Palaeo BioChar

The creation of biochar (or Terra Preta) is a technique with huge potential for carbon sequestration. 

However, this is not a new innovation - it turns out it's been used for thousands of years by the pre-Columbian Amazonians.  So although it has a huge potential for present and future climate change mitigation, I'm going to investigate terra preta in a palaeo-geoengineering context.
Here's a little video which is quite good at explaining the concept, it focuses on Australia and features the first Australian agrichar farm:

But aside from that, instead of reviewing a scientific paper on the matter I read a book titled 'Amazonian Dark Earths: origin, properties, and management' - I say read, I selected the chapters I thought would be relevant and read those, because it was a long book..

In this part of the world, the terra preta is called 'Amazonian dark earth'(ADE), or 'terra preta de índio'; which I prefer, it sounds much more exotic. 
After much debate in the scientific world, there is now general consensus that ADE is anthropogenic, though whether it was created intentionally or not is still unknown as practices used to form them are not fully understood.
Most known sites are 500-2500 years old, though it is possible earlier sites have disappeared due to soil erosion, sea level rise, mineralization or other environmental factors, as the area has been inhabited for 11,000-10,000 years.
So this means, maybe the population began to rapidly expand 2500 years ago, or, this was the beginning of fully developed sedentary lifestyles.  But unlike usual farming practices, it shows the same sites were used for at least 10 years rather than rotating crops every few years.

So what actually caused enrichment of the soil?  There are several hypotheses:

1. A result of ashfall from the nearby Andes volcanoes
2. From sedimentation in tertiary lakes or ponds
3. Wind blowing in fertile soils from afar
4. As a result of food preparation, cooking and cleaning
5. From burning of 'discard areas', aka a large pre-Columbian dump.

The majority of evidence points to hypothesis 4 and 5.  The blackness of the Earth is a product of incompletely combusted organic matter, and bones from fish and game as well as shards of pottery confirm these were probably discard areas.  The fact they cover wide areas of land suggests settlements were either very large, or the population shifted around over time.  Large populations would have required productive agricultural areas for their livelihoods; and seeing as there is no evidence for slash and burn (they only had stone axes!) is seems likely they knew burning their rubbish produced fertile soils.


So, the carbon sequestration part.  Although obviously these people were blissfully unaware of global warming as we know it, their methods of soil improvement have stimulated a modern day innovation of creating biochar as a means of storing carbon. 
Pollen analysis of ADE sites shows palms were important in formation; as were river grasses, shellfish shells and household refuse, which all led to primary or secondary sequestration in ADE.  It has been calculated that 1 hectare of ADE stores 10Mg or C per year, creating a carbon reservoir of >200Mg C/ha within 25-50 years.  Additionally, 14C dating of some soils indicates around 10cm accumulating per decade, which obviously works out as 1cm of this soil being formed per year.  Which I think is pretty good considering tropical soils are reknowned for high erosion and depleted nutrients.

So in conclusion, it is clear that the pre-Columbian Amazonians deliberately created this soil, but as a way to improve their crops as opposed to storing carbon.  Fire played a central role in the formation of the soil, and household waste was the key to creating such fertile land.  In terms of using this ancient idea in the present day, there is the same problem as with afforestation - farmers are not likely to sacrafice their land in order to store carbon, unless there is some sort of reward available or profit to be made.  It is estimated it will take 4-6 years before creating ADE will benefit the farmer, I guess it just depends if they are willing to wait this long to reap the benefits.  Overall I think there is a lot of potential for this technique to be successful at reversing global warming.

Lehmann, J., D. Kern, B. Glaser and W. Woods(eds) (2003) Amazonian Dark Earths: Origin, Properties, Management, Amsterdam: Kluwer Academic Publishers.