Ca ions with Sr ions at the frog neuromuscular junction. The solutions with 0.5 mM concentration of Ca ions (calcium solution) or 1 mM concentration of Sr ions (strontium solution) were used where baseline neurotransmitter release (at low-frequency stimulation) is equal. Decay of paired-pulse facilitation of EPC at calcium solutions with increase of interpulse interval from 5 to 500 ms was well described by three-exponential function consisting of early, first and second components. Facilitation at strontium solutions was significantly diminished due mainly to decrease of early and first components. At the same time, EPC facilitation with rhythmic stimulation (10 or 50 imp/s) at strontium solutions was significantly increased. Also more pronounced decrease of NE response 3rd phase, reflecting potassium currents was detected under rhythmic stimulation of 50 imp/s at strontium solutions comparing to calcium solutions. It was concluded that facilitation sites underlying first and early components had lower affinity to Sr ions than to Ca ions. The enhancement of frequency facilitation at strontium solutions is mediated by two mechanisms: more pronounced broadening of NE action potential and increase of bivalent cation influx due to feebly marked activation of Ca(2+)-dependent potassium current by Sr ions, and slower dynamics of Sr(2+) removal from NE axoplasm comparing to Ca(2+).     

Relation Between Calcium and Strontium Transport Rates as Determined Simultaneously in the Primary Root of Zea mays

Root segments of Zea mays 55 mm long, were exposed to nutrient containing ⁸⁵Sr and ⁴⁵Ca tracers. Translocation rather than uptake was measured, using a newly-designed glass compartmentation system and validated tracer analytic model. Ca transport from solutions containing between 0.25 and 5.0 mm Ca was only slightly affected by concentration, but translocation from 0.25 to 0.05 mm solutions was markedly reduced. Maximum transport of strontium from nutrient containing 0.05 mm Ca was twice that from 2.5 mm Ca, and also twice the maximum calcium transported. Thus, under the condition simulating calcium depletion, i.e., 0.05 mm Ca, greater amounts of strontium were transported. In these cases the solutions also contained stable strontium at concentrations between 0.25 and 5.0 mm. In simultaneous determinations, the ratio of Sr to Ca moved was exactly equal to the ratio of their concentrations in nutrient solution, and there was no evidence of discrimination. Dinitrophenol reduced transport of Sr and Ca to an equivalent extent, amounting to between 2 and 9% of non-treated control levels.     

Localisation of mineral uptake by roots using Sr isotopes

To assess the contribution of deep soil horizons to the mineral supply of trees, we investigated the natural variation in the⁸⁷ Sr/⁸⁶Sr isotopic ratio of plant-available strontium with soil depth. In three sites of North-western Spain, this ratio increased with soil depth. The comparison of isotopic ratios of tree leaves and roots at different depths showed that most of the Sr accumulation in Eucalyptus globulus and Pinus pinaster growing on shallow and poor soils in this rainy climate originated from the upper soil layers. As Ca and Sr behave similarly in the soil-plant system, this conclusion can be applied to Ca. This superficial uptake is attributed to the low availability of Sr and Ca in the soil as well as to the shortness of the drought period as compared to the length of the growth period. This technique appears to offer a promising way of studying relative root distributions in soils and plant competition for nutrients.     

Strontium and paleodietary research: A review

The measurement of skeletal strontium/calcium (Sr/Ca) ratios can provide information on the proportion of meat and vegetable foods in the diets of prehistoric peoples. This information is based in the well-documented reduction of Sr/Ca ratios in terrestrial food chains. The reduction, and therefore the paleodietary technique, is complicated by (a) differences in Sr/Ca ratios entering food chains, and (b) metabolic considerations such as age, pregnancy, etc. Changes in Sr/Ca ratios during interment may also obscure biological Sr/Ca levels. The theoretical basis of the technique, its complications, and practical use by anthropologists are reviewed, in an attempt to define the quality of information currently and potentially available from Sr/Ca analyses.     

