Dominic Woolf has recently published a thirty page study on biochar titled ‘Biochar as a soil amendment: A review of the environmental implications’
This report is a bit too long to republish in its entirety but I think it is useful to copy the summary conclusions. I will likely copy other portions with commentary as appropriate.
It is worth reading in its entirety as it will bring the reader completely up to date with the present apparent state of the art. My one comment is more in line with my own proposed methodology for production as likely employed by the Amazonians. It is that industrial techniques such as metal kilns or barrels are isolating the feedstock during the burn phase.
The corn culture earthen kiln should not only be reasonably efficient but also more inefficient in terms of the completeness of the burn. I suspect that this matters in practice and I would hate to miss something important by sticking to modern kilns.
I sense that a collapsed earthen kiln will leave a high quality biochar blended with an appropriate amount of soil, whereas a metal kiln will produce a uniform powdered char product with far less variation and perhaps much less biological utility. We need to compare both protocols in the field to see what we should be striving for.
In conclusion, we can say that biochar appears, given the current state of knowledge, to have potential both for greenhouse gas mitigation and as a soil improver. Considerable uncertainties remain, however, about its applicability to different soils and crops and about how much biochar production is feasible with respect to constraints on economics, land availability and competing demands for biomass (including direct incorporation into the soil). The uncertainties and areas requiring further research are outlined below:
• A maximum of 1 PgCyr-1 biochar might be produced from agricultural residues (if all current global agricultural residues were converted to biochar). In practice, this figure will be constrained by cost, suitability of different residues, requirements to incorporate residues into the soil, and other competing demands.
How much biochar might be produced from agricultural residues once such constraints have been taken into account is a matter for further research.
• Estimates of how much biomass might be produced by dedicated cropping remains a highly debated question. At the low end, figures from Sims et al (2006) suggest that between 0.06 - 0.7 PgC yr-1 might be realistically achievable by 2025. At the high end, figures from Smeets et al (2007) suggest that up to 46 PgC yr-1 might be achievable if we were to transform the planet into a large factory farm. More detailed studies at the local level will be required to ascertain the true potential for dedicated production of biomass.
• Other potential sources of biomass include shifting cultivation, forestry residues, sewage and waste streams such as food waste and paper/cardboard. Further research will be required to ascertain the combined potential of all possible sources of biomass for biochar production.
• How rapidly biochar may oxidise in different environments is still largely unknown, although its observed recalcitrance under many conditions gives reason for optimism that the rate of decay of black carbon in soil will be sufficiently slow to make it a useful form of carbon sequestration.
• Co-production of biochar and energy is clearly possible (as demonstrated by the fact that pyrolysis technologies designed for energy production alone produce a residue of char). However, there is a conflict between maximising energy or biochar production. For a 45% yield of char, a maximum of 32% of the available energy from the biomass will be recoverable. For a 20% yield of char on the other hand, a maximum of 72% of the available energy from the biomass will be recoverable. The optimisation between biochar and energy production will require balancing considerations of climate change mitigation, energy demand, economics and engineering, and requires further research and development.
• Whilst a beneficial effect of biochar soil additions on crop yields has been demonstrated for a small number of soil/crop combinations, its utility in a wide range of soil/crop types (particularly in temperate zones) remains to be demonstrated. This will require consideration not just of its effect on nutrient cycles, but also on hydrology.
• The effect of biochar production on nitrous oxide emissions is largely an unknown factor. Although there is a possibility that biochar additions may reduce N2O direct emissions from soils, and may also reduce indirect N2O emissions by
reducing nitrate run-off, neither of these possibilities has been adequately demonstrated under a range of different agricultural conditions. There is also the possibility that, if biochar is produced by dedicated cropping with application of mineral nitrogen fertilizer, the direct and indirect N2O emissions from this fertilizer will lead to an increase rather than a decrease in net N2O emissions.
• Biochar has the potential to either alleviate pressure on land use (by increasing crop yields) or to become a competing demand for land (in the case of dedicated cropping for biomass feedstock). Either way, the role of biochar in establishing a comprehensive land use strategy that meets the environmental, social and economic needs of the 21st century is in need of further consideration.
• It is possible that biochar may help to reduce nutrient run-off from soils and the associated problems of eutrophication and hypoxia of both inland and coastal waters. In what soils and under what conditions this might in fact be achieved remains to be shown. There is evidence that under some conditions, biochar may have the opposite effect of increasing leaching of applied mineral fertilizers.
• Despite its potential to reduce greenhouse gas emissions, the widespread land application of biochar might also have a detrimental effect on global warming by increasing the radiative forcing due to albedo. The extent to which this may be a problem, and the extent to which this may be mitigated by strategies such as maintaining a dense vegetation canopy over darkened soils requires further research.
Given the serious potential impacts of anthropogenic climate change, and the significant potential of biochar as a mitigation strategy, the uncertainties outlined above need to be resolved with some urgency.