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The Impact of High CO₂ on Plant Photosynthesis
The Value of Ammonia Fertilizer in a High CO₂ Atmosphere
 
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BASIC PREMISES. When plants to do photosynthesis they utilize a fixed about of machinery to do so, which is schematically presented here as the figure to the right. What is only now being recognized (ref) is that the very same machinery is used to convert nitrates in the environment to the nitrites that the plants need to make their proteins.

Thus we could redesign our schematic to show, below, the partitioning of the fixed amount of machinery between photosynthesis and nitrite (NO₂^-) production. The fixing of CO₂ is usually equated with "growth" because the produced sugars become cellulose and starches, which constitute most of the plant' biomass.

Today with our concerns about global warming as caused by increasing levels of CO₂ in our atmosphere, we are interested in the impact this might have on plant growth, which is the base of our food pyramid. Heretofore, it was assumed that increasing CO₂ in the environment surround the plant would increase the rate of plant growth. This was indeed borne out by short-term growth studies. However, agronomists have recently checked this out with long-term studies and found otherwise! While there was an initial surge in growth, that growth soon slowed and eventually fell below the rates found in the control group of plants that were in normal atmospheric concentrations of CO₂.

HYPED CO₂ LEVELS. The reason for the slowdown in plant growth in an atmosphere enriched in CO₂ becomes evident when one considers the proportion of the machinery available for producing nitrite. As CO₂ increases, that for making nitrite decreases. With continuing increasing deficit of nitrogen building blocks, the plant merely cannot produce the enzymes and other proteins needed to grow rapidly. In the diagram to the right, you will note that the "sugar" is less (smaller letters than previously shown, above), and the nitrite is similarly less. With lessened amount of nitrite, the synthesis of more of the machinery slows. So actually the boxy schematic for the machinery should be drawn smaller!

Is there any easily feasible way that this problem can be averted? Happily there is! Merely bypass the plant's need to convert nitrate to nitrite. Afterall, the plant takes the nitrite and reduces it to ammonia which enters the N-pool through the glutamic dehydrogenase gateway. So rather than fertilize the plant with nitrates, use ammonia, which many farmers already use. Thus we get more ammonia into the plant, and the photosynthetic machinery can devote itself fully to photosynthesis.

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