2. • Soil organic carbon (SOC) is one part in the much larger global
carbon cycle that involves the cycling of carbon through the
soil, vegetation, ocean and the atmosphere .
• The SOC pool stores an estimated 1500 PgC in the first meter
of soil, which is more carbon than is contained in the
atmosphere (roughly 800 PgC) and terrestrial vegetation (500
PgC) combined (FAO and ITPS, 2015)
• This phenomenal SOC reservoir is not static, but is constantly
cycling between the different global carbon pools in various
molecular forms (Kane, 2015).
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What is SOC ?
6. Global carbon cycle
•Organic material
transformation process
results in a complex
biogeochemical mixture of
plant litter compounds and
microbial decomposition
products in various stages
of decomposition.
(Von Lützow et al., 2006;
Paul, 2014)
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9. SOIL CARBON SATURATION
The concept of soil carbon saturation implies that the soil carbon
stock has reached its maximum carrying capacity for storing soil
carbon inputs (Six et al., 2002; Stewart et al., 2007).
This threshold, which depends on many factors including
inherent and dynamic soil properties and their interactions with
abiotic factors, is also referred to in literature as the maximum
carbon stabilization capacity (Beare et al., 2014).
Soils that are depleted of SOC have the greatest potential to
gain carbon, but also have the least propensity to do so. Since
the majority of soils around the world are far from their saturation
thresholds, there is great potential for increased carbon inputs
and management that protects existing stocks to maximize soil
carbon sequestration(Kane, 2015).
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10. SOC is divided into different pools
as a function of its physical and chemical stability
Pool Turnover Period
Fast pool (labile/active pool) 1–2 years.
Intermediate pool 10-100 years
Slow pool (refractory/stable pool) 100 to >1 000 years
Pyrogenic SOC : formed from partially carbonized (e.g., pyrolyzed) biomass
during wildfires which is present in many ecosystems (Schmidt and Noack, 2000)
(FAO and ITPS, 2015; O’Rourke et al., 2015):
11.
12. Soil organic matter
SOM contains roughly 55–60 percent C by mass. In many soils, this
C comprises most or all of the C stock – referred to as SOC –
except where inorganic forms of soil C occur (FAO and ITPS, 2015).
The protection of OM against decomposition by the described
mechanisms decreases in the order:
chemically protected > physically protected > biochemically
protected > non-protected (Kogel-Knabner et al. (2008),
SOM can be divided into different pools
Active pools - turnover in months or few years;
Passive pools - turnover in up to thousands of years.
16. Stabilization of C in soil
The three mechanisms depend on a number of biotic, abiotic and
management factors that shape their soil carbon stabilization efficacy.
(Six et al., 2006; Kane, 2015).
21. Conclusion………..
The shear size of the soil carbon pool and the
annual flux of carbon passing through the soil
are two of the reasons that SOC can play a
significant role in mitigating GHG emissions.
Currently, there is not enough research on the
topic of anthropogenic impacts on the
formation and loss of soil inorganic C to be
able to assess its sequestration potential and it
is only the organic C pool that is under
consideration for inclusion in various emission
reduction schemes.
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23. Introduction
Most current SOC models employ first-order kinetics for
decomposition processes, and therefore the models predict
linearity between C input level and SOC level at equilibrium
(i.e., steady-state; Paustian et al. 1997)
Many long-term agroecosystem field experiments, in which
treatments give different levels of C inputs, show soil C
stocks that appear linearly related to the average amount of
C returned to the system (e.g. Huggins et al. 1998b; Kong et
al. 2005; Paustian et al. 1997a).
Saturation limits to individual SOC pools have been proposed
by other researchers, due to silt + clay protection (Hassink
1996, 1997), soil structure (physical protection within
aggregates), and the biochemical complexity of the organic
compounds (Baldock and Skjemstad 2000).
