Acquisition and allocation of resources in response to elevated CO2 and nitrogen fertilization in two perennial C3 grasses

Acquisition and allocation of resources in response to elevated CO2 and nitrogen fertilization in two perennial C3 grasses Hannah Abigail Kinmonth-Schultz Chairperson of the Supervisory Committee: Assistant Professor Soo-Hyung Kim College of Forest Resources Atmospheric concentrations of carbon dioxide (CO2) have increased over the last century, likely due to human activity, and will likely continue to increase. Plants will respond to increased CO2 by increased rates of photosynthesis, biomass, and carbon stored in plant tissue; however, the response may differ between plant species and functional groups. Therefore, composition of communities may be altered. Fast growing invasive plants may be best suited to take advantage of the increase in CO2. The effect of CO2 on plant growth must also be considered in conjunction with other human mediated resource changes. Nitrogen is of particular interest because it is often a component of run-off from urban and agricultural areas into natural lands, as well as being a component of wet and dry atmospheric deposition; it facilitates invasive plant establishment; and plant growth is impacted by the ratio of carbon to nitrogen in plant tissue. Here we describe chambers that were built to elevate CO2 and the results of a study in which two plant species were subjected to elevated CO2 and nitrogen. Phalaris arundinacea is a common invader of wetlands throughout North America, while Glyceria striata is a native species that has been found to suppress germination of P. arundinacea seedlings. Both are rhizomatous, C3, perennial grasses. These species were subjected to either ambient or elevated CO2 (~320 ppm above ambient), and two nitrogen treatments – full strength Hoagland’s solution or a modified solution containing 1/8 the nitrogen components, over two repetitions of a closed-top chamber experiment in the Douglas Research Conservatory, at the University of Washington Botanic Gardens, Seattle, WA, USA. The chambers were effective in maintaining elevated CO2 levels, however improvements to their design are discussed. While nitrogen was the strongest contributor to differences in growth in both species, CO2 contributed to an increase in aboveground biomass, primarily in stem and leaf biomass. The CO2 effect was enhanced in the high nitrogen pots while the increase was more pronounced for G. striata. For both species elevated CO2 led to an increase in root biomass across nitrogen treatments, and for P. arundinacea, a decrease in allocation to rhizomes in the high nitrogen pots. Elevated CO2 led to a decrease in specific leaf area for P. arundinacea. For P. arundinacea the carbon-to-nitrogen ratio increased in the low nitrogen pots, both because of an increase in carbon, and because of a decrease in nitrogen. The same pattern occurred for G. striata; however, this was due only to a decrease in nitrogen. P. arundinacea increased its fructan-to-carbon ratio in the low nitrogen pots, with an even higher increase under elevated CO2. Most responses to CO2 occurred during the first experimental repetition, which had higher light levels than the second. We conclude that elevated CO2 likely increased the competitive ability both species, but for somewhat different reasons. It may increase the competitive ability of P. arundinacea by increasing aboveground growth in high nitrogen environments. Phalaris arundinacea responds to low nitrogen by increasing carbon allocation to its rhizomes, primarily to non-structural carbohydrates such as fructans. Elevated CO2 magnifies this trend. Elevated CO2 may also enhance the competitive ability of G. striata, and G. striata does not accumulate carbon, but rather stores nitrogen in its rhizomes when nitrogen is available.