Here is something on the Ag level I DO know something about and can contribut my 2cents to.
Soil Building vs Depletion is a simple calculation of Inputs vs Outputs More inputs than outputs, soil builds. More outputs than inputs, soil depletes. So what are the outputs and inputs?
In large part, 3 very common elements Carbon, Hydrogen and Oxygen are used for the life process, and all are present in the atmosphere as gases. Nitogen also is necessary and makes up a large part of living systems, also in great supply in the atmosphere. But it is present as N2, a diatomic molecule that is quite stable, and needs to be "ficed" into Nitrous Oxide or Ammonia to be useful for living things. Legumes (beans) do that naturally, the Haber process does it industrially.
C,H & O provide the energy source for all living things, in plants through Photosynthesis from energy collected from the sun. It is stored in two other types of molecules, Carbohydrates and Fats.
The next important element necessary for the energy system is Phosphorus, not a gas and not present in the atmosphere to any significant degree. P is necessary to move the e
energy around as part of a molecule Adenosine Tri-Phostphate. This powers all the uphill reactions of all living things, plant or animal. A couple of systems, the Krebs Cycle and Electron transport chain do the energy transfer job. These are pretty complex systems, but overall pretty efficient for what they need to do.
Finally, there are a whole bunch of other elements required to build a plant or animal, Sodium, Potassium, Magnesium, Calcium, Iron, Sulfur, Chlorine etc. Chlorine is the only gas here, but it is too reactive to hang out in the atmosphere. It is present as a salt in various inorganic and organic molecules, or as an ion in salt solutions with water.
Now that we have a primer here on the system, where is it all coming from in any given biome to make all the living things in that neighborhood? As noted, the C, H, O, & N all come from the atmosphere. C combined with O2 as CO2, H&O combined as H2O, water. O2 & N2 also present. Everything else comes from the land or water environment they grow in.
Now, in a closed system, all the trace elements recycle. Dead animals and plants give back the trace elements to the soil. They also hand them back while living when they excrete. Thus
**** and
**** are good fertilizers.
However, no biome can produce more living things if it is a closed system without more inputs of all types. Similarly, if there are more outputs than inputs, any given biome will be reduced in what it can support. What we do now is take biomass from one place where it grows, and move it somewhere else to get eaten by people and the animals they raise for food. After that, the excretions are moved out to sea through the rivers. They don't get back to where they originated from. So over time, the soil depletes.
In the H-G era, this did not happen. Everything stayed pretty much where it was, just moving around a little bit. With the development of Ag and Cities, the stuff started moving away from where it was, then downstream and out to sea. Constant depletion for the last 10,000 years.
You can build soil in some places if you are careful with your inputs and outputs, but you can't rebuild it everywhere. The available minerals have been washed out to sea, and getting them back is quite hard because they are so diluted. Good in theory for sea creatures, but they have other problems to deal with also, like the ocean heat content. For Homo Sap, we live on land and the total available stuff is depleted. We have supplanted what was available through mining, but like coal, phosphate mines are pretty played out also.
Homo Sap numbers will reduce and the rest of the biome will recover, over time. Quite a long time. What it took 10.000 years to wash out to sea will likely take 1M years to get back as the ocean inundates the land and the stuff that makes life possible gets redeposited. In the meantime, Homo Sap numbers will decrease to whatever the land still can support in total. That number is not too large, relatively speaking. I can't tell you the exact number, but my WAG would be about 1:1000. How long it will take to drop that low also unknown. I do know however the Reindeer on St. Matthews Island accomplished this task in a very short time indeed.
RE