Strontium:cakium concentration ratios in otoliths of herring larvae as indicators of environmental histories

Synopsis Elemental analyses, using wave-length dispersive electron microprobe techniques on otoliths from reared Atlantic herring larvae, Clupea harengus, showed trace quantities of strontium relative to that of calcium, and an inverse relationship between Sr/Ca concentration ratios and rearing temperature. These data are consistent with those for coral aragonite, in that there appears to be an inverse temperature effect on physiological incorporation of strontium in the otolith aragonite. Our determinations of Sr/Ca concentration ratios of lab-reared herring larvae showed that the deposition of strontium relative to calcium and the rearing temperature were related, where: T (° C) = −2.955 [Sr/Ca] × 1000 ± 19.172. This principle thus makes it possible to use Sr/Ca concentration ratios in fish otoliths to delineate past temperatures experienced by an individual. Further, combining electron microprobe analyses with scanning electron microscope (SEM) examinations of daily increments in the same otolith makes it possible to reconstruct the temperature history for an individual fish on a time scale of days. An example of the application of the technique to an approximately six-month-old field-caught herring larva is given, and the limitations of the technique are discussed.     

Shoot Sr concentrations in relation to shoot Ca concentrations and to soil properties

This work was aimed to investigate whether shoot Sr concentrations of plant species are related to respective Ca concentrations and to soil properties and to compare the Sr-Ca observed ratios (OR), defined as the quotient of the ratios Sr/Ca in shoots and in the soil solution or in the extractable form, among species and soils. Ten pasture plant species were grown in pots (1-L volume) filled with eight soils differing in the various physicochemical characteristics. Each pot received 50 mg Sr except those of the soil with the highest cation exchange capacity (C.E.C.) that received 100 mg Sr per pot. For each soil, shoot Sr concentrations of species were linearly and positively related with the respective Ca concentrations. C.E.C, organic matter content and Ca in the soil solution or in the extractable form were the only soil properties that were related, all negatively, with shoot Sr concentrations. The ratio of extractable Sr and Ca was positively and linearly related with the ratio of Sr and Ca. in the soil solution. OR was affected by both species and soils. Most of OR values of all species in all soils ranged between 0.8 and 1.5, except for the grass Agrostis capillaris which had the highest values for most of soils. This indicates that Agrostis capillaris compared to other species, takes up proportionally more Sr than Ca.     

The relative uptake of Ca and Sr into tree foliage using a whole-watershed calcium addition

The use of strontium isotopes and ratios of alkaline earth elements (i.e., ⁸⁷Sr/⁸⁶Sr and Ca/Sr) to trace Ca sources to plants has become common in ecosystem studies. Here we examine the relative uptake of Ca and Sr in trees and subsequent accumulation in foliage. Using a whole-watershed Ca addition experiment at the Hubbard Brook Experimental Forest in N.H., we measured the uptake of Ca relative to Sr in foliage and roots of sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), American beech (Fagus grandifolia), and red spruce (Picea rubens). Vegetation was analyzed for Ca and Sr concentrations and the ⁸⁷Sr/⁸⁶Sr ratio. A comparison of the Ca/Sr ratio in the vegetation and the Ca/Sr ratio of the applied mineral allows for the calculation of a discrimination factor, which defines whether Ca and Sr are incorporated and allocated in the same ratio as that which is available. A discrimination factor greater than unity indicates preferential uptake of Ca over Sr; a factor less than unity reflects preferential uptake of Sr over Ca. We demonstrate that sugar maple (SM) and yellow birch (YB) have similar and small discrimination factors (1.14 ± 0.12,1σ and 1.16 ± 0.09,1σ) in foliage formation and discrimination factors of less than 1 in root formation (0.55–0.70). Uptake into beech suggests a larger discrimination factor (1.9 ± 1.2) in foliage but a similar root discrimination factor to SM and YB (0.66 ± 0.06,1σ). Incorporation into spruce foliage occurs at a much slower rate than in these other tree species and precludes evaluation of Ca and Sr discrimination in spruce foliage at this time. Understanding the degree to which Ca is fractionated from Sr in different species allows for refinement in the use of ⁸⁷Sr/⁸⁶Sr and Ca/Sr ratios to trace Ca sources to foliage. Methods from this study can be applied to natural environments in which various soil cation pools have different ⁸⁷Sr/⁸⁶Sr and Ca/Sr ratios. The results reported herein have implications for re-evaluating Ca sources and fluxes in forest ecosystems.     

