here are multiple benefits to seque

here are multiple benefits to sequestering C in forest and
agricultural soils, beyond the obvious benefit of offsetting CO2
emissions. Lal (2007) summarized collateral soil C sequestration benefits on improved soil quality, increased soil productivity, reduced risk of soil erosion and sedimentation,
decreased eutrophication and water contamination. Soil organic matter is about 58% C with a C:N ratio between 10
and 12 (Stevenson, 1994). Increasing SOM increases both C
and N concentration in the soil. Many physical, chemical,
and biological characteristics associated with productive soil
are due to high SOM content (Doran, 2002; Doran et al.,
1998; Janzen et al., 1998). Soil aggregation and aggregate stability are improved by SOM (Gollany et al., 1991; Pikul et al.,
2005; Six et al., 1998; Tisdall, 1996; Tisdall and Oades, 1982).
Increasing SOM also improves water infiltration, water-holding capacity, aeration, bulk density (Gollany et al., 1992; Olness and Archer, 2005), penetration resistance and soil tilth.
Soil organic matter plays an important role in determining
soil chemical properties including pH, nutrient availability
and cycling, cation exchange capacity and buffer capacity
(Tisdall et al., 1986). Management strategies that increase
SOM (e.g., reducing tillage increasing soil coverage) also
aid in reducing soil erosion, which preferentially removes
the SOM-rich top-soil (Cihacek et al., 1993; Gregorich
et al., 1998; Lal, 2003).
The vast majority of SOM originates from plant inputs,
although this material may pass through several trophic levels
prior to acquiring the characteristics of stable SOM. Conversion of plant biomass begins with decomposition; thus, decomposition studies provide insight into early steps of humification.
Field and laboratory incubation studies suggest that it is common for 50% or more of the initial plant biomass input to
decompose within the first year (Broder and Wagner, 1988;
Burgess et al., 2002; Buyanovsky and Wagner, 1997; Johnson
et al., 2004; Schomberg et al., 1994; Stott and Martin, 1990).
The rate of decomposition in the field depends on climatic conditions (moisture and temperature), particle size, biomass to
soil contact, biomass orientation, and plant biochemical composition (Aerts, 1997; Ghidey and Alberts, 1993; Johnson
et al., 2007a).
2.3. Charcoal/black C
Charcoal or black C, a unique recalcitrant form of C, is
found in many soils, especially those with history of burning
activities. In the literature, terminology referring to this type
of C includes ‘‘charcoal,’’ ‘‘char,’’ ‘‘bio char,’’ ‘‘black C’’
and ‘‘agro-char.’’ In this review, we use ‘‘charcoal’’ as a generic
term for this form of recalcitrant C and use ‘‘bio char’’ to specifically refer to biologically active charcoal resulting from
biomass pyrolysis. Charcoal results from incomplete combustion (insufficient oxygen) of biomass C (Goldberg, 1985) and
can contribute to C sequestration (Fowles, 2007). The physical
and chemical properties of charcoal vary tremendously from
fly-ash burning to bio char from pyrolysis (Goldberg, 1985).
Charcoal is rather ubiquitous in soils, resulting from natural
or intentional burning of biomass (Schmidt and Noack,
2000). Charcoal can represent 10e35% of the total SOC and
is highly recalcitrant to microbial and chemical decomposition
(Skjemstad et al., 2002).
One of the advantages of using bio char as a soil amendment is that C can be locked in the soil for centuries, perpetuating enhanced plant growth and the ability to store and
recycle C more efficiently (Fowles, 2007; Lehmann et al.,
2006). It has been suggested that converting from ‘‘slash
and burn’’ to ‘‘slash and char,’’ which is more C and nutrient
conservative, could improve soil quality of Oxisols (Lehmann
et al., 2002). Adding charcoal in addition to NPK fertilizer
improved plant growth and doubled grain yield compared to
using inorganic fertilizer alone on a Brazilian Oxisol
(Christoph et al., 2007). Bio char has the capacity to reduce
CO2 emissions, making the system C-neutral or in some cases
C-negative (Fowles, 2007). Bio char formed under the proper
conditions has remarkable nutrient affinity and enhances the
cation exchange capacity of soil, as well as biological
processes that lead to improved soil structure, water storage,
and soil fertility (Fowles, 2007). Bio char can be infused
with other nutrients (i.e., N as ammonium bicarbonate) to
act as a slow release fertilizer (Day et al., 2002, 2005) and
potentially decrease leaching and runoff (Fowles, 2007). Bio
char could also adsorb pesticides and other potential pollutants (Lehmann et al., 2006), as well as reduce N2O and
CH4 emission from agricultural fields (Fowles, 2007). The
feedstock and pyrolysis conditions of thermochemical bioenergy platforms can be manipulated to produce bio char.
Generation of bio char may require sacrificing some of the
energy produced to retain more C sequestration value
(Johnson et al., 2007b).