10
$\begingroup$

I understand that all plant matter releases CO2 as it decomposes and also when it burns in, for example, a forest fire. My assumption, upon which this question is based, is that the CO2 release is the same in each case.

My main question is, what drives the production of all greenhouse gases in those two situations, e.g. methane in decomposition, NOx in combustion to help determine which is worse in terms of the total radiative effects of the greenhouse gases?

I appreciate the answer is likely to vary according to type of plant matter, the degree of anaerobic decomposition and the intensity of fire. Sub question: Are there any benchmarks that indicate which is worse in some specific situation or controlled trial?

$\endgroup$
  • 1
    $\begingroup$ Do you mean CO₂, or total greenhouse gases as adding up their radiative effects? $\endgroup$ – gerrit Feb 1 '16 at 18:27
  • $\begingroup$ I meant the latter, I presume its a tall order, as I assume that the CO2 is the same either way. I've modified the question for clarity. $\endgroup$ – Puffin Feb 1 '16 at 18:31
  • 2
    $\begingroup$ The CO₂ is not necessarily the same either way, as CH₄ and other greenhouse gases can burn to form CO₂ and H₂O and others. It's a good question, somewhat multidisciplinary between Earth Science, Chemistry, and Sustainable Living. $\endgroup$ – gerrit Feb 1 '16 at 18:33
  • 4
    $\begingroup$ Decomposition of a plant does not always release all of the carbon bound in the plant. Rather, it may also be stored for a long time as organic carbon in the soil, or it may be washed into the ocean and deposited in sediments for geologically long times. $\endgroup$ – Wolfgang Bangerth Feb 2 '16 at 2:58
7
$\begingroup$

Another way of looking at the question:

I take 2 standard wheelbarrows of bits of plants from my garden. One wheelbarrow is used to fill the compost bin. The other goes to the bonfire. How much ash do I have from burning compared to how much compost from the compost bin. Any gardener will know that the amount of compost will be much larger than the amount of ash. So now we need to compare the chemical composition of the ash with that of the compost. For Wood Ash there are quite a few research articles. Most of the carbon atoms in wood ash will be in the form of Calcium Carbonate (CaCO3). Percentage in the ash will vary widely but lets take a median of 50%. Ash as a percentage of original organic matter will be maybe of the order of 8% by weight.

For an original 100kg of organic matter (yes, its a big wheelbarrow) we would have 8kg of ash. In this ash would be 4kg of CaCO3. The carbon atoms would be 12% of that (relative atomic masses). So 100kg of organic bits gives you 0.48kg of carbon atoms left in the ash when you burn it.

There seems to be fewer research articles on composting but we found the following approximations. For 100kg of organic matter there will be about 9% organic carbon (9kg). Composting will turn 100kg of raw material into around 30kg of compost in which the organic carbon content will be around 25% ( 7.5kg of carbon). These figures are median "pot luck selection" as results can vary widely. We can however see that composting retains massively more (7.5kg) of carbon atoms compared with burning (0.48kg). The difference will be in the atmosphere.

$\endgroup$
  • 1
    $\begingroup$ That's a very intuitive answer, thank you. May I ask you to have a look at this vaguely related Q &A? earthscience.stackexchange.com/questions/9322/… It gives a much lower value for the "compost" case apparently because it assumes the carbon and nitrogen reactions proceed until to exhaustion. The gardener doesn't see this because they are interested in the intermediate "30kg" stage and presumably the compost would continue to react "if" continually mixed rather than being buried. $\endgroup$ – Puffin Apr 17 '17 at 15:54
  • 2
    $\begingroup$ I would add another step to this mind-experiment -- if you put the compost on the ground, or if you put the original plant matter on the ground and never called it "compost" separately, what then? Soil carbon can be in a labile (fast-turnover), slow-turnover, or very slower turnover pool -- each are needed for longterm soil ecology, and possibly we could design composting systems to put more carbon into the slow pools. Here's a summary: soilquality.org.au/factsheets/labile-carbon $\endgroup$ – cphlewis Apr 17 '17 at 20:40
  • 1
    $\begingroup$ @cphlewis. Yes, different composting techniques and differing soil chemistry will give widely different results and in the longer term we would of course like to track all the carbon atoms. No doubt there are many cycles in soil of differing timescales. Any gardener with an allotment will know this and you can see interesting things if you dig up the soil in undisturbed natural woodland and observe it over time (I have managed a "wild" wood for the past 17 years). If you want to manage carbon from plant waste some research into slow pools does look like a useful research area. $\endgroup$ – user7733 Apr 19 '17 at 19:28
  • 1
    $\begingroup$ @Puffin Fill a compost bin and leave it for a long time, say 20-30 years. Still plenty of stuff left. I have seen this in neglected gardens when a formerly active gardener becomes elderly/incapable/housebound. It does not all disappear, go to dust, turn to gas, well not in 3 decades anyway. Still plenty of nice brown soil like compost in the bin. So consider how much carbon is found in "old" soil. Looks like its is about 10% by weight according to chemical analysis.So, yes, much less than the 25% from about 1-2 years worth of typical garden composting. $\endgroup$ – user7733 Apr 19 '17 at 19:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.