Accumulation and subcellular distribution of cations in relation to the growth of potassium-deficient barley

Abstract Growth of barley (Hordeum vulgare L., cv. Georgie) was insensitive to soil K content above about 150 mg kg^?1, but at lower levels it declined. The reduction in yield was greater in soils containing approximately 10 mg Na kg^?1 than in soils with about 90 mg kg^?1 of Na. Growth was unaffected by changes in shoot K concentration above 75 mol m^?3, but declined at lower concentrations, and the decrease was less in plants grown in soils with high Na. Growth responses were not simply related to tissue K concentrations because plants grown in soils with extra Na had higher yields but lower K concentrations. When soil Na was low, plants accumulated Ca as tissue K declined, but when Na was provided this ion was accumulated. Plant Mg concentrations were generally low but increased as K decreased. The Ca and Mg were osmotically active. There were highly significant inverse linear relationships between yield and either the Ca or Mg concentrations in the shoots. X-ray microanalysis was used to examine the compartmentation of cations in leaves from barley plants (cv. Clipper) grown in nutrient solutions with high and low K concentrations. In plants grown with 2.5 mol m^?3 K, this was the major cation in both the cytoplasm and vacuole of mesophyll cells. However, in plants grown with 0.02 mol m^?3 K it declined to undetectable levels in the vacuole, although it was still detectable in the cytoplasm. In all plants, Ca was mainly located in epidermal cells. The implication of the results for explaining responses to K. in terms of compartmentation of solutes is discussed.     

Influence of the spatial layout of vegetation on the stability of slopes

Background and aims

Plant nutrient uptake is affected by environmental stress, but how plants respond to cation-nutrient stress is poorly understood. We assessed the impact of varying degrees of cation-nutrient stress on cation uptake in an experimental plant-mineral system.

Methods

Column experiments, with red pine (Pinus resinosa Ait.) seedlings growing in sand/mineral mixtures, were conducted for up to 9 months. The Ca and K were supplied from both minerals and nutrient solutions with varying Ca and K concentrations.

Results

Cation nutrient stress had little impact on carbon allocation after 9 months of plant growth and K was the limiting nutrient for biomass production. Measurement of Ca/Sr and K/Rb ratios allowed independent estimation of dissolution incongruency and discrimination against Sr and Rb during cation uptake processes. The fraction of K in biomass from biotite increased with decreasing K supply from nutrient solutions. The mineral anorthite was consistently the major source of Ca, regardless of nutrient treatment.

Conclusions

Red pine seedlings exploited more mineral K in response to more severe K deficiency. This did not occur for Ca since Ca was not limiting plant growth. Plant discrimination factors must be carefully considered to accurately identify nutrient sources using cation tracers.     

Calcium Oxalate Crystal Formation and Growth in Two Legume Species as Altered by Strontium

Young plants of Phaseolus and Canavalia were grown in nutrient solutions where calcium had been partially or totally replaced by either strontium or sodium. The deleterious effect of this replacement on growth and development was greater with strontium than with sodium. As the calcium content of the nutrient solution was decreased fewer calcium oxalate crystals were formed in leaf tissues. There were fewer crystals formed when calcium was replaced by strontium than by sodium. Changes in solubility characteristics of the crystals indicated that they had incorporated strontium.     

