Soil and the plants growing in it are more self-sufficient than we think

Here's an excellent webinar on the role of biological activity in soil health.
http://www.conservationwebinars.net/webinars/relevance-of-soil-biology-in-assessing-fertility-and-soil-health

In summary: the decomposers in warm, moist, biologically active soil with a sufficient organic matter reserve produce not only much of the nitrogen needed for plant growth, but also most of the CO2 needed by those plants. CO2 availability from the atmosphere can limit plant growth.

My application: Maintaining a closed canopy with little air mixture enhances stratification of CO2 to keep it close to plant leaves, where we want it. This is easy to do once plants are large, but more difficult in a vegetable garden with seedlings. I would think planting vegetables in standing residue (especially from grains) would facilitate the CO2 stratification almost as well as a living, mature crop canopy would, by minimizing wind-driven convection. One of the main reasons plants lose water is through opening their stomata to take in CO2. If we can foster higher levels of CO2 around plant leaves, we can reduce water loss due to transpiration. This is a further extension of the micro-climate concept that I have not heard much about.

What are your thoughts?

Great post and webinar Izzy, thank you for finding and sharing this.

As was pointed out, the microbial CO2 remains close to the level of the soil.
The concentrations decrease the farther from the soil (ground zero if you will) till they actually become dissipated at what would be a tree's leaf height.
I agree with your assessment of this microbial CO2 being important for vegetables.
Medical Marijuana growers, Orchid growers and other greenhouse users have long added CO2 via compressed gas release in their grow houses, during the day light cycle.
During the night time, the CO2 systems are shut down, since plants do not take up CO2 in the dark cycle.

I would have to do some quantitative experiments with your idea of using standing residues of grain production to be able to comment on effectiveness of that strategy.

CO2 levels can be detected, as you surmised, under the leaf canopy of squashes, these large leaves do create a microclimate conducive to CO2 harboring.

Respiration in plants is CO2 in, O2 out during the day, when cells are activated by sunlight and O2 in, CO2 out during the night hours.
Therefore it is not relevant that CO2 levels remain high through out a 24 hour period.
Water loss is going to occur during either portion of active plant respiration.
Air temperatures are more significant for excess loss of water than the actual respiration cycles of plants.
This is one of the reasons the atmosphere inside a greenhouse has higher humidity levels than the outdoors, as well as the containment factor, the higher heat level contributes to plants respiring at higher rates thus more moisture is put into the air by the plants themselves.

To reduce transpiration of H2O you would need to have control of the temperature, the CO2 level during the day and the O2 level during the night.
This can be and is done in the environment of a green house, it is harder to control, in fact it may be impossible to control in the out door environment.

Experimentation with your theories is called for to be able to reach a conclusion and then proved by reproducible results.