Role of Ca-ox in controlling Ca/Sr discrimination and 44Ca/40Ca fractionation of calcium

Background/Question/Methods

Cation tracers such as Ca and strontium (Sr) ratios and Ca-isotopes (⁴⁴Ca/⁴⁰Ca) are gaining interest as tools to identify sources and pathways of calcium (Ca) cycling in ecosystem studies. The mechanisms that influence these elemental and stable isotope ratios remain poorly resolved during cycling through ecosystem pools and especially among different soluble, structural and crystalline forms of cations. We synthesized Ca-oxalate (Ca-ox), a ubiquitous biomineral found in terrestrial ecosystems and measured discrimination for Ca over Sr and ⁴⁰Ca over ⁴⁴Ca. However, inorganic synthesis of Ca-ox may not be directly comparable to in situ Ca/Sr and ⁴⁴Ca/⁴⁰Ca bulk values due to complex physiological processes during Ca-ox crystal formation in plants. So, at high and low-Ca status sites with differences in Ca-ox accumulation in Douglas-fir tissues in the Coast Range of Oregon we investigated plant pools and soils to compare whole-plant level, organ-level and tissue-level Ca/Sr discrimination and ⁴⁴Ca/⁴⁰Ca separation.

Results/Conclusions

The formation of this Ca oxalate is a newfound mechanism for biological discriminating for Ca over Sr and fractionating for Ca isotopes. We determined that inorganic crystal formation exhibited preference for Ca over Sr (discrimination factor for Ca was 5.4), and for ⁴⁰Ca over ⁴⁴Ca (the ⁴⁴Ca-enrichment factor was -1.46 ‰ for the precipitation of Ca-ox). Our preliminary findings indicate that tissue-level Ca/Sr discrimination factors were higher in magnitude than whole-plant or plant organ-level discrimination at both sites, primarily due to Ca-ox accumulation. There was a positive linear relationship between Ca-ox concentrations and Ca/Sr values (R² = 0.55) across all tree tissue types and sites indicating that Ca-ox accumulation related to tree Ca supply status can have a significant impact on bulk tree tissue Ca/Sr values. Organ-level ⁴⁴Ca/⁴⁰Ca separation factors were equal or greater in magnitude than whole-plant and plant tissue-level separation factors, indicating that movement of Ca between plant organs on xylem exchange sites is an important influence on bulk ⁴⁴Ca/⁴⁰Ca values. Using a multi-tracer approach, the ecosystem pools at two sites were comparable in directionality on bivariate plots of Ca/Sr and ⁴⁴Ca/⁴⁰Ca suggesting consistent patterns within this species across sites with different Ca-status, however, continued mechanistic understanding of discrimination and fractionation is necessary in order to use these tools as tracers of cations in terrestrial ecosystems.