the system boundaries be possibly negative. With biochar, not only the soil carbon
is stored for decades, but also biomass productivity in the ecosystems increases, due
to soil quality improvement, with a secondary cascading effect of additional carbon
added in soils (e.g., Lehmann and Joseph 2009; Lehmann et al. 2006; Gaunt and
Cowie 2009).
Chemically, biochar has a high content of fixed carbon, ranging from 50 to 90%
with volatiles and ash amounts ranging from 0–40% to 0.5 and 5%, respectively
(e.g., Verheijen et al. 2010; Weber and Quicker 2018). The high carbon amount and
the existence of aromatic rigid structural compounds, which are hexagonal compact
carbon units linked together and deprived of oxygen and hydrogen, are the common
features of recalcitrant biochar feedstocks which can derive from a wide range of
biomasses, through a scope of thermochemical pyrolytic and gasifying conversions.
The residence time of biochar versus its mineralization rate in the soil is a key
variable for the establishment of scenarios of the global reduction in carbon
emissions. Under a conservative perspective, the composition of biochar is about
80% of stable aromatic recalcitrant carbon and 20% labile mineralizable carbon
which can be released into the atmosphere as carbon dioxide in the first years of
application in soil. This release is very dependent on the weight of these fractions in
the biochar composition.
Modeling analysis shows that a maximum reduction in carbon emissions would
require a mean residence of biochar in soil higher than 100 years, considering a
system of 50% conversion rate for biochar production from slow biomass pyrolysis
or carbonization. A mean residence time of 50 years returns 96% of carbon to the
atmosphere over a period of 200 years, whereas a return of 28% would correspond
to a residence time of 500 years (e.g., Lehmann et al. 2010).
The concretization of the whole potential of biochar feedstock in environmental
management, depends essentially on a sustainable development of a paradigm of
slash-and-char. The later consists of biomass conversion through, for example,
technologies of carbonization, torrefaction, pyrolysis, or gasification of biomass
feedstocks, such as from agricultural and forestry wastes such as forest residues, mill
residues, field crop residues, and urban wastes. Those slash-and-char practices should
thereby replace traditional slash-and-burn practices of biomass residues in the field.
A myriad of conversion technologies has in common the heating and/or the
oxidation of biomass feedstock under the partial or total absence of oxygen. The
yields of biochar in these processes range from 20 to 60% and carbon contents of
biochar higher than 50%, depending on operative variables and feedstock (e.g.,
Schenkel et al. 1999; Lehmann 2007; Meyer et al. 2011; Enders et al. 2012; Lee
et al. 2013).
The stability or longevity of carbon in/and biochar is another important matter,
insofar that only a long residence time ensures relevant carbon sequestration. Over
century lifecycles, biomass mineralization dynamics is more sensitive to variations
in the amount in labile carbon, say from 0 to 5%, than to decrease, for example,
from 1000 to 500 years in mean residence times. As aforementioned, most carbon
in biochar is highly stable and has an average time of 1000 years or longer at 10 °C
mean annual temperature (e.g., Roberts et al. 2010).
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8 Fundamentals of Global Carbon Budgets and Climate Change
is stored for decades, but also biomass productivity in the ecosystems increases, due
to soil quality improvement, with a secondary cascading effect of additional carbon
added in soils (e.g., Lehmann and Joseph 2009; Lehmann et al. 2006; Gaunt and
Cowie 2009).
Chemically, biochar has a high content of fixed carbon, ranging from 50 to 90%
with volatiles and ash amounts ranging from 0–40% to 0.5 and 5%, respectively
(e.g., Verheijen et al. 2010; Weber and Quicker 2018). The high carbon amount and
the existence of aromatic rigid structural compounds, which are hexagonal compact
carbon units linked together and deprived of oxygen and hydrogen, are the common
features of recalcitrant biochar feedstocks which can derive from a wide range of
biomasses, through a scope of thermochemical pyrolytic and gasifying conversions.
The residence time of biochar versus its mineralization rate in the soil is a key
variable for the establishment of scenarios of the global reduction in carbon
emissions. Under a conservative perspective, the composition of biochar is about
80% of stable aromatic recalcitrant carbon and 20% labile mineralizable carbon
which can be released into the atmosphere as carbon dioxide in the first years of
application in soil. This release is very dependent on the weight of these fractions in
the biochar composition.
Modeling analysis shows that a maximum reduction in carbon emissions would
require a mean residence of biochar in soil higher than 100 years, considering a
system of 50% conversion rate for biochar production from slow biomass pyrolysis
or carbonization. A mean residence time of 50 years returns 96% of carbon to the
atmosphere over a period of 200 years, whereas a return of 28% would correspond
to a residence time of 500 years (e.g., Lehmann et al. 2010).
The concretization of the whole potential of biochar feedstock in environmental
management, depends essentially on a sustainable development of a paradigm of
slash-and-char. The later consists of biomass conversion through, for example,
technologies of carbonization, torrefaction, pyrolysis, or gasification of biomass
feedstocks, such as from agricultural and forestry wastes such as forest residues, mill
residues, field crop residues, and urban wastes. Those slash-and-char practices should
thereby replace traditional slash-and-burn practices of biomass residues in the field.
A myriad of conversion technologies has in common the heating and/or the
oxidation of biomass feedstock under the partial or total absence of oxygen. The
yields of biochar in these processes range from 20 to 60% and carbon contents of
biochar higher than 50%, depending on operative variables and feedstock (e.g.,
Schenkel et al. 1999; Lehmann 2007; Meyer et al. 2011; Enders et al. 2012; Lee
et al. 2013).
The stability or longevity of carbon in/and biochar is another important matter,
insofar that only a long residence time ensures relevant carbon sequestration. Over
century lifecycles, biomass mineralization dynamics is more sensitive to variations
in the amount in labile carbon, say from 0 to 5%, than to decrease, for example,
from 1000 to 500 years in mean residence times. As aforementioned, most carbon
in biochar is highly stable and has an average time of 1000 years or longer at 10 °C
mean annual temperature (e.g., Roberts et al. 2010).
302
8 Fundamentals of Global Carbon Budgets and Climate Change