Because plants have chosen to specialize into nitrogen-eating and non-nitrogen eating. So in nature there isn't a problem because these plants mix.
Grass puts Nitrogen into the soil. There are other plants, but grass is the most well-known. Things like corn or grain take nitrogen out, and use it. In nature these plants grow centimeters apart, so there is no problem. There's a nitrogen cycle. However if you want much denser grain, you can only have grain.
So now we've got agriculture, and it requires density. Large plots of land where you've got nothing but grain. Nitrogen goes out, doesn't come back in. Grain grows for 2-3 years, then refuses. It dies, because it cannot use the nitrogen from the atmosphere. It can only get it from the soil.
So we found a solution a very long time ago (it's in the bible, surprisingly, although we have figured out slight improvements since then). Do agriculture in a 7 year cycle: leave blank, grain, grass + animals, grain, grass, grain, grass + animals. There are many cycles used in various places, but they're all quite similar.
Fertilizer shortcuts the process: so why does it increase the yield 3 times ? Because you can cut out the grass years and the blank years. Not because the plants grow faster, because you can grow 3x as many of them.
> Not because the plants grow faster, because you can grow 3x as many of them.
This is against current consensus. The consensus is that plants do grow faster when you put fertilizer in the soil and the yield increase is because of that, not because of the "grass years".
There are plenty of studies on this, one example:
> Crop yields were increased by 19–41% (rice) and 61–76% (rapeseed) during the two years of rice-rapeseed rotation under NPK fertilization compared to PK fertilization across the study sites. Yield responses to fertilization were ranked NPK > NP > NK > PK, illustrating that N deficiency was the most limiting condition in a rice-rapeseed rotation, followed by P and K deficiencies.
Bottlenecks are only exclusive when the processes run serially.
All of water, CO2, and nitrogen (not an exclusive list) can stunt growth, and increasing the levels of any one of them can accelerate it.
Your own link points to this, referring as it does to the "most limiting condition", and the fact that addressing P and K deficiencies also increases yield, but not as much as also addressing deficiencies in N.
True, but I'm willing to bet that N deficiency is pretty wild spread world-wide, including in natural soil. Probably the second dominant stunt factor on plants (after water), and way before CO2.
Nitrogen deficiency is the factor that would promote the most growth, but at the same time, it's fair to think of all plants as constantly CO2 starved.
We would be pretty miserable at 800ppm CO2, but plants would love it.
