45
2.2 Dynamics of SOC
The magnitude of organic C in the soil system is a dynamic balance between the rate
of C gains (i.e., inputs of dead plant, animal, and microbial residues) and the rate of
C losses from decomposition, mineralization, leaching, and erosion processes (FAO
2001, 2004; Walcott et al. 2009). Under aerobic conditions, most of the C entering
the soil is released to the atmosphere through heterotrophic respiration. According
to FAO (2001, 2004) reports, only about 1% of C entering the soil (55 Pg C year
−1
)
accumulates into more stable fractions (i.e., 0.4 Pg C year
−1
) with long mean residence times (MRTs). The turnover rate of the different SOC components also
depends on their composition and complex interactions between the biological,
chemical, and physical processes in the soil (Post and Kwon 2000; Walcott et al.
2009). Some components, such as lignin and charcoal, are difficult to digest biochemically, while others, such as carbohydrates and proteins, break down rapidly.
Therefore, there is a continuum of soil organic compounds in terms of their decomposability and turnover times (MRTs). Walcott et al. (2009) presented a simple
approach that separates SOC pools into fractions depending on how fast the components are broken down and replaced (i.e., recalcitrance). These SOC pools are:
(a) Fast, labile, or active SOC pool, which has a short turnover time (i.e., days to
years), with fast decomposition
(b) Slow, stable, or humus SOC pool, which has a longer turnover time (i.e., years
to decades and centuries), with slower decomposition
(c) Passive, refractory, or recalcitrant SOC pool, which has a much longer turnover
time (e.g., centuries to millennia)
The amount of SOC in the different pools is controlled by complex interaction of
climatic, pedologic, agronomic, and biotic factors. Soil type, depth, and texture are
crucial in terms of soil properties. It has been established that the content of SOC is
generally greater at the surface and diminishes exponentially with depth (Walcott
et al. 2009). This is because organic materials that input to forest and agricultural
soils (i.e., litter fall, exudates, leachates, dead roots, crop residues, manures, and
fertilizers) mostly reside in the upper layers, with only small amounts penetrating
much deeper. However, in some Vertisols where the shrink-swell nature of the soils
encourages downward movement of organic matter, high SOC levels can be found
at depths greater than 50 cm. Moreover, soil texture plays a role in the stabilization
of organic compounds; hence, textural variations can have significant effects on
SOC content in the different pools. In fine-textured soils, about 30% of SOC tends
to be found in the passive pool (in the form of charcoal and physically protected C),
while in the coarse-textured soils it is only about 4%. It has also been reported that,
in some instances, decomposition of humus in the slow C pool is slower in clays and
silts than in coarse sandy soils and that the presence of Fe
2+
, Al
3+
, and Ca
2+
in clays
can help to protect soil humus from further decomposition (Walcott et al. 2009).
Climate is another important determinant of SOC content in the different pools.
Biological processes, such as the amount of organic matter inputs, as well as the
Land Use Changes and Sustainable Land Management Practices for Soil Carbon…
2.2 Dynamics of SOC
The magnitude of organic C in the soil system is a dynamic balance between the rate
of C gains (i.e., inputs of dead plant, animal, and microbial residues) and the rate of
C losses from decomposition, mineralization, leaching, and erosion processes (FAO
2001, 2004; Walcott et al. 2009). Under aerobic conditions, most of the C entering
the soil is released to the atmosphere through heterotrophic respiration. According
to FAO (2001, 2004) reports, only about 1% of C entering the soil (55 Pg C year
−1
)
accumulates into more stable fractions (i.e., 0.4 Pg C year
−1
) with long mean residence times (MRTs). The turnover rate of the different SOC components also
depends on their composition and complex interactions between the biological,
chemical, and physical processes in the soil (Post and Kwon 2000; Walcott et al.
2009). Some components, such as lignin and charcoal, are difficult to digest biochemically, while others, such as carbohydrates and proteins, break down rapidly.
Therefore, there is a continuum of soil organic compounds in terms of their decomposability and turnover times (MRTs). Walcott et al. (2009) presented a simple
approach that separates SOC pools into fractions depending on how fast the components are broken down and replaced (i.e., recalcitrance). These SOC pools are:
(a) Fast, labile, or active SOC pool, which has a short turnover time (i.e., days to
years), with fast decomposition
(b) Slow, stable, or humus SOC pool, which has a longer turnover time (i.e., years
to decades and centuries), with slower decomposition
(c) Passive, refractory, or recalcitrant SOC pool, which has a much longer turnover
time (e.g., centuries to millennia)
The amount of SOC in the different pools is controlled by complex interaction of
climatic, pedologic, agronomic, and biotic factors. Soil type, depth, and texture are
crucial in terms of soil properties. It has been established that the content of SOC is
generally greater at the surface and diminishes exponentially with depth (Walcott
et al. 2009). This is because organic materials that input to forest and agricultural
soils (i.e., litter fall, exudates, leachates, dead roots, crop residues, manures, and
fertilizers) mostly reside in the upper layers, with only small amounts penetrating
much deeper. However, in some Vertisols where the shrink-swell nature of the soils
encourages downward movement of organic matter, high SOC levels can be found
at depths greater than 50 cm. Moreover, soil texture plays a role in the stabilization
of organic compounds; hence, textural variations can have significant effects on
SOC content in the different pools. In fine-textured soils, about 30% of SOC tends
to be found in the passive pool (in the form of charcoal and physically protected C),
while in the coarse-textured soils it is only about 4%. It has also been reported that,
in some instances, decomposition of humus in the slow C pool is slower in clays and
silts than in coarse sandy soils and that the presence of Fe
2+
, Al
3+
, and Ca
2+
in clays
can help to protect soil humus from further decomposition (Walcott et al. 2009).
Climate is another important determinant of SOC content in the different pools.
Biological processes, such as the amount of organic matter inputs, as well as the
Land Use Changes and Sustainable Land Management Practices for Soil Carbon…