[Terrapreta] Interesting article

[David Wardle]

Not wanting to sound contrary, but I cannot agree with Gunther's comments 'the conclusion that the addition of char to soil could be offset by the increased decomposition of litter, is wrong' or 'After a century or two, all the litter would be expected to be metabolised, charcoal or not.'
Firstly, our litter bags contained humus and not 'litter'. Secondly, there is ample evidence (indeed you can find it in undergraduate textbooks on soil science) that there are significant pools of soil organic carbon (including in humus) that themselves are quite recalcitrant and take many centuries or millennia to break down. Therefore the claim about 'after a century or two' is not true.

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From: folke Günther [mailto:folkeg at gmail.com]
Sent: den 5 maj 2008 09:40
To: Terra Preta; Richard Douthwaite; Exergigruppen; David Wardle
Subject: Re: Interesting article

Now, I have received the full article and a supplement describing the investigation method (attached). Actually, nothing changes my conclusions from yesterday; The inevitable decomposition of litter is hastened by the addition of charcoal, but since this would happen anyway, the conclusion that the addition of char to soil could be offset by the increased decomposition of litter, is wrong.
FG
2008/5/4 folke Günther <folkeg at gmail.com<mailto:folkeg at gmail.com>>:
I haven't received the pdf yet, but since there as been quite a discussion,I will go through the background. When I have got the pdf, I will go through it and give some more details.
The basic is this:

 1.  Burying charcoal in the soil always sequesters carbon (carbon dioxide) from the atmosphere

    *   This is true disregarding the route this charcoal may have taken to the soil. I.e. even if the 'footprint' of this special  charcoal is larger than its actual content of coal (say that it is flown around the world), burying the charcoal mean that this certain amount is eliminated fro the atmosphere.
    *    I.e. If you burn it, it will return to the atmosphere, If you bury it, it will stay in the soil for thousands of years

 1.  Charcoal increase soil metabolism.

    *   This may not come as a surprise to any of the members of this list.The reasons for that, and its effects, have been discussed, and easily observed, for a long time.

 1.  If you mix litter and charcoal, the litter will decompose faster than if it is not mixed with charcoal.

    *   This was confirmed by the study. A large part (25% i the first two years) of the litter was metabolised by the microorganisms. I do not know if a simultaneous increase of the plants living in, on or near the bags was observed. One could expect  that.

 1.  In bags with only litter, some metabolism would be observed, although smaller than in the litter mixed with charcoal.

    *   This is perfectly normal.

 1.  Thus, the presence of charcoal increase the rate of litter decomposition.

    *   Why am I not surprised?
    *   Jumping to the conclusion, however, that the presence of charcoal in the soil would be offset by the increased metabolism of litter, is wrong. After a century or two, all the litter would be expected to be metabolised, charcoal or not.

 1.  Therefore, the sequestration effect of charcoal is not counteracted by increased soil metabolism, since the SOM (Soil Organic Matter) is ephemeral in comparison to the charcoal, and will decompose anyhow. However, the in increased metabolism is reflected in a change in litter decomposition rate.
It would be very interesting to have the real figures, since that might allow a calculation of the metabolism increase. A friend observed a surprisingly high increase in the decomposition of a small compost heap when charcoal was added

2008/5/2 folke Günther <folkeg at gmail.com<mailto:folkeg at gmail.com>>:
In the latest number of Sciene, (2 May), David Wardle, Marie-Charlotte Nilsson och Olle Zackrisson delivers an article: "Fire-Derived Charcoal Causes Loss of Forest Humus", where they claim that charcoal particles remaining after fire increase the microbial activity so they break down humic particles at a rate that counteracts the carbon sequestration effect of the carbon.


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Folke Günther
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