Wednesday, January 9, 2013

The Shocking Carbon Footprint of Compost

Most people think of composting as a very "green" thing to do, but few realize that composting actually generates a significant amount of the potent greenhouse gases (GHG), methane and nitrous oxide.  Under current landfill regulations, requirements to exclude water minimizes the breakdown of organic matter and requirements to capture and burn methane mean that even that option has a better carbon footprint than composting (thanks to Fred Krieger for pointing out this advance in the landfill arena).  The even better option is anaerobic digestion which I will describe at the end of this post.

These Emissions Are Not A Scientific Surprise

To a microbiologist, it is not surprising that these gases would be generated during composting. Methane and nitrous oxide are formed by certain microbes when there is not enough oxygen available (anaerobic conditions). In the middle of a large-scale compost pile there are micro-sites without oxygen. This occurs even in a pile turned frequently for aeration. This is particularly true during the "hot" phase of the composting process which kills pathogens and weed seeds. During the period of very high oxygen demand, some parts of the pile will run short and the anaerobic organisms will make methane and nitrous oxide.

An Example

The graph above is based on one typical study of GHG emissions during composting (Hao et al 2001).  This was from active composting of cattle manure - a common procedure in which the pile is aerated by turning it frequently using a tractor (its fossil CO2 emissions are shown in green above.)

The first column represents how much carbon or nitrogen was emitted in various forms per metric ton (Mton) of manure. We can't even see the 0.19 kg nitrous oxide-N at this scale. Methane is 8.1 kg C and the fuel is 4.4 kg C.

The second column shows how much the emissions contribute to a net increase in greenhouse gases in the atmosphere (Carbon dioxide is "carbon neutral" because it was recently pulled out of the atmosphere by a plant - thus no net GHG contribution.  Methane and nitrous oxide are multiplied by 21 and 310 respectfully because of their higher radiative forcing potential).

The third column shows the GHG contribution per metric ton of finished compost (after 21% loss of mass - much as water).  The total "carbon footprint" of the compost is now 233.4 kg CO2-C/Mton.  For those more familiar with English units and expression as CO2 this would be 2167 lbs CO2/Ton.

How Much Compost Is Typically Used?

When compost is used in farming, it is normally applied in large quantities.  According to the University of California, Davis cost and return studies, a typical organic crop would receive between 2 and 10 tons of compost per acre.  Thus a mid-range use of 5 tons/acre would represent a carbon footprint of 10,833 pounds (CO2 equivalents).  This is without including the fuel footprint of hauling the compost to the field and spreading it.

How Big Is That Footprint?

To put this in perspective, the carbon footprint of this amount of compost used on one acre of a crop would be equal to the various other carbon footprints described below:
  • The carbon footprint of manufacturing 2,580 pounds of synthetic urea-nitrogen fertilizer (at 4.2 lbs/CO2 per lb) 
  • The "embedded carbon footprint" of that urea for fertilizing 12.9 acres of corn at 200 lbs/acre
In other words, the footprint of the applied compost is shockingly large.  It is certainly not a practice one would want to see on a large scale.

Waste Is A Terrible Thing To Waste

Why bring this up?  Because there is a superior use for manures and other organic waste streams.  When waste is processed in an anaerobic digester,  most of the carbon in the is intentionally converted to methane, and then the methane is burned as a form of renewable energy.  The emissions are carbon neutral and the energy generated offsets fossil carbon use.  As with compost, the remaining fiber that is left after digestion can still be used for soil improvement or other uses.

Anaerobic digesters require a substantial, initial capital investment and are non-trivial to operate, but they are clearly the best way to deal with most organic waste streams.  They also pay for themselves over time.  Modern municipal water treatment facilities tend to have these digesters as do some large-scale dairies and CAFOs (confined animal feed operations).

The largest onion processor in California (Gills Onions) installed a digester for its substantial stream of trimmings.  Gills eliminated a troublesome odor and disposal issue, they now offset much of their energy demand, and they are ahead financially after paying back the initial investment. This is a great example of how "doing the right thing" from a greenhouse gas perspective can also be a sound, bottom-line option.

