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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)?

Alex
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4 Answers4

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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

Linza Itkonen
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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.

Ghanima
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    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 May 26 '15 at 05:10
  • 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 May 26 '15 at 08:17
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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).

FreeText
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    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 Jan 10 '19 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 Jan 10 '19 at 23:42
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    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 Jan 11 '19 at 01:52
  • 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 Jan 11 '19 at 04:22
  • 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 Jan 11 '19 at 04:30
  • Thanks @FreeText! I am sustainably minded, so my question is in the context of already doing A LOT of composting. I wonder often about the chemistry on our 'farm' and if I take some product and sell it in the market, will I necessarily have to buy other molecules to replace the ones I sold. I know that the carbon comes mostly from the air, but I know too that it goes back to the air. The conversation with @LShaver is very notable, as it touches on the base of my confusion - I want to understand every part of these processes and not for political reasons ;p – Alex Jan 11 '19 at 13:36
  • Well, if you want to get technical about what's in your compost, you can find out what's good for particular crops and add additional stuff. E.g. egg shells for calcium, coffee grounds have nitrogen. To get the richness of what you're composting available for the plants, you need the bacteria to break down the more complex hydrocarbon chains, and as thoroughly as possible. It will continue when distributed, but far less efficiently. This tends to be the priority for composting. Soil is primarily organic matter, i.e. hydrocarbons, so you're not losing much (miniscule amounts, really). – FreeText Jan 11 '19 at 23:24
  • I think what you're more likely to lose are minerals, which are more rare and are taken up by what you grow. Generally adding back any vegetative waste, especially from the kitchen is good. After that, find out what the crops like, and then where to get it, e.g. from various waste products. Good luck! – FreeText Jan 11 '19 at 23:25
  • @LShaver, your article link cites a paper that doesn't actually support the point. Even the paper mentions it as a possibility (but really as if something changes then something may change), but is not proven by, or the point of, the paper. You might be right but neither actually proves it or tries to prove it. If anything, the cited paper proves that "ancient carbon" is stable. What they did find is old carbon was mineralised, not gasified. But let me know if you have any other links or references. Maybe we should ask this as a new question? – FreeText Jan 16 '19 at 18:20
  • Take a look at this quote from the paper: "Although tree mortality is a relatively rapid event (instantaneous to several years long), decomposition of tree stems can take decades. Therefore, the CO2 release from the decomposition of dead wood adds to the atmospheric carbon pool over decades, whereas natural regeneration or in-growth occurs on a much shorter timescale." – LShaver Jan 16 '19 at 18:33
  • Yes, I'll go with that. Yes, there is gas released through composition. But the net gas released does not equal the gas captured over lifetime. After decomposition, most of the carbon remains in hydrocarbon chains as solids. It becomes the soil. Again, grass to soil. The soil is the (decomposed) grass, i.e. holds the (now shorter) hydrocarbon chains. What makes forests carbon sinks, to respond to your initial comment, is not just that the tree is around for a long time, and not just that decomposition takes a long time, but also after decomposition the carbon remains in the soil. – FreeText Jan 17 '19 at 19:42
  • https://en.wikipedia.org/wiki/Soil_horizon When they say "organic compounds", think hydrocarbon chains. It's primarily carbon (hence "carbon-based life form"). It's carbohydrates, lipids, proteins and nucleic acids; all of which are hydrocarbon chains, plus some other atoms like oxygen, etc. This is where the carbon goes. To circle back to the original question, you don't have to worry about offgasing since there's a plethora of carbon, and a lot of it came from the atmosphere. New growth will do the same. It's the other stuff, minerals, etc., that's more rare (and add more robust flavour). – FreeText Jan 17 '19 at 19:58
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See also: https://www.sciencedaily.com/releases/2008/02/080225072624.htm Perhaps turn your garden into a carbon sink?

Braeburn8
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