Theory 61/Nitrogen cycle/Onset phase
Fixation Starts on a Need Threshold
The claim
Biological nitrogen fixation begins when the ratio of nitrogen demand to available supply exceeds a threshold, because fixation is expensive.
01Mechanism
Fixation costs carbon; plants fix only when the alternative is worse.
02Prediction
Fixation should track the ratio.
03Refutation
Fixation tracking nitrogen pool would remove demand.