You are welcome to comment here and/or to email me at  For notifications of future posts you can follow me on twitter ( @grapedoc )

References on GHG emissions during composting:

Hao, X., Chang, C., Larney, J., Travis, G. 2001. Greenhouse gas emissions during cattle feedlot manure composting. Journal of Environmental Quality 30:376-386.    Osada, T., Kuroda, K., Yonaga, M. 2000 Determination of nitrous oxide, methane, and ammonia emissions from swine waste composting process.  Journal of material cycles and waste management 1:51-56    Hellebrand, H.1998. Emission of nitrous oxide and other trace gases during composting of grass and green waste. Agric. Engng Res. 69:365-375     Sommer, S., Holler, H.2000. Emission of greenhouse gases during composting of deep litter from pig production – effect of straw content. The Journal of Agricultural Science 134_327-335    Hao, X., Chang, C., Larney, F. 2004. Carbon, nitrogen balances and greenhouse gas emission during cattle feedlot manure composting.  Journal of Environmental Quality 33:37-44    Jackel, U., Thummes, K, Kampfer, P. 2005. Thermophilic methane production and oxidation in compost. FEMS Microbiology Ecology 52:175-184. (looking for microbes which might help reduce the methane emissions from composting)     Hellmann, B., Zelles, L., Palojarvi,A, Bai, Q. 1997.  Emission of climate-relevant trace gases and succession of microbial communities during open-windrow composting.  Applied and Environmental Microbiol 63:1011-1018

Compost image from University of Rhode Island


  1. In other words, manure is going to produce a lot of methane one way or another, and when you talk about large quantities, capturing that methane and burning it as fuel is preferable to simply releasing it into the atmosphere. Of course, that means you won't be able to use it directly as a fertilizer, which seems like a bummer, until you really look at the amount of damage that methane would do, and how much lower the embedded carbon footprint of synthetic fertilizers would be?
    Is that a fair summary?

    Now, I do have a criticism -- you have a nice chart and lots of detail about the carbon footprint of manure, but nothing about the construction and operation of the anaerobic digester. Even if the methane burned is itself carbon-neutral, it would see that to make a real comparison, we'd need a sense of the total construction and operating carbon cost of the facility, and factor in the production carbon cost of the replacement synthetic fertilizer as described above. The digester might still come out ahead, but I think the difference would be less dramatic -- and the comparison more accurate.

  2. What do you figure is the smallest size grower/dairy where a digester would be viable?

    1. Michael,
      apparently that depends on lots of factors like government programs and the rate at which utilities will buy the electricity. Here is a recent article on the question from Cornell

  3. Of course composting is probably better than what many people do, which would be nothing. In our town a local farmer is trying to erect a digester to handle the waste from his cattle. Last spring, although the digester was about 2 miles out of town lawns in town were littered with "Move the Digester!" signs. The farmer is now looking at other locations. I'm not sure if people were concerned about the smell and maybe the amount of heavy truck traffic that would be noisy and tear up the roads. Hopefully he'll find a great spot to build his facility.

    About an hour North of my farm you'll find Fair Oaks dairy farm. A very large dairy that has recently fitted their digester to harness CNG for their truck fleet. Last spring when I toured the dairy (something all people can do, with a pig farm opening soon as well) I was told that 60 tankers of milk leave Fair Oaks every day. When the trucks are all converted to CNG and basically running off cow poo they claim they will cut diesel fuel consumption by 1,000,000 gallons each year. At today's prices they are putting nearly $4M/yr in their pocket while cutting emissions greatly.

  4. As a backyard composter, I have often wondered about the large scale use of compost to produce energy, and whether or not this was an efficient and economically viable option. Do you foresee municipal anaerobic digesters for energy production? Possibly fed with drop off or curbside pickup of organic waste? Or is such production only viable as an on-site option?

    1. I'm an ag-oriented person so I'm sure I don't understand the logistics of municipal waste. Perhaps there could be collection points and ideally those would send the waste via pipe to digesters. I'm sure that would be super expensive after the fact. The issue with municipal, even what comes through the sewers, is that you are at the mercy of lots of people who could put nasty things into the system out of ignorance. This has always been a limitation on the use of sewage sludge for crops or even landscapes. A major challenge is how to pull the valuable things (like N, P and K) out of waste streams without the heavy metals, organic solvents, pharmaceuticals, human pathogens.....

