Scientists dispute virtues of ‘black gold’
May 2nd, 2008 - 3:31 pm ICT by admin -
London, May 2 (IANS) The virtues of biochar — or “black gold” — in the soil, which was being touted as a possible carbon sink to counteract global warming, has been disputed by scientists. A new study suggests that the supposed benefits of biochar (charcoal derived from wood) may be exaggerated. When charcoal was mixed with humus, there was a substantial increase in soil micro-organisms (bacteria and fungi), the study found.
These microbes carry out decomposition of organic matter (carbon) in the soil. And consistent with this, they found that charcoal caused greatly increased losses of native soil organic matter, and soil carbon, for each of the three forest stands.
Much of this lost soil carbon would be released as carbon dioxide, a greenhouse gas.
Therefore, while it is true that charcoal represents a long-term sink of carbon because of its persistence, this effect is at least partially offset by the capacity of charcoal to greatly promote the loss of that carbon already present in the soil.
The study also suggested that the effect of biochar on the loss of carbon already in the soil needs to be better understood before it can be effectively applied as a tool to mitigate human-induced increases in carbon-based greenhouse gases.
These findings by David Wardle, Marie-Charlotte Nilsson and Olle Zackrisson of Swedish University of Agricultural Sciences, Ume
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May 4th, 2008 at 6:21 am
From Christoph Steiner a major Terra Preta researcher to the terrapreta List:
” The study of Wardle et al was simple and provides valuable data. 10 year
studies are a rare opportunity but other conventional means of C
sequestration (conservation tillage etc) were studied much longer. That
charcoal increases the decomposition of labile soil organic matter (SOM)
is a logical consequence of increased microbial biomass and activity.
This is the example of humus rich Swedish forest soils. The Terra Preta
example is different (low respiration rates in absence of an easily
degradable organic substance). Chernozems are a other example. Charcoal
can led to the formation of very persistent SOM and this in environments
and soils with low carbon sequestration capacity. Nobody proposed to
apply charcoal as a C sink in humus rich soils. It can be a mean of
carbon sequestration in depleted soils (e.g. southeastern US). Due to
agriculture most soils have lost 50% of there original carbon content.
The recalcitrance of charcoal allows SOM build up beyond the carrying
capacity of a soil.
In some cases increased decomposition might be even desired. Composting
of manures and other green biomass would be increased and emissions of
CH4 and N2O reduced. Organically applied nutrients might be faster
available for plants if applied with charcoal and leaching of nitrogen
reduced.
This study proves once again the recalcitrant nature of charcoal and
shows that we have to do much more research in the field to determine
appropriate applications for charcoal as a C sink. I am confident that
there are many management options.”