Stable isotopes are useful tools to describe how elements move through plants, as transport mechanisms often favor certain isotopes. In the past, stable isotope research on plants has focused primarily on agricultural plants and the transport of elements with high concentrations. Only recently have improvements in mass spectrometry made natural-abundance measurements of heavier metal isotope systems, including magnesium (Mg) and calcium (Ca), possible. Work to characterize these systems in plants is ongoing, and no study measures both Ca and Mg in the same plants. The new potential to study Ca and Mg in plants motivates this thesis, as does the application of stable isotopes to paleodietary study. Ca and Mg isotopes within dinosaur tooth enamel have been used to hypothesize trophic level differences, spatial niche partitioning, feeding height stratification, and preferential feeding on plant groups and plant parts. Without a more complete understanding of isotopic fractionation in plants, however, it is difficult to support these conclusions.

In this thesis, I aim to advance understanding of Mg and Ca isotope systems through a survey of plant materials gathered from two field sites in the California redwoods. I observe Ca and Mg isotope signals (1) at ecosystem scale (from water to soil to plants and across plant species), (2) within plants, across different plant parts, and (3) across leaves up the height of a single 107-meter redwood tree. When considering Ca and Mg isotopes separately, differences in isotopic ratios occur across multiple scales (location, species, plant parts, leaf height), complicating paleodietary interpretations. A model for Mg and Ca transport is proposed as a framework to quantify some of these differences. When considering Ca and Mg isotope measurements together, species form distinct clusters, which may be explained by the proportion of different pools of nutrients, mediated by the presence of potassium (K). Thus, the dual measurement of the two isotope systems in plants and tooth enamel emerges as a promising tool to inform paleodietary reconstructions, for dinosaurs as well as other animals, and to improve understanding of Ca and Mg biogeochemical cycling. Future work should clarify the role of K in fractionation and further classify plant material in Mg-Ca isospace.

Potassium (K), magnesium (Mg), and calcium (Ca) are vital macronutrients in plant biochemistry, regulating plant growth, nutrient and metabolite transport, responses to environmental stresses, etc. K and N fertilizers are frequently applied to crops while Mg and Ca are usually applied when deficiency is expected. This thesis investigates how varying K concentrations affect nutrient uptake in plants, particularly in Arabidopsis thaliana, using a controlled hydroponic experiment setup. Plants regulate potassium uptake through specialized transport systems. When potassium is limited (<0.1 mM), plants tend to depend on high-affinity transport systems (HATS) to uptake potassium, whereas low-affinity transport systems (LATS) dominate when potassium levels are plentiful (>0.1 mM). Previous experiments revealed that at 0.10 mM external potassium, plants exhibited δ⁴¹K isotopic signatures in line with HATS, employed under stressed conditions without showing significant reductions in plant K acquisition relative to plants grown in more replete potassium environments. To address the gaps from the previous experiments, we examined intermediate potassium concentrations to evaluate whether a gradual transition or a step change would occur between HATS and LATS. Our results demonstrate that δ⁴¹K values become more negative with rising potassium availability. Plants grown at 0.33 mM potassium displayed intermediate δ⁴¹K, suggesting the concurrent operation of both transport systems. In contrast, δ⁴⁴/⁴⁰Ca and δ²⁶Mg remained stable across varying potassium conditions, indicating that their uptake is not dependent on external potassium availability. Additionally, based on the observed relationship between δ⁴¹K and δ15N, potassium availability may have an influence on nitrogen source utilization in plants (NO3 or NH+4). These findings demonstrate how nutrient availability affects uptake mechanisms and isotopic fractionation, providing new insights to achieve more sustainable agriculture practices.