Drivers of plant nutrient acquisition and allocation strategies and their influence on plant responses to environmental change

Photosynthesis is the largest carbon flux between the atmosphere and terrestrial biosphere and is constrained by ecosystem biogeochemical cycles. Terrestrial biosphere models exhibit strong divergence in simulated carbon and nitrogen fluxes under future environmental conditions. Divergence across model products may due to the high sensitivity of terrestrial biosphere models to the formulation of photosynthetic processes, coupled with uncertainty in the photosynthetic acclimation response to changing aboveground and belowground environments. Photosynthetic least-cost theory provides a promising framework for understanding such photosynthetic responses to changing environments; however, empirical tests of the theory are rare, limiting our ability to assess whether the theory is suitable for implementing in future iterations of terrestrial biosphere model products. Here, I present four experiments designed to test assumptions of photosynthetic least-cost theory. Experiment chapters are flanked by a general introduction chapter and general conclusion chapter. The first experimental chapter quantifies structural carbon costs to acquire nitrogen in Glycine max and Gossypium hirsutum grown under four nitrogen fertilization levels and four light availability levels in a full factorial greenhouse experiment. I find that carbon costs to acquire nitrogen in both species increase with increasing light availability and decrease with increasing fertilization, though responses to fertilization in G. max were markedly less than G. hirsutum. The second experimental chapter quantifies leaf nitrogen and photosynthetic traits in upper canopy leaves of deciduous trees growing in a 9-year nitrogen-by-sulfur field manipulation experiment. I find evidence for nitrogen-water use tradeoffs with increasing soil nitrogen availability, evidenced through a negative relationship between leaf nitrogen content and ratio of leaf intercellular CO2 concentration to atmospheric CO2 concentration (leaf Ci:Ca) and stronger increase in leaf nitrogen content with increasing soil nitrogen availability than leaf Ci:Ca. The third experiment investigates variance in leaf nitrogen content across a climate and soil resource availability gradient in Texan grasslands, showing that effects of soil resource availability and climate on leaf nitrogen content are driven by changes in leaf Ci:Ca. Finally, the fourth experiment quantifies leaf and whole plant acclimation responses in G. max grown under two atmospheric CO2 levels, with and without inoculation with Bradyrhizobium japonicum, and across one of nine nitrogen fertilization treatments in a full factorial growth chamber experiment. I find that photosynthetic responses to CO2 were independent of soil nitrogen fertilization and inoculation treatment, though the positive effect of elevated CO2 on total leaf area and total biomass was stronger with increasing fertilization and in inoculated pots under low fertilization. Experiments included in this dissertation provide consistent support for patterns expected from photosynthetic least-cost theory. The use of multiple experimental approaches allowed me to examine mechanisms driving patterns expected from the theory and investigate whether these patterns occur in the field across environmental gradients. Findings from these chapters challenge common paradigms in plant ecophysiology, providing empirical evidence suggesting that including photosynthetic least-cost frameworks in terrestrial biosphere models may improve longstanding divergence in simulated carbon fluxes across terrestrial biosphere model products.