So instead of what happens today when a tree dies and rots, the tree died and just piled up in the forest. I always think it's amazing to imagine what a forest must have looked like during the Carboniferous with all these dead trees just piled high. Trees were essentially like today's plastic: non-biodegradable refuse that stuck around for millennia. Eventually all those trees got buried and turned into fossil fuels (mostly coal).
And, interestingly, this process let to huge climate change back then, too - basically the reverse of what's happening now (and at a much slower pace). Trees sucked all this carbon out of the air and reduced global temperatures by about 8C.
Edit: One other note - there were of course still forest fires during the Carboniferous. It's believed they would have been incredible raging infernos with so much undecomposed combustible material.
The tree absorbs CO2 from the atmosphere and combines it with water (H2O) from the soil to produce more complex chemical compounds (such as carbohydrates). This process uses sunlight as the source of energy, and is better known as photosynthesis. The chemical compounds thus produced are used by the tree as structural material (the wood), or as stored energy (basically, reverting the photosynthesis reaction, aka "burning", releases the energy).
If the tree matter gets consumed by animals, those animals are going to use the stored energy, and ultimately re-release the stored carbon as CO2. If the tree just dies and is allowed to decompose, bacteria and other microorganisms will do the same: they'll eat the carbohydrates that the tree produced, and they'll breathe out the resulting CO2. If the tree is burned as fuel, the CO2 gets re-emitted directly.
So, to truly remove that CO2 from the atmosphere, the tree needs to be felled once it's stopped growing, and the resulting wood stored someplace where it can't be decomposed by microorganisms. Burying it is one solution to this (and this is what happened during the carboniferous era, when much of our coal and oil got created: dead trees fell in shallow swamps and were quickly buried under a layer of silt which prevented the decomposition).
Again, I'm no expert, so I'm happy to stand corrected if I've made any mistakes above.
There are some active land management techniques which do something similar with shrubs/plants/grasses which generate an enormous amount of soil and therefore trap a lot of carbon.
Technically it's the 2xCO_2 -> 2xO_2+2C and the Carbon that sticks around becomes the wood. (also insert Water and energy into that equation. Photosynthesis is crazy).
And yes, if it gets burned or decomposed that carbon can get released back into the air (usually as CO2) but if you instead bury it you essentially lock it into solid form. And over years and years that becomes oil!
Question, though: isn't this something you learned in school?
After the tree dies, it would only go back into the atmosphere if the trunk was burned.
To nitpick: or if it decomposes in other ways.
This process is why you have methane production in swamps and conventional landfills.
- Shallow burying the wood would not stop emissions from escaping into the atmosphere. Doing so would deprive it of oxygen which would cause the production of methane which is even worse than co2
- Wood is way less dense than coal so even if the idea worked you would only be able to store a fraction of the amount of co2
- Its hard to imagine the sheer scale of collecting all the wood to ship back to the mines. As mentioned wood is much less dense than coal so the effort to ship the wood to coal mines would be much more difficult than shipping the coal out of the mines. Not to mention that the wood would be coming from a much wider area.
There are ideas about burning wood to produce electricity, capturing the CO2 exhaust and pumping that deep underground. The concept is called Bio-Energy with Carbon Capture and Storage (BECCS).
https://en.wikipedia.org/wiki/Wood#Chemistry_of_wood https://en.wikipedia.org/wiki/Carbon-based_life
Its a complex ecosystem with the plants at the center, and trees, since they have the deepest roots do the best at promoting carbon uptake. Of course, if you clearcut a forest the soil ecosystem dies and the carbon eventually comes back out of the ground.
However, you can hijack the process a little and keep the carbon locked away by doing things. You could use the wood as lumber or you could burn the wood in a special way which produces biochar.
The clue to net carbon capture is to plant new forests and make sure the forest stays alive. While the forest lives, all the CO2 is safely sequestered.