Effects of strontium on the absorption of calcium, magnesium and phosphate ions from the ovine reticulo-rumen

The net absorption rates of strontium ions from the ovine reticulo-rumen, isolated in situ in trained conscious animals, were measured under controlled conditions. A linear positive response was obtained from the addition of Sr²⁺ ions to the artificial rumen fluid. This increase in the absorption of Sr was reflected in an increase in the plasma Sr concentration. In contrast to the discrimination observed elsewhere in favour of the absorption of Ca relative to Sr, the absorption rate of Sr from the reticulo-rumen was significantly greater than that of Ca, from solutions containing the same molar concentration. A graded increase in the Sr concentration in the ruminal fluid from 1 mmol/l to 4 mmol/l led to a corresponding reduction in the absorption rate of Ca but an increase in that of phosphate. The latter result is similar to that observed when the intra-ruminal concentration of Ca²⁺ ions is increased. It is suggested that Ca and Sr share a common pathway for absorption from the reticulo-rumen and that this may involve coupling with the absorption of phosphate ions. Accepted: 18 January 2000     

Test of soil extractants for their suitability in predicting Ca/Sr ratios in leaves and stems of sugar maple seedlings

Differential uptake and translocation of Ca and Sr in organisms have been reported, calling into question the use of Sr to track Ca cycling in the environment. We investigated the relationship between Ca/Sr ratios in soil extracts of various strengths (H₂O, NH₄Cl, and NH₄EDTA) and seedlings of sugar maple (Acer saccharum Marsh.) grown from natural regeneration on 37 sites. Our objectives were to determine if Ca/Sr ratios in soil extracts are correlated with those in sugar maple tissues, and what soil extractant best duplicate plant tissue Ca/Sr ratios. Leaves had higher Ca/Sr ratios than stems and the extractants did not produce equal Ca/Sr ratios: H₂O had the lowest Ca/Sr, and NH₄EDTA the highest. The relationships between soil extract Ca/Sr ratios and leaf and stem Ca/Sr ratios were significant and linear, but the slopes differed among extractants. The lowest slope (0.45) was observed for the water extract/leaves and the highest (2.15) for the NH₄EDTA extract/stem with discrimination factors ranging from 0.22 with NH₄EDTA to 1.59 for water. Leaf extracts were more strongly correlated with soil Ca/Sr than stem extracts (R ² of 0.57–0.7 vs. R ² of 0.45–0.6, respectively). These findings support the use of Ca/Sr ratios in plants to track their source of soil Ca, but they highlight the need to calibrate the relationships for the plant tissue and soil extractant used.     

Evaluation of the mobility and discrimination of Ca, Sr and Ba in forest ecosystems: consequence on the use of alkaline-earth element ratios as tracers of Ca

A comprehensive understanding of Ca cycling in an ecosystem is desirable because of the role of this element in tree mineral nutrition and its status as a major base cation on the soil exchange complex. The determination of the origin of Ca in forests is particularly indicated in regard of important changes linked to acid inputs and intensive logging. Natural strontium isotopes are increasingly used as tracers of Ca in forest ecosystems for qualitative and quantitative assessments. Nevertheless this method is limited to relatively simple systems with two sources of nutrients. Some recent studies coupled Sr/Ca or Sr/Ba ratios to Sr isotopic measurements in order to solve more complex systems. Such method has however associated with it some uncertainties: this approach assumed that Ca, Sr and Ba behave similarly throughout the ecosystem and does not take into account the Ca biopurification processes occurring in some tree’s organs which can alter element ratio. The present work focuses on two deciduous species covering large areas in Europe: European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.). In order to test the similarity of behaviour between Ca, Sr and Ba, their concentrations were measured extensively in the major compartments of two forest ecosystems. In parallel, the discrimination process inside tree organs was studied in 23 stands for beech and 10 stands for oak. We found that Sr and Ca behave similarly in all soil and tree compartments. By contrast, Ba and Ca appear to have contrasting behaviours, especially in streams, soil solution and soil exchange complex (no correlations between element concentrations). Sr/Ba and Ba/Ca ratios must therefore be used with care as tracer of Ca. The Ca biopurification is absent in roots and slight in bole wood but is large in bark, twigs and leaves. The discrimination factors (DF) between wood and leaves are characteristic of the two species studied and do not change significantly as a function of the soil Ca status (acidic or calcareous soils). Therefore, strontium–calcium DF can be used as a correction factor of the Sr/Ca ratio of leaves when this ratio is used in connection with Sr isotopic ratios. This correction allows to solve systems of tree nutrition with more than two sources of Ca.