Ca isotopes (44Ca/40Ca) and calcium (Ca) and strontium (Sr) ratios are increasingly being used to understand forest Ca cycling, although there remains much to learn about processes controlling fractionation in plants. For this study, we hypothesized that the formation of Ca-oxalate in foliar tissues fractionates Ca isotopes and Ca/Sr and contributes to variation in the patterns observed in plants. We first synthesized Ca-oxalate from an aqueous solution in the laboratory and determined a fractionation factor (aCa-oxalate-aqueous Ca) -1.46 per mil, confirming that Ca-oxalate formation is isotopically depleted relative to aqueous Ca. Oxalate crystal formation selectively accumulated at a rate 35% higher for Ca than Sr at the levels evaluated in our experiment. We then sampled Douglas-fir (Pseudotusga menziesii) needles at two sites in the Coast Range of Oregon that differed in Ca both among sites and across needle age classes, reflecting differences in soil Ca status. We used a chemical sequential extraction technique (water, 2N acetic acid, 2N hydrochloric acid) to isolate soluble-Ca, structural-Ca and Ca-oxalate foliar pools, respectively.
Results/Conclusions Nearly all of the difference in foliar Ca levels among sites and needle age classes was attributable to Ca-oxalate concentrations which increase with soil Ca status and with leaf age. 44Ca/40Ca of foliar Ca-oxalate was isotopically depleted relative to other foliar pools, particularly in young needles. Isotopic mass balance calculations revealed that the high-Ca and low-Ca sites had similar bulk leaf 44Ca/40Ca, but that the isotopic contrast between the soluble-Ca and Ca-oxalate fractions was greater at the low-Ca site. Variations in Ca/Sr values were greater across sequentially extracted leaf cation pools than across leaf age classes. The high degree of variation in Ca/Sr ratios, especially at the high Ca site, appears to be driven by the accumulation of Ca in oxalate in leaf tissues over Sr. These data highlight a problem in using Sr as a proxy for Ca in forest studies, particularly with respect to plant physiology and nutrient dynamics, and where oxalate is an important biomolecule shaping forest Ca cycling. These preliminary results suggest that foliar Ca oxalate dynamics are an important control of plant Ca isotope signatures and Ca/Sr ratios, and draw attention to questions about Ca physiology in trees.
