**4. Carbon sequestration**

Carbon sequestration is the process of transferring carbon dioxide (CO2 ) from the atmosphere into stable terrestrial carbon (C) pools.

The process can be driven naturally or anthropogenically. The anthropogenically driven sequestration ensures that there is no net gain in the atmospheric C pool because the CO2 sequestered comes from the atmosphere. There are basically two types of sequestration: abiotic and biotic. The abiotic techniques involve injection of CO2 into deep oceans, geological strata, old coal mines and oil wells. The biotic component on the other hand, involves managing higher plants and micro-organisms to remove more CO2 from the atmosphere and fixing this C instable soil pools. Biotic sequestration is further subdivided into oceanic and terrestrial sequestration. Oceanic sequestration involves C capture by photosynthetic activities of organisms such as phytoplankton, which converts the C into particulate organic material and deposits such on the ocean floor. This type of sequestration is reported to fix about 45 Pg C/year [18].

Terrestrial sequestration involves the transfer of CO2 from the atmosphere into the biotic and pedologic C pools. This is accomplished by the transfer or sequestration of CO2 through photosynthesis and storage in live and dead organic matter. The major terrestrial C sinks include: forests, soils and wetlands.

#### **4.1. Carbon sequestration in soil ecosystem**

Soil carbon sequestration is defined by Olson et al. [19] as:

*the process of transferring carbon dioxide from the atmosphere into the soil of a land unit through plants, plant residues, and other organic solids, which are stored or retained in the unit as part of the soil organic matter (humus)* [19]*.*

According to the Soil Science Society of America, it is the storage of carbon in a stable solid form in the soil as a result of direct and indirect fixation of atmospheric CO<sup>2</sup> [20]. The direct fixation involves natural conversion of CO<sup>2</sup> into soil inorganic compounds such as calcium and magnesium carbonates while the indirect sequestration takes place when plants produce biomass through the process of photosynthesis. This biomass is eventually transferred into the soil and indirectly sequestered as soil organic carbon after decomposition. Subsequently, some of this plant biomass is indirectly sequestered as soil organic carbon (SOC) during decomposition processes. The amount of carbon sequestered in the soil reflects the long term balance between carbon uptake and release mechanisms. Many agronomic, forestry and conservation practices, including best management practices lead to a beneficial net gain in carbon fixation in soil. The carbon sequestered under direct fixation is also referred to as soil inorganic carbon (SIC) while C fixed indirectly is called soil organic carbon (SOC) [5].

Carbon can also be sequestered in soil through the accumulation of humus onto the surface layers (usually 0.5–1 m depth) of soil or anthropogenically through land use change or adoption of right management practices (RMPs) in agricultural, pastoral or forest ecosystems [5]. Soils in managed ecosystems tend to have a lower SOC pool than those in natural ecosystems due to oxidation or mineralization, leaching and erosion [5]. Globally, soils are reported to the have the capacity of sequestering 0.4–0.8 Pg [21].

The sequestration of carbon in soils depends on a number of factors depending on whether it is abiotic or biotic. Abiotic soil C sequestration depends on clay content, mineralogy, structural stability, landscape position, soil moisture and temperature regimes [22]. Biotic soil C sequestration on the other hand depends on management practice, climate and activities of soil organisms [23, 24].

#### **4.2. Carbon stock in forest soils**

carbohydrates. These organic compounds are then used in making the plants structural components (also known as biomass) and generating the energy needed for metabolic activities. The maximum amount of carbon that can be produced, otherwise known as gross primary productivity (GPP), depends on the plant's ability to produce these compounds through photosynthesis. The biomass produced through photosynthesis is utilized by the plants themselves in generating the energy needed for metabolic activities in a process called respiration. The difference between the GPP and respiration is called the net primary productivity (NPP).

NPP is determined by the potion of solar radiation captured by the plants and used for the photosynthesis (also known as photosynthetically active radiation (PAR), the leaf area index, the light use efficiency (the ratio of primary productivity to absorbed PAR) of the vegetation and autotrophic respiration [12]. The higher the NPP the more carbon is transferred to stable

The process can be driven naturally or anthropogenically. The anthropogenically driven sequestration ensures that there is no net gain in the atmospheric C pool because the CO2 sequestered comes from the atmosphere. There are basically two types of sequestration:

cal strata, old coal mines and oil wells. The biotic component on the other hand, involves

fixing this C instable soil pools. Biotic sequestration is further subdivided into oceanic and terrestrial sequestration. Oceanic sequestration involves C capture by photosynthetic activities of organisms such as phytoplankton, which converts the C into particulate organic material and deposits such on the ocean floor. This type of sequestration is reported to fix about 45 Pg

tosynthesis and storage in live and dead organic matter. The major terrestrial C sinks include:

*the process of transferring carbon dioxide from the atmosphere into the soil of a land unit through plants, plant residues, and other organic solids, which are stored or retained in the unit as part of the* 

) from the atmosphere

into deep oceans, geologi-

from the atmosphere and

through pho-

from the atmosphere into the biotic and

NPP is generally believed to be 45% of the GPP [16].

