In studies of the nitrogen fixation of Elliott's milkpea, researchers found that increasing carbon dioxide increases nitrogen fixation at first, but then decreases it, even to a level that is below the baseline level. If a program were being designed to maintain high levels of nitrogen fixation, the best course of action would be to add
A) even more carbon dioxide gas.
B) more nitrogen gas.
C) iron, molybdenum, and other micronutrients to the soil.
D) a combination of oxygen and nitrogen gases.
E) a competing species.

Question

Quizplus · Accepted Answer

The decrease in nitrogen fixation at high levels of carbon dioxide suggests that the plant may be limited by the availability of micronutrients needed for nitrogen fixation. Adding iron, molybdenum, and other micronutrients to the soil can alleviate this limitation and maintain high levels of nitrogen fixation. Adding more carbon dioxide or nitrogen gas, or a combination of oxygen and nitrogen gases, would not address the underlying limitation of micronutrient availability. Similarly, introducing a competing species would not necessarily improve nitrogen fixation in Elliott's milkpea.

In Studies of the Nitrogen Fixation of Elliott's Milkpea, Researchers