Physiological and growth responses of grassland species to elevated carbon dioxide and increased nitrogen supply with emphasis on symbiotic nitrogen fixers vs. non-fixers

Although rising atmospheric concentrations of carbon dioxide (CO ₂) and increasing nitrogen (N) deposition are well-documented global changes, their interactive effects on vegetation are not well understood. This thesis characterizes the physiological and growth responses of perennial grassland species to combinations of atmospheric CO₂ and N treatments. Mechanisms operating at leaf, whole-plant, and community levels were studied using species adapted to N-limited habitats with emphasis on species capable of accessing atmospheric N through symbiotic N₂ fixation.; The response of leaf-level gas exchange was measured for 13 species in field plots exposed to ambient (368 μmol mol−1) and elevated (560 μmol mol−1) CO₂ concentrations combined with unamended and enriched (+4 gN m−2 yr −1) N treatments. All species showed pronounced photosynthetic acclimation resulting in minimal stimulation (7%) of photosynthesis with CO ₂ enrichment. Elevated CO₂ decreased stomatal conductance (24%), leading to increases in intrinsic water-use efficiency. Increased N supply did not affect leaf-level responses to elevated CO₂. The substantial acclimation of photosynthesis was associated with decreases in stomatal conductance and leaf N in response to CO₂ enrichment.; To further investigate the effect of N availability on the CO₂ response, growth and physiological responses to elevated CO₂ were compared between an N₂-fixer and a non-N₂-fixer across a range of N additions in a growth chamber study. The N₂-fixer derived 32% more N from symbiotic N₂ fixation and accumulated 80% more biomass, regardless of N addition, under elevated compared to ambient CO₂. In contrast, the growth response to CO₂ enrichment of the non-N₂-fixer was limited at low N.; The hypothesis that N₂-fixers will alleviate N-limitations on the CO₂ responses of plants and communities was evaluated in multi-species field assemblages. Photosynthesis and plant and soil N status were enhanced by the presence of an N₂-fixer, however this did not facilitate greater responses of non-N₂-fixers to elevated compared to ambient CO₂.; Interspecific variation in acclimation of photosynthesis to CO₂ enrichment and N availability, and the contrasting growth responses of N₂-fixers and non-N₂-fixers, will be important determinants of the response of vegetation to future environments. Furthermore, species interactions may critically modify plant and community responses to these global changes.