[Terrapreta] limitations on charcoal as a C sink

[Christoph Steiner]

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.