Carbon sequestration is the process of transferring carbon dioxide (CO2

abiotic and biotic. The abiotic techniques involve injection of CO2

managing higher plants and micro-organisms to remove more CO2

pedologic C pools. This is accomplished by the transfer or sequestration of CO2

Terrestrial sequestration involves the transfer of CO2

Soil carbon sequestration is defined by Olson et al. [19] as:

**4.1. Carbon sequestration in soil ecosystem**

*soil organic matter (humus)* [19]*.*

pools in the soils [17].

C/year [18].

forests, soils and wetlands.

**4. Carbon sequestration**

6 Carbon Capture, Utilization and Sequestration

into stable terrestrial carbon (C) pools.

Carbon is stored in forest ecosystems mainly in biomass and soil and to a lesser extent in coarse woody debris [25]. The carbon stock in forest soils play a large role in global carbon cycle due to the large expanse of forest ecosystems estimated at 4.1 billion hectares globally [26]. It has been estimated that, globally, the forest ecosystem contains about 1240 Pg C [27]. Out of this amount, the plants (vegetation) contain about 536 Pg C while the soil is believed to contain up to 704 Pg C. This is a very significant amount.

The forest ecosystems contain more than 70% of global soil organic carbon (SOC) and forest soils are believed to hold about 43% of the carbon in the forest ecosystem to 1 m depth [2].

However, unfortunately this high carbon content inherent in natural forest soils is easily depleted by decrease in the amount of biomass (above and below ground) returned to the soil, changes in soil moisture and temperature regimes and degree of decomposability of soil organic matter (due to difference in C:N ratio and lignin content) [14]. Anthropogenic activities such as conversion of forests to agricultural land also deplete the soil organic carbon (SOC) stock by 20–25% [28]. Deforestation is reported to emit about 1.6–1.7 Pg C/year (about 20% of anthropogenic emission [29].

### **4.3. Carbon stock in agricultural soils**

According to the IPCC agricultural soils have the potential of sequestering up to 1.2 billion tonnes of carbon per year. However, it has been estimated that already about 50% of agricultural soils have been degraded globally, a situation that creates an opportunity for sequestering atmospheric carbon in the soil for a long period of time [8].

**5.3. Ancillary benefits**

**5.4. Carbon inventories**

sequestration also generates these benefits.

**6. Challenges of carbon sequestration in soils**

residence time in the ecosystem. These pools include:

reality. Some of these challenges include:

Apart from climate change mitigation and improving forest land productivity, carbon sequestration in soils (of different ecosystems) also have several ancillary benefits. Some of these include: improvement in water holding capacity and infiltration, provision of substrate for soil organisms, serving as a source and reservoir of important plant nutrients, improvement of soil structural stability among others [13]. According to [34] the environmental benefits associated with soil carbon sequestration is 40–70% higher than the productivity benefits. Based on these reasons, therefore, any policy, strategy or practice that increase soil carbon

Carbon Sequestration in Soils: The Opportunities and Challenges

http://dx.doi.org/10.5772/intechopen.79347

9

The obligation on countries, that are parties to the UNFCC, to deposit their independent nationally determined contributions (INDCs) requires a comprehensive estimation and valuation all carbon sink and sources in the terrestrial and other sectors. These estimation and valuation of carbon in the LULUCF sector will be incomplete if the contribution of soil carbon is excluded due to its large percentage (36–46%). Carbon inventory is a process of estimating changes in the stocks (emission and removals) of carbon in soil and biomass periodically for various reasons [35].

Although there are a lot of opportunities in leveraging carbon stock and sequestration potential in the soil of different ecosystems, there are numerous challenges making this difficult in

**a.** *Measurement and verification*: the stock of carbon in soils is difficult, time-consuming and expensive to measure. Changes within the range of 10% are very difficult to detect due to sampling errors, small-scale variability and uncertainties with measures and analysis [36]. The annual incremental stock of carbon in soil is very small usually within 0.25–1.0 t/ha [37]. It is even more difficult to account for little gains or losses in soil carbon at various scales due to methodological difficulties such as monitoring, verification, sampling and depth [38]. Even if these small changes (gains or losses) are detected, it is not easy to link such changes to management or land use practice in a given context. The capacity of the soil to sequester and retain carbon is also finite as it reaches a steady state after sometime.

**b.** *Carbon pools*: sequestered carbon exists in the soil in different pools with varying degree of

long residence time ranging from decades to thousands of years.

**i.** Passive, recalcitrant or refractory pool: organic carbon held in this pool has a very

**ii.** Active, labile or fast pool: carbon held in this pool stays in the soil for much shorter period due to fast decomposition. The residence time normally ranges from 1 day to a year.

The potential of sequestering carbon in agricultural land is huge as over one third of the world's arable land is in agriculture [30]. Agricultural land could sequester at least 10% of the current annual emissions of 8–10 Gt/year [31].
