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The boom-bust cycle answer I took for my question: Sustainability of Regular Global Wildfires due to Oxygen Rich Atmosphere
pends on the atmosphere being rich in oxygen but poor in CO2. I, however, would like the biosphere of is world to be comparable in mass to that of earth, so the total amount of carbon on the planet's surface should be similar to ours.

How can I get plants and animals to keep the majority of the carbon trapped in plants/plankton?

Obviously, this system backfires (literally) for the plants every wildfire/oceanic die-off, but why/how would the plants keep all the carbon to themselves without animals just releasing it all immediately?

I thought about their equivalent to cellulose being a very carbon-dense polymer that uses little water, but I wouldn't know what polymer to use or how that would work.

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    $\begingroup$ Trapped in plants / plankton as opposed to what? Being loose CO2 in the atmosphere? Isn't that how it is on earth now? Hardly any CO2 is in the atmosphere! $\endgroup$ – Willk Apr 4 at 23:24
  • $\begingroup$ Carbon in the atmosphere: 800 gigatons. Carbon in the biosphere: 550 gigatons. Carbon in the soil + detritus: 1500 gigatons. Carbon in the ocean: 40,000 gigatons (of which about 1000 gigatons in the surface layers). Here is a picture. Coal reserves (= carbon underground): 1100 gigatons. Carbon in petroleum reserves (= carbon underground): 200 gigatons. Please note that photosynthesis would be drastically slowed down if the concentration of CO2 would be two or three times lower than what we have; you don't want to reduce it too much. $\endgroup$ – AlexP Apr 5 at 0:14
  • $\begingroup$ ... Ah, and about those animals. On land, plants count for over 92% of the biomass, animals for less than 8%. I don't know the numbers for the oceans, you'll need to look them up. $\endgroup$ – AlexP Apr 5 at 0:17
  • $\begingroup$ @AlexP I this is xenobiology; the efficiency of earth photosynthesis isn't particularly relevant. Also, you misinterpreted my problem with animals. I am not sayong they STORE carbon; I'm saying they RELEASE carbon. $\endgroup$ – Aezyc Apr 6 at 1:44
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This sounds like the carboniferous period of Earth. Fungi hadn't evolved yet that could break down Wood, so trees died, fell over, and DIDN'T rot. Global swampy conditions meant there was an abundance of anoxic environments for plants to get buried and not rot. Much of the coal on Earth was laid down in this time (Thus the name). This was also the period when we had giant insects on Earth because the O2 concentration was higher. Plants keep the carbon because they can't digest the wood. If you need to, you can mix up the reasons that the plant matter doesn't break down (plants concentrate heavy metals and animals haven't evolved that can handle it, etc.) Eventually something might evolve to break the cycle, but that doesn't have to be any time soon.

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Actually very easy, just make conditions where plants or animals can't be processed by other living things once they die. For example, there could be a lot of floating ocean algae that cover the whole surface and sink to the bottom of the deep sea faster than anything can process the leftovers... leading to most of the carbon being permanently trapped at the bottom of the ocean and ultimately stored in sediment. Of course, there would need to be a slow and steady stream from down there to keep the whole system up and running. Alternatively, you could simply have a lot of plants, like a loooot of plants with very little active CO2 intake (either because of low light level or lacking CO2). When everything is trapped in living plants there won't be much left in the atmosphere

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Make the plants hard to eat, both while alive and after death.

This can be simply tough molecules -- trees can take centuries to decay even on earth -- or heavy metals, or toxic compounds.

Such animals as manage must cope with this by not growing too large and requiring more food than they can cope with.

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