Tuesday, May 13, 2008

Right observation, wrong conclusion

Some time ago, I got an enthusiastic e-mail from a friend.

He told me he have had a compost heap at home
(Really a heap of leaves, not the neat thing you often find in a residential garden. It had been staying there for years, with just a slight decease in volume).
After having repeatedly listening to me talking of the benefits of adding char to compost and soils, he thought that he should give it a try. So he took some leftover barbeque char and put in the compost heap.

That was last autumn. He forgot everything until some weeks ago when he happened to stroll by. The heap had disappeared! Instead of the heap of semi-mouldered leaves, there was a much smaller heap of something rather looking like soil, with lots of earthworms in it. Plus the char.

Then he went to the computer and mailed me: --“ I am a believer …!


In last week’s number of Science, (2 May), David Wardle, Marie-Charlotte Nilsson och Olle Zackrisson delivers an article: "Fire-Derived Charcoal Causes Loss of Forest Humus". They have done exactly the same observation, that charcoal increases soil meabolism, using a controlled measurement method during a ten-year long test. They used three types of 1 gram bags containing respectively charcoal, humus and humus + charcoal, letting them stay in the soil for up to ten years.

The investigation revealed that the reduction in weight of the bags containing both charcoal and humus was much larger than the bags containing only charcoal or only humus.
Just like my friend with the compost heap found out.

So long, so well. A fair conclusion would have been that the charcoal increases the soil metabolism. Not mentioned in the report, but the also found increased vegetation around the bag with humus + char.

Sadly, they did not stay with that. They also jumped to a conclusion where they claim that the increased microbial activity break down humus particles at a rate that counteracts the carbon sequestration effect of the carbon.

The latter conclusion, however, is wrong.
Since the humus particles would have been broken down later anyhow, a faster breakdown will not counteract the carbon sequestration made by the incorporation of carbon particles in soil.
Just like the compost heap of my friend; waiting some more years would have changed an un-charred heap into the same composted state as the one with char added.

The decomposition rate of the humus in the soil has nothing to do with the carbon sequestration capacity of the charcoal. This capacity is only influenced by the longevity of the charcoal in soil (which is many thousands of years)