      That is why I stick to thinking about manures - it is so much simpler even as complex as it is.

    2. Perhaps with engineered bacterial pumps in the future?

  5. Robbin,
    Good questions. The vast majority of manure is applied as manure to crops that are not for direct human consumption because of the pathogen issue. It is far from an ideal fertilizer because it tends to be higher in P than right to balance the N, so the best way to use it is to supplement with synthetic N. If manure is stored (as it often needs to be) there are emissions depending on the nature of the storage and the temperature, so it gets very complicated to estimate.

    I agree that the manufacturing footprint of the equipment has to be factored in. For instance, I have solar as a hedge for electricity costs, but I know it will take 20 years or so to pay back the carbon footprint of making the panels (so I'm not at all smug about being "green"). I'm sure that then total net footprint would be less dramatic, but with the magnitude of the composting footprint I cannot imagine it coming out better.

    The reason I point this out is that there is a tendency for people to believe that composting is a purely wonderful thing. Take the food scrap program in SF. They run big trucks up and down steep hills to gather the waste, drive those trucks 40-50 miles to a compost facility. Make GHG there, and then haul the compost 100 miles to put on vineyards who then claim to be super sustainable. It is absurd. Ideally they would just have a sanitary sewer system that could gravity transport the waste to anaerobic digesters at the water treatment plants. Next better they could use digesters for the food scraps and generate some energy.

    The other reason I point it out is as just one more reason why organic cannot expand to a mainstream solution. First of all, there isn't enough manure or other such things to fertilize any significant part of ag - maybe 5%, and even that isn't a great idea. I have recently seen a new USDA technology that is able to capture a good deal of the N from manure and that would be a great pre-process before digestion.

    The really great solution would be to use a renewable energy source to generate the hydrogen for Haber-Bosch and make synthetic nitrogen with no carbon footprint. The economics of that just got tougher with cheap natural gas, but there is not intrinsic reason that one has to get the hydrogen to make synthetic nitrogen from fossil sources.

    1. You should consider making this comment (wrapped in with the comment to which you responded) an edit to your post.

    2. Shannan,
      I agree that the discussion would be good not buried in the comments, but that gets to be a super long post and it is already longer than what experts say is ideal. Three years ago I did a follow-up post after seeing what issues were raised (on Sustainablog at the time). Maybe I should do that again here.

  6. I guess this means we should cut down all of the forests to stop the massive number of leaves from composting. The Amazon Forest must release more gases with composting than all the coal plants in the world! It is time to deforest the Amazon Basin. Look at all those BIG trees in California releasing all of the methane as the dead stuff composts. Cut them trees down and save California. But trees absorb CO2. CO2 is plant food. If we cut down all of the trees CO2 goes up. There is no solution. If there are no humans left in the world it won't matter - the solution!

  7. Leaf litter in a forest would not be at all like a commercial composting operation. It would be far more well aerated which would favor decomposers that do not generate these gases. You have not heard of forest floors getting hot like a compost pile. Seeds also survive there just fine. Composting is a human activity with not that much natural precedent. So is an anaerobic digester, but at least we can harvest the energy from that.

  8. Talk about driving a stake through the heart of the sacred cow (as it were) of the "organics" movement....

    Not that I'm against composting per se: We have a tiny CSA farm and a small herd of hobby milk cows, and it would be foolish of us not to use that manure on the fields. But we--as you say--are not at all "smug about being green"--in fact, I've become quite hostile to the pretensions of the organics movement.

    This piece got me to thinking about my experiences working at an organic farm some years ago. One of my jobs was to pile and turn donated vegetable wastes in an (expensive, government-funded) composting facility, and to spread piles of composted manure that were dropped off every fall from a local dairy farm.

    As I reflect back on it, I wonder about the futility of it, given the realities of the nitrogen cycle, the details of which give me a headache, nonetheless here are my thoughts up for your evaluation, if you wish to respond.

    Organics advocates like to claim that cow manure is a "natural, sustainable" source of nitrogen, as opposed to the evils of "synthetic, unsustainable, synthetic" nitrogen from the ammonium factory. But when I thought about it after leaving my job at the organic farm, I realized: cow manure is not at all a "source" of N: it is more like a carrier of N.

