Hi Norman,

I’ve also wondered a bit about that, although maybe I’m wrong but intuitively it makes sense to me when viewed from purely an energetics point of view in the plant. Please take all that I say underneath with a pinch of salt since I’m not an expert in these fields.

So how it seems to me: 1) When plants have start photosynthesizing completely they are able to readily make sugar.
2) Amino acids are sugar but with an amine (NH2) group attached to it. Therefore if you have the sugar molecules you just have to attach the NH2. Initially this can take up a lot of energy as plants take up NO3 and NH4 and they need to be converted. NO3 takes more energy as it first needs to be reduced to NH4. So it is less energetically favourable. Once steps 1 and 2 are working very efficiently plants can “afford” to send more of the sugar from step 1 to the soil organisms and forge symbiotic relationships.
3) When the symbiotic relationship is underway quite nicely, the plant exchanges sugars for water, minerals and organic compounds. These organic compounds are prefabricated parts. So if they for instance supply the plants with amino acids that are already made, the plants don’t have to use as much ATP in step 2, and the energy can be used for other more energetically expensive processes (lipid production?). Also I’m pretty sure plants can take up Amino acids from the soil. Therefore if you have quite a big microbial turnover, there should be amino acids and proteins etc in the soil as they die and their cell contents are released.

The key resource here I suspect would be a source that shows the different amount of ATP used in Lipid synthesis vs Amino acid synthesis. I remember someone from my undergrad studies speaking about how lipids are more expensive to produce than amino acids, so I never really questioned the step 2 to step 3, it sort of just made sense to me. I will however do a bit of digging to see if I can find something.

Maybe someone else has a better answer.