Does a fallen tree left to decay release essentially all of its stored carbon as $\small\mathsf{CO_2}$ to the atmosphere? Or is the carbon uptaken by termites and bacteria, preventing its release to the atmosphere?
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2$\begingroup$ Some carbon ends up as methane in organic decay. Not in combustion. Not sure about the quantities. $\endgroup$– GimelistCommented Nov 12, 2016 at 22:34
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$\begingroup$ There might be a considerable difference in the isotope ratios. Bacteria tend to use light isotopes of each element first. $\endgroup$– daniel.heydebreckCommented Nov 14, 2016 at 9:07
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1$\begingroup$ If the tree completely decomposes all its accumulated carbon has to go somewhere, either into to the soil, water, or atmosphere. In some environments with shallow soil development I would suspect that a large portion of the carbon will end up in the atmosphere. termites, bacteria and fungi are short lived and any carbon they take up will be rapidly released into the environment. It would be good to know what the motivation for the question is. CH4 released by animals is a very strong greenhouse gas as well. $\endgroup$– user824Commented Nov 16, 2016 at 17:56
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$\begingroup$ A few tons per hectare per year is sequestered in the soil. $\endgroup$– Keith McClaryCommented Nov 17, 2016 at 3:47
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1$\begingroup$ Related question: earthscience.stackexchange.com/questions/7424/… $\endgroup$– PuffinCommented Dec 20, 2016 at 18:55
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7 Answers
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I can't quantify the answer, but intuitively, nearly all of a burning tree's carbon is converted to $\small\mathsf{CO_2}$ - assuming the fire is hot enough. You are correct in assuming that termites and bacteria convert some of the carbon to other organic species, as do fungi, yeasts, moulds, and slow weathering. In addition, some of the rotting tree's carbon is converted to soluble inorganic ions, humic, fulvic and tannic acids. So no, organic decay would not yield the same $\small\mathsf{CO_2}$ as burning.
This would make an interesting school or 1st-year uni science experiment. Burn some wood in a sealed environment, measure the $\small\mathsf{CO_2}$ evolved, then repeat the experiment with wood in a wet organic-rich isolated biome, and compare the results. I suspect it would make for an interesting paper with potential climate change policy implications.
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$\begingroup$ You'd never get the wet wood to decay fast enough to measure the CO2. $\endgroup$– MaxWCommented Nov 12, 2016 at 22:09
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1$\begingroup$ OK, so it is an experiment that mighjt take a few years, but it is still do-able. $\endgroup$ Commented Nov 13, 2016 at 4:30
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1$\begingroup$ Forest fires usually leave stumps and roots unburned. $\endgroup$ Commented Nov 17, 2016 at 3:48
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$\begingroup$ @GordonStanger it is possible to break down wood faster if you do use ants/termites for the first step,and it will act as an fertilizer for bacteria and fungi.it will still take some time but not years(you mention this in your answer)and it is doable as a school project. $\endgroup$ Commented Oct 14, 2018 at 6:02
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$\begingroup$ What about all the carbon soot? It seems like burning converts more to solid carbon which isn't converted to CO2. $\endgroup$– mchidCommented Oct 29, 2022 at 16:43
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The key word in your query is the word "essentially". Termites and bacteria die too. So whatever carbon was initially converted from the tree into termites or bacteria ends up mostly going to $\small\mathsf{CO_2}$ also.
Of course some carbon gets converted to carbonate and that can combine with calcium to form the mineral calcite. The tree can also get buried in mud and over eons get converted to coal or oil.
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1$\begingroup$ I suppose to the extent it biodegrades, the bateria etc mostly burn the organic material for energy rather than building cells, thus releasing most of the carbon to the atmosphere (and the rest soon follows as these creatures biodegrade). $\endgroup$– GrumdrigCommented Dec 4, 2016 at 7:58
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When a forest is used for sustainable fuel, there is a range of age classes in the forest, some old, some middle aged and so on. Wood harvest takes the oldest trees leaving young trees to take their place. Young trees growing vigorously consume more CO$_2$ than the old ones. In such a forest, old trees not used for fuel will decompose over time producing some CO$_2$ and a smaller amount of methane. An even smaller amount of carbon will be left in the soil.
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If organic matter is buried - such as in landfill - it will decay anaerobically, resulting in $\small\mathsf{CH_4}$ (methane) rather than $\small\mathsf{CO_2}$ (Carbon dioxide). Methane is about 30 times more harmful as a global warming gas. Hence the reason why it is important not to landfill organic matter such as paper, card or waste food. However landfill of plastics (which is generally derived from a fossil fuel) is carbon sequestration, resulting in a net reduction of global warming gasses. So waste must be sorted before incineration - incinerating plastics produces/releases much $\small\mathsf{CO_2}$ and drives global warming
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The idea that decomposers that consume the wood's carbon just die and release it is foolish. These decomposers are at the base of a complex food web that moves that carbon through a huge network. I haven't seen a study to quantify the amount of carbon released to the atmosphere, but it is an important question. I have seen many studies trying to get at the question of how much carbon is incorporated into the soil, but each situation is so different that it is hard to get a clear answer.
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It's not so much the amount of $\small\mathsf{CO_2}$ released but the rate at which it would be released that contributes to the Greenhouse effect. Burning will release CO2 faster than any form of natural decomposition
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2$\begingroup$ What causes the greenhouse effect is the accumulated amount of CO2 in the atmosphere. The speed of additional amounts is not that relevant. $\endgroup$ Commented Oct 11, 2018 at 13:58
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$\begingroup$ Surely the quicker CO2 is released, the more it builds up, assuming the rate of CO2 absorption remains constant? $\endgroup$– GrotbagsCommented Oct 12, 2018 at 11:05
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Burning wood for home heating could actually be a carbon credit. Wood offsets combustion of oil, natural gas, propane, and the use of coal fired generation of electricity. Wood left to rot, as the previous post note, is carbon neutral.
