Theory 85/Energy Limits/Nitrogen fixation
Fixing Nitrogen Is Expensive
The claim
Biological nitrogen fixation costs a fixed fraction of photosynthetic energy, so its occurrence is set by the ratio of nitrogen need to carbon surplus.
01Mechanism
Nitrogenase consumes ATP and reducing power; the plant fixes only when nitrogen limitation exceeds the carbon cost.
02Prediction
Fixation should track the ratio across soils.
03Refutation
Fixation tracking nitrogen need alone would remove the carbon term.