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Geology, Palaeontology & Archaeology / Questions on Terra Preta
« on: 04/04/2007 03:33:02 »
I have just joined this forum and am interested for the same reasons as Erich et al. One possibility that appeals to me is that in Australia we regularly have to perform "hazard reduction burning" around the urban/bushland interface to minimise the risk of burning people and houses through bushfires. I would like to think that part of the carbon in the fuel removed could be sequestered and Terra Preta sounds like an admirable application to economically support this labour-intensive clearing operation.
If anybody is worried by the removal of nutrients from these interface zones, don't be; Australian East Coast sandstone country in particular is very infertile and its flora is well adapted to nutrient paucity. Indeed the primary problem in maintaining biodiversity here is nutrient contamination from urban runoff which promotes choking weed infestation. Already some mechanical clearing is used for fire hazard reduction and the resultant vegetable matter (woody shrubs, branches, leaves) is chipped for garden mulch. To do so on a much larger scale instead of control burning would require an economic base.
I would like to raise a few scientific questions which I hope somebody can help with:
1) Elemental carbon is apparently very stable at ambient temperature and bushfires have been shaping the Australian landscape for 40000 years or more. Although traces of charcoal do appear in alluvial sediments, would we not expect massive deposits of charcoal in stream beds etc? Where has it all gone? Surely not all of it was burned in subsequent bushfires?
2) Is the terra preta carbon actually charcoal or a partially-charred matrix of polyaromatic tar or creosote compounds? Would these not be as toxic to soil organisms as thery are to humans?
3)Am I on the right track in visualising terra preta char as simply slowing down the release of "ash bed" nutrients over several years? How would the nutrients have been refreshed for sustainable agriculture?
4)It appears from my reading that the oxidation of elemental carbon (char, graphite, diamond etc.) is inhibited, as in metals, by a tenacious film of oxide preventing further oxygen ingress. For carbon of course, that oxide is gasseous CO and CO2 in the free state but is adsorbed strongly to any clean carbon surface at ambient temperature. At temperatures above about 300°C however it is desorbed, leading to the familiar exothermic oxidation chain reaction. The question is whether any ambient temperature biological agents such as enzymes can strip away these oxides to slowly oxidise charcoal?
5)Elsewhere I have read of terra preta as being self-propagating, presumably through bacterial or fungal migration and presumably at ambient temperature. What is the evidence for this and what mechanism is postulated?
6)One feasible mechanism for propagating terra preta might be to bury dense piles of fresh vegetation at its edge, covering with earth to insulate and exclude excess air. Through bio-heating due to innoculation with microorganisms and moisture, this deposit might spontaneously combust in the manner disasterously familiar to farmers storing hay or silage. Is there any archaelogical evidence for this process in precolumbian Amazon settlements, or any reported fertiliser effect around extinguished but once-smouldering modern silage pits?
Lots of questions, any answers?
Murray.
If anybody is worried by the removal of nutrients from these interface zones, don't be; Australian East Coast sandstone country in particular is very infertile and its flora is well adapted to nutrient paucity. Indeed the primary problem in maintaining biodiversity here is nutrient contamination from urban runoff which promotes choking weed infestation. Already some mechanical clearing is used for fire hazard reduction and the resultant vegetable matter (woody shrubs, branches, leaves) is chipped for garden mulch. To do so on a much larger scale instead of control burning would require an economic base.
I would like to raise a few scientific questions which I hope somebody can help with:
1) Elemental carbon is apparently very stable at ambient temperature and bushfires have been shaping the Australian landscape for 40000 years or more. Although traces of charcoal do appear in alluvial sediments, would we not expect massive deposits of charcoal in stream beds etc? Where has it all gone? Surely not all of it was burned in subsequent bushfires?
2) Is the terra preta carbon actually charcoal or a partially-charred matrix of polyaromatic tar or creosote compounds? Would these not be as toxic to soil organisms as thery are to humans?
3)Am I on the right track in visualising terra preta char as simply slowing down the release of "ash bed" nutrients over several years? How would the nutrients have been refreshed for sustainable agriculture?
4)It appears from my reading that the oxidation of elemental carbon (char, graphite, diamond etc.) is inhibited, as in metals, by a tenacious film of oxide preventing further oxygen ingress. For carbon of course, that oxide is gasseous CO and CO2 in the free state but is adsorbed strongly to any clean carbon surface at ambient temperature. At temperatures above about 300°C however it is desorbed, leading to the familiar exothermic oxidation chain reaction. The question is whether any ambient temperature biological agents such as enzymes can strip away these oxides to slowly oxidise charcoal?
5)Elsewhere I have read of terra preta as being self-propagating, presumably through bacterial or fungal migration and presumably at ambient temperature. What is the evidence for this and what mechanism is postulated?
6)One feasible mechanism for propagating terra preta might be to bury dense piles of fresh vegetation at its edge, covering with earth to insulate and exclude excess air. Through bio-heating due to innoculation with microorganisms and moisture, this deposit might spontaneously combust in the manner disasterously familiar to farmers storing hay or silage. Is there any archaelogical evidence for this process in precolumbian Amazon settlements, or any reported fertiliser effect around extinguished but once-smouldering modern silage pits?
Lots of questions, any answers?
Murray.