Nitrate Assimilation under Elevated CO2 and Leaf N2O Emissions

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The PIs have found that high carbon dioxide levels interfere with the ability of wheat plants to use nitrate fertilizer as a nitrogen source. Were this a general phenomenon, it would offer a mechanistic explanation for several other unresolved issues including (a) why the stimulation of photosynthesis by high carbon dioxide levels declines over time, (b) why high carbon dioxide levels inhibit plant respiration, and (c) why nitrogen availability determines ecosystem response to carbon dioxide enrichment. The PIs have also discovered that wheat leaves emit significant amounts of nitrous oxide, a greenhouse gas that has an effect approximately 250 times greater than carbon dioxide and that contributes to ozone degradation in the upper atmosphere. The PIs preliminary results indicate that leaf nitrous oxide emissions could account for as much as 12% of the nitrous oxide generated worldwide.The proposed research will survey a range of plant types for carbon dioxide inhibition of nitrate assimilation and leaf nitrous oxide emissions to determine whether they are common occurrences. It will evaluate some of the possible mechanisms behind this inhibition. It will assess the importance of several possible sources of nitrous oxide emissions including shoot nitrate assimilation, root nitrate assimilation, and production by microbes around the root. It will evaluate these phenomena in the field through a collaboration with an ongoing study that is examining soil nitrous oxide emissions from wheat fields in the Yaqui Valley of Mexico. The results of this research will be far-reaching, providing fundamental information on the mechanisms responsible for plant responses to elevated carbon dioxide as well as for greenhouse gas emissions from plants.