    The only true source of N on this planet is the atmosphere, and that N can become "fixed" in a limited number of ways: through lightning and rain, through the symbiotic relationship between N-fixing bacteria and legumes, and through the Haber-Bosch process of combining atmospheric N with H drawn from methane (natural gas as primary source). The lightning/rain path is very limited, so throughout most of the history of agriculture N came from the legumes and secondarily through the manures and decomposition products of the plants and animals on up the food chain that are dependent on the legumes.

    Once the Haber-Bosch process was established in the early 20th century, everything changed: We now had a new way to fix N to thereby increase agricultural output and literally grow the population with it.

    The manure dumped off at the organic farm, as well as the vegetable wastes dumped off by the cafeterias and restaurants, ultimately come from so-called conventional farms that use "synthetic" N as a source. The dairy farmer uses bagged NPK on his hay crops, and the supplier of his grain uses NPK on his corn crops; the farmers that grow the crops that supply the restaurants and cafeterias also use NPK.

    Therefore (and here's my point), the "organic" farm is just as dependent on synthetic N as if they were applying bagged NPK on the fields themselves! In fact, I wonder if it wouldn't have been more efficient and economical for them just to apply the N directly in the form of "synthetic" bagged NPK than to launder it through cows first...

    I'd go a step further and say that "organic" farmers who think they are not just as dependent on synthetic nitrogen as anyone else on this planet are deluded.

    Is this a fair assessment?

    1. Michael,
      Yes, it is a fair assessment. The good thing about organic was its novel concern (for the time) about improving soil quality. The reason it can never be more than a niche is that its method of soil improvement is so dependent on imported N,P,K and organic matter from off-site. Only legumes make their own new contribution to the N supply (and Azospirillum on some other crops like sugarcane). This post is in preparation for one questioning the environmental "advantage" of organic on several fronts. The Soil Association and Rhodale have made claims that organic could solve climate change by sequestering carbon in the soil. Both forgot to factor in the methane from composting which completely undoes their advantage.

  9. Does vermicomposting reduce the carbon footprint?

  10. Another interesting post. My comments would be:

    1. Compost adds carbon to the soil, which is good for long-term soil health, whereas synthetic N fertiliser doesn’t – though perhaps the latter does also produce more nitrous oxide when applied to fields at typically higher N rates than in organic farming? Presumably in an efficient AD process you’d be looking to maximise the extraction of your carbon for the methane, so you’d have less in the digestate? And in large-scale grain-feedlot systems, how much of that digestate actually does get back to the grain fields anyway (and at what energetic or GHG cost)?

    2. Synthetic N fixation requires a lot of fossil energy, and if you used renewable sources instead to get the H you’d need a whole lot more energy still. Until somebody has figured out how to produce vastly more clean and renewable energy than at present, which may be never, wouldn’t it be wise to try to minimise our dependence on synthetic N fixation and increase the use of biotic N fixation wherever possible?

    3. You don’t necessarily need compost or livestock to grow crops – there are many different ways to farm organically. Importing manure from non-organic holdings isn’t really one of them, and ought to be strictly controlled under organic standards. As with most things, the more the ‘alternative’ system tries to play the same large-scale game as the mainstream, the more difficulties it gets into. What I’d infer from your post is that on a crowded human planet it’s difficult for all forms of agriculture to do their job of trying to extract good crop yields without seriously screwing around with the carbon and nitrogen cycles. But probably the best way of doing it is to grow crops where people live and return everything you possibly can to the soil, preferably in ways that minimise methane and nitrous oxide production. Sound reasonable?

  11. Chris,
    Yes, compost is great for building soil quality but so is the spent fiber after a digester. I'm planning on writing a post about "what is the best way to build soil quality." What I will be saying mainly applies to row crops (the far far biggest area), and there the best way to build soil quality (carbon being part of that) is to avoid disturbances like tillage, to let crop residues remain on the surface, and to use cover crops to better imitate the perennial grassland biome that built fantastic soils in the Midwest before the advent of plow-based farming.

