I make several compost piles, flip them regularly, add some additives and apply them to my crops.

I often cover the piles when its rainy, will this affect the offgassing? Some of my neighbors apply some of some things to their plants (coffee cherry shells, for example) but pre-composted - basically letting it decompose around the plant. Will this have an effect on how much CO2 is released (I suspect yes, since its in thinner layers, while the others are in mountains)?

4 Answers 4


It is. However, the aerobic process of composting produces CO2, which is better than the anaerobic processes which result from burying compostable materials under layers of sand (preventing access to air), such that it creates methane. If the methane isn't captured (and in most landfills on earth, it still is not) then it causes climate change at a rate of about 25 times that of the same amount of CO2. If you compost at home, you also prevent emissions from the transport of your biowaste from your home to the landfill, freeing up truck space for things that actually must go to the landfill (surprisingly little, these days).

Your composting purposes (soil augmentation for crops) are sound; covering the compost when it rains will prevent the compost from becoming too moist, which will change bacterial processes to favor anaerobic decomposition, but it also deprives the compost of oxygen. If you want your compost to continue aerobically (which, you probably do), then you can cover it, but definitely uncover after rain is done, and mix it thoroughly.

Your neighbors applying directly to the soil in a thin layer makes it offgas less CH4 (methane) and more CO2, since the process is exposed to more oxygen. It also mostly solves the problems of the middle of the heap becoming anaerobic or the compost becoming too moist.

Have you checked the pH of your compost? This is a pretty good indicator of how the decomposition is going. Anaerobic decomposition tends towards acidity, which in turn kills aerobic bacteria.

Reads! https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html

Most of the good part of this handbook are in the electronic preview on Google Books: https://books.google.com/books?hl=en&lr=&id=MX_jbemODmAC&oi=fnd&pg=PA1&dq=compost+process+modeling+emissions&ots=WdZEojafha&sig=lXEB72mn1u4K4EB9zAgiYNlM758#v=onepage&q&f=false


Short answer: yes they do.

It's completely natural that decomposing organic materials release CO2 and other greenhouse gases like methane and nitrogen oxides in some quantities. Both during compostation (which is usually done at normal atmosphere) and after putting it in the field.

I highly doubt that the mentioned different procedures really lead to significant offgasing in the long run. If you intend to reduce offgasing then decomposing would need to be performed in a closed environment. But even if you produce and harness the methane - using it will yield the same ammount of CO2. Significant difference would be not to release methane into the atmosphere which is a stronger greenhouse gas than CO2.

  • 1
    This depends on exactly what is meant by 'adding', Yes, they produce CO2, but no more than would be produced by the material decaying in some other manner. But the CO2 is not strictly added: rather, it's part of the long-term carbon cycle that all life depends on, so it keeps going around the CO2-> oxygen/plant matter->animal/bacterial food->CO2 loop.
    – jamesqf
    Commented May 26, 2015 at 5:10
  • Yes, relating to carbon/carbondioxide it's a null-sum game. Either way it will be generated and is part of the cycle just as you say. However during decay of organic waste also other gases are produces (such as methane and NOx) which are also greenhouse gases. Especially methane could be "harvested" if generated by closed fermentation plants, burned to CO2 (while using the energy). So in waste disposal it does matter whethe it's done "open air" or in closed systems. (I understand that the answer is rather short and could be improved though.)
    – Ghanima
    Commented May 26, 2015 at 8:17

What's the intent of the question? Is it a pollution/greenhouse gas (GHG) concern? Or a scientific question?

Good composting needs lots of oxygen. It will decompose better uncovered (faster, more completely, better for the plants it will feed), as long as it's warm enough. In regions with cold winters, the process will stop if it gets too cold, and covering it for the season is usually necessary.

In terms of GHGs, the gases release are minor compared to what they absorbed in their lifetime, and using it as fertilizer is much better than alternative fertilizer (which must be produced and transported).

As a scientific question I can see the interest, as a practical, sustainability question, e.g. re: GHGs, please compost as much as possible. Turning the compost helps the oxygen absorption, but also distributes the bacteria throughout to maximise their efficiency (so they always have fresh food). Most of the bacteria is below the surface layer where they are protected from the elements, and have sufficient moisture and correct temperatures (not too hot, not too cold).

Composting also releases heat - the bacterial breakdown of the cellulouse is "exothermic" - but it's not a concern for global warming. I don't mean this to be sarcastic, but rather that it terms of sustainability (and my confusion regarding the question) composting is very good either by the fertilizers it displaces, or the pollution from the systems to dispose of the material otherwise. Decomposition will happen if you do it or you let nature run its course). Best composting needs lots of oxygen (and nitrogen).

  • 1
    In terms of GHGs, the gases release are minor compared to what they absorbed in their lifetime -- how could this be? Eventually all of the carbon a plant absorbs will be released back to the atmosphere if it's left to decompose aerobically. Only burying it would keep the carbon out of the atmosphere.
    – LShaver
    Commented Jan 10, 2019 at 21:52
  • "Eventually all of the carbon a plant absorbs will be released back to the atmosphere". Well, that's just it - it doesn't, it sits on the land and goes into the soil (to be used by other plants, grows fruit, gets eaten, goes back into the ground...). It's how forests are carbon sinks.
    – FreeText
    Commented Jan 10, 2019 at 23:42
  • 1
    Not to pile on, but as an experiment, if you take grass clippings you can compost it into soil. Grass is basically cellulose and water. Cellulose is just a carbohydrate (carbon + hydrogen), and water is, well, H2O so nothing bad or to add there. If it all went to gas, it wouldn't make soil. Sorry, but there seems to be a repeated idea on this site that decomposition is similar to burning, and it's not; I'm not really sure where that idea comes from. It's odd. Which isn't to say no gas is released during decomposition (e.g. CO, CO2, CH4), but it's a small fraction of the whole.
    – FreeText
    Commented Jan 11, 2019 at 1:52
  • The difference is in the rate. The carbon in the soil from decomposed matter is eventually used by other organisms which then also decompose. Each molecule of carbon will eventually be cycled through the atmosphere. Organisms only hold onto it for as long as it takes them to live to maturity, die, and fully decompose. Take a look at this article. What effectively makes old growth forests into carbon sinks is that trees hold carbon for a long time, and then take a long time to decompose. The difference with burning is that it's fast.
    – LShaver
    Commented Jan 11, 2019 at 4:22
  • I think we are essentially in agreement -- burning does in moments what natural processes of decomposition take decades or centuries to do. Thus on human timescales forests act as carbon sinks.
    – LShaver
    Commented Jan 11, 2019 at 4:30

See also: https://www.sciencedaily.com/releases/2008/02/080225072624.htm Perhaps turn your garden into a carbon sink?


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