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24. Linear model
•Proposed by Jenny (1941) and other early workers)
•The simple first-order decay model
•assumes that the amount of C entering a C pool is
independent of the pool size
•decomposition rates are directly proportional to the
size of the pool
25. Carbon saturation model
Hassink and Whitmore, 1997
•The C saturation model has a whole soil saturation
limit (Cm) due to inherent physicochemical
limitations.
•The fundamental relationship between C addition
levels and soil C levels at steady-state is
asymptotic.
•expressed as a simple modification to the C input
term
26. Mixed model
•where, k1 and k2 is decomposition constant
•a is partitioning coefficient for losses from C1, and transfer of decomposition
products to the more stable pool C2
Stewart et al., 2007
27. Contd…..
As C2 approaches saturation, the amount of
decomposed products from C1 transferred into
C2 decreases and ceased at saturation point.
At this point SOC accumulation in C1
proceeds according to the linear model
28.
29.
30.
31.
32. Conclusions
Soils that show
1. no increase in SOC stock with increasing C
input levels
2. high C content soils having decreased C
stabilization efficiency
suggest an upper limit, or saturation limit to
whole soil C.
33. Objective: Carbon saturation in the silt and
clay particles in soils with contrasting
mineralogy
CASE STUDY - 2
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34. Introduction
The goal of this study was to elucidate whether or not the silt and
clay fractions are saturated in temperate and subtropical soil from
Chile and Mexico, respectively.
C-saturation depends on the potential of silt and clay fractions to
stabilize soil organic matter (SOM) from microbial mineralization
(Baldock and Skjemstad, 2000)
The C pool of a soil fraction displaying linear accumulation is
interpreted as not being influenced by C saturation, while the
opposite is true for a C pool exhibiting a plateau or asymptotic
influence (Diekow et al., 2005; Stewart et al., 2008).
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38. Discussion
As expected, C in 2:1 soils was higher than that in
1:1 soil, indicating that the nature of clay type plays
an important role in SOC accumulation (Feller and
Beare, 1997)
Vertisols dominated by smectitic mineral clay have
been reported to contain greater amounts of organic
matter than soils dominated by kaolinite, feldspars
and quartz (Ngole and Ekosse, 2009)
However, subtropical Ferrasols, having a mixed clay
type, showed higher SOM than temperate soils. This
result reflects influence of management practices
involving hand tilling and occasional plowing by
animal traction with decades of maize and beans
cultivation, leading to greater amount of C in
39.
40. Conclusions
The results of the present study confirm early
findings that individual particles such as clay
fraction can achieve a maximum C accumulation
or saturation.
When this maximum is reached, further inputs of
new organic matter cannot be physically protected
in the clay fraction and organic compounds
progressively accumulate in the silt fraction and
as uncomplexed free materials, namely particulate
(POM) or free organic matter in the sand-sized
fraction (Matus et al., 2008).
41. Contd……
On average, the clay fraction of temperate soils was
saturated at 1.9±0.1 g C kg-1 in 2:1 soils and 1.0±0.1g C
kg-1 in 1:1 soils, while in subtropical soils they were
saturated at 13.6±0.7 g C kg-1.
C accumulation in the silt fraction was explained in terms
of partial reactivity and habitat pore space for microbial
growth. Thus, the silt fraction allows linear C
accumulation in similar proportions across SOC levels.
the low C in < 2 μm fraction in temperate soils having 2:1
clay compared to subtropical Ferrasols (mixed clay) can
be explained by the stabilization of C within the micro-
aggregates (Caner et al., 2010).
42. I can fight Climate change -
SOC
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Editor's Notes
Soil organic carbon sequestration is the process by which carbon is fixed from the atmosphere via plants or organic residues and stored in the soil.
It infers that soil carbon stabilization curves are not infinitely increasing, and that when a C saturation level is reached, SOC sequestration comes to an end, soils stop being a net carbon sink and may become a net carbon source. As such, SOC sequestration has spatial and temporal limitations and is a reversible process (Paustian et al., 2016).
C stabilization efficiency decreased in soils with high SOC content.