    But as for these questions, composts and manure can generate methane and nitrous oxide in soils after application as can plowing under a green manure or cover crop. Lots of it depends on soil characteristics and rainfall patterns. Delivering fertilizers just when plants need them is the ultimate goal, and that is almost possible with delivery through something like a drip system. In rain fed crops it is tricky, but there are ways to optimize via different N forms and precision application.

    Yes, as much biological N fixation as possible is the best approach. That is why soy is a very good thing to have in a rotation and legume containing cover crop mixes make a lot of sense.

    When it comes to renewable energy for making N, what I imagine is a highly decentralized system at the farm or maybe elevator level where a wind farm or solar array makes ammonia when it can throughout the year and that is converted to urea or something stable for storage until it is needed in the spring. It might be a pipe dream, but I believe it is possible. I am aware of some groups doing research on pieces of this.

    Actually, the organic standards don't exclude manure from non-organic sources. If they did, the potential size of organic would be even smaller. Animals don't make N, they just are somewhat inefficient in extracting all of it from their food. Biologically fixed N is the only new N in an organic system.

    I go with your last statement except the emphasis on growing it where people live. For instance, if you want to grow fruits and vegetables where people live in the Northeastern US or Chicago, the growing environment is not very suitable for lots of crops and people end up with super intense spray programs and still poor yields. I say grow crops where they grow the best so that your land-use-efficiency is as high as possible and you get the qualities you need (e.g. good bread wheat does the best in terms of quality in North Dakota and Montana but not many people live there) . Transport it efficiently. Minimize waste - including the waste of perfectly good fruits and vegetables that don't meet over-the-top cosmetic requirements.

  12. It’s true of course that composts/manure can generate GHGs after application, but so can synthetic fertilisers. Both can also generate nitrate pollution of water – but synthetic fertilisers are generally worse. You’re no doubt right that the best method of reducing N pollution would be drip systems and timed precision applications, but how close are we to that? Cassman et al reported in a 2002 paper that the average annual number of N fertiliser applications in US maize was 1.8 – maybe that’s changed a lot in the last 10 years, I don’t know, but it looks a long way away from precision application. To compare organic and conventional systems fairly I think you need to look at what farmers in both systems actually do, and not at what farmers in your preferred system might ideally do.

    Soil Association organic standards here in the UK require a derogation to use non-organic manure, and state that “You must only use non-organic manure and plant wastes to complement your soil fertility management. You must use them only occasionally and when other ways of maintaining soil health and fertility are insufficient.” Admittedly there’s a loophole there and it would be interesting to know how much that derogation is invoked – although if organic farmers are using tons of manure from conventional farmers, it doesn’t say a lot for the conventional farmers’ N management either. Ideally we need to move towards in situ biotic fixation, but it’s not easy.

    In preindustrial times, populations clustered mostly where local resources made it feasible for them to do so. Nowadays we can live wherever we choose (or more commonly where it's chosen for us) but if we want to maximise biotic nutrient cycling that causes a problem. Maybe humanity’s grand experiment with urbanisation and cereal transport will prove successful, but I wouldn’t bet against deurbanisation and a relocalisation of agriculture in the long term.

  13. Chris,
    I think you would agree that within both the conventional and organic farming communities there are subsets of growers who are most progressive. There are not really good statistics available on this as far as I know. Even tillage practice and cover cropping are poorly tracked. I do know that sales of auto-steer systems are significant, but how many growers use that to do controlled wheel traffic paired with placement of N only in uncompressed soils is hard to know. Still, if only 5% of growers are using current best-known practices, that would >25x the total for Organic and even more in a rain-fed row crop setting.

    Perhaps in the later part of this century if global population declines there could be de-urbanization, but the world will have to also deal with a population heavily weighted to old people - a trend that is already beginning in many countries.

  14. Aerobic composting is very expensive and inefficient. A Washington State study shows that as little as a 5% mistake in carbon nitrogen ratio can cause a 30-50% loss of nitrogen through evaporation. You will lose 10-15% just during the distribution process alone. I am always stunned when people want to implement expensive complex solutions when there is an incredibly simple solution. Just bury the organic matter in place. Soil is the best anaerobic decomposer ever invented. And it holds the NPK very well. Dah


Please send comments if you wish. Sorry about the word verification, but I'm getting tons of spam comments