Plant and Soil 225: 299–310, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 299 A comparison between Ca and Sr cycling in forest ecosystems Anne Poszwa 1,3 , Etienne Dambrine 1 , Benoît Pollier 1 and Olivier Atteia 2 1 INRA Nancy, Cycles Biog´ eochimiques, 54280 Champenoux, France. 2 Institut EGID, Universit´ e Bordeaux III, 33 405 Talence, France. 3 Corresponding author ∗ Received 6 July 1999. Accepted in revised form 11 July 2000 Key words: Acer pseudoplatanus L., calcium, Fagus sylvatica L., Karst, Picea abies L., strontium Abstract In favourable conditions, the 87 Sr/ 86 Sr isotope ratios of the Sr delivered by rain and soil mineral weathering differ. Assuming that Ca and Sr behave similarly in forest ecosystems, several authors have used the 87 Sr/ 86 Sr variation in forest compartments to calculate the contribution of rain and mineral weathering to Ca fluxes and pools. However, there are a number of experimental reports showing that Ca and Sr may behave differently in the soil and in the plant. We have tested this Ca–Sr analogy in the field by measuring the variation of Sr and Ca concentrations, fluxes and pools in spruce, beech and maple stands on granite, sandstone and limestone. Results show that (1) variations of Ca and Sr concentrations are generally correlated at each level of the ecosystems. (2) In spruce on acid soils, a preferential uptake of Ca over Sr occurs (Aubure spruce Sr/Ca = 0.8×10 −3 ; soil exchangeable Sr/Ca between 2 and 6×10 −3 ). On calcareous soils, a preferential uptake of Sr over Ca by spruce may occur. (3) In spruce and beech on acid and calcareous soils, a preferential translocation of Ca over Sr from roots to leaves occurs ((Sr/Ca) in leaves was between 10 and 90% of that in roots). (4) The biological cycling of Ca and Sr leads to an enrichment of the upper soil layers in Ca and Sr. Compared to Sr, Ca accumulates in the upper layer of acid soils because Ca cycling through litterfall is favoured over Sr cycling, and possibly because of the selectivity of acid organic exchangers for Ca. Introduction Calcium (Ca) is a major element and is essential for plants. Amounts of Ca in plants fluctuate between 5 and 50 mg g −1 of the dry weight according to species, organ and stage of development. Calcium accumu- lates in old plant parts (Asta, 1992; Marschner, 1986; Mostafa and Ulrich, 1976). In most forest ecosystems, the strontium (Sr) and Ca absorbed by trees originate from two primary sources: atmospheric inputs and weathering of primary soil minerals. In favourable conditions, the 87 Sr/ 86 Sr isotopic ratio of atmospheric deposition and weathering differ enough to measure the contribution of each source to plant, soil exchangeable and stream- water Sr (Wickman, 1996). The chemical structures of strontium (Sr) and calcium (Ca) are similar. The ∗ FAX No: 383394069. E-mail: poszwa@nancy.inra.fr ionic and hydrated radii of Ca, 0.099 nm and 0.6 nm, respectively, are close to those of Sr, 0.113 nm and 0.5 nm, respectively (Faure, 1986). Assuming that Ca and Sr behave similarly, the results obtained for Sr have been applied to Ca (Åberg, 1995). The 87 Sr/ 86 Sr variation with soil depth of root available Sr has been used to study the contribution of specific soil horizons to Ca uptake (Dambrine et al., 1997; Wickman and Jacks, 1992). Using the same assumption, relative con- tributions of the Ca deposited in rain and released by weathering to forest pools have been calculated (Jacks et al., 1989). There is a large set of reports suggesting that Ca and Sr may behave differently in the soil-plant system. Sr/Ca exchange isotherms on organic substrates indic- ate almost no preferential adsorption on carboxylate groups at neutral pH, while a preference for Ca over Sr (Vanselow selectivity coefficient Kv Ca→Sr = 0.3) was obtained at low pH (Baes and Bloom, 1988). Con-