**3. Evidence that carbon is sequestered in the soil and terrestrial ecosystems**

the atmosphere (through mineralization of soil organic matter). It has been reported that mineralization of only 10% of the soil organic carbon pool globally can be equivalent to about

This underscores the need to preventing carbon loss (emission) from the soil resource. Globally, the soil contains a large carbon pool estimated at approximately 1500 Gt of organic carbon in the first 1 m of the soil profile [2–4]. This is much higher than the 560 Gt of carbon

carbon stock, the soil is preventing carbon dioxide build up in the atmosphere which will

There is huge opportunity of sequestering atmospheric carbon in the soil for a long period of time because already 24% of global soils and 50% of agricultural soils are degraded globally [7]. Because most of agricultural soils are already degraded, they are estimated to have the

Despite the huge carbon deposit in soil ecosystem globally, research efforts in sequestration has been primarily focused on geological and vegetation carbon capture and storage while

This chapter will trace the origin of carbon sequestration idea as a potential climate mitigation measure as well as review the conceptual basis and mechanism of carbon capture and sequestration in soils. The benefits and challenges facing carbon sequestration in soils are also discussed extensively. Finally, some proven management practices and strategies used in enhancing the soil carbon stock under forest and agricultural ecosystems are outlined. The chapter concludes by emphasizing the need for the scientific community to resolve most the

it in terrestrial ecosystems, including the soil was first proposed by Dyson in 1977 [10]. He

up in the atmosphere. This is in light of evidence that the photosynthetic turnover is 20 times

ing of fast growing trees on a massive scale on marginal land or growing and harvesting

sphere could be minimized. This could be a short gap measure to hold the atmospheric CO2

trees in a large scale plantation as a potential strategy for halting the continuous CO2

swamp-plants and converting them into humus or peat the concentration of CO2

in the atmosphere can be minimized by sequestering

concentration in the atmosphere outweighs the benefits

[10]. He therefore concluded that by plant-

emission without 'drastic

could be absorbed by

build

in the atmo-

into the atmosphere is inevitable in the light of continued dependence

[6]. By holding this huge

concentration in

(C) found in the biotic pool [5] and twice more than atmospheric CO2

potential of sequestering up to 1.2 billion tonnes of carbon per year [8].

the atmosphere until when more effective strategies are found [4].

giving less attention on the role of soil as a viable carbon sink [9].

challenges making widespread adoption of this initiative difficult.

on fossil fuels. Therefore, a strategy was needed for reducing CO2

shutdown of industrial civilization'. He proposed that the excess CO2

**2. Genesis of the carbon sequestration idea in terrestrial systems**

Carbon sequestration in soils can be a short term solution of reducing CO2

30 years of anthropogenic emissions [1].

4 Carbon Capture, Utilization and Sequestration

compound the problem of climate change.

The idea that the concentration of CO2

realized that the danger of rising CO2

larger than the annual increase in atmospheric CO2

and that increased CO2

The soil is reputed to contain the largest terrestrial carbon pool estimated at approximately 2344 Gt (1 gigaton = 1 billion tonnes) of organic carbon in the first 3 m, 1500 Gt in the first 1 m and 615 Gt stored in the top 20 cm of the soil profile [2–4]. By holding this huge carbon stock, the soil is preventing or delaying carbon dioxide build up in the atmosphere which will compound the problem of climate change. Considering the fact that only 9 Gt of C is added to the atmosphere yearly through anthropogenic activities from fossil fuels and ecosystem degradation [4], the soil can be counted on as an effective carbon sink that renders vital climate regulation services.

Conversely, the soil also emits CO2 back to the atmosphere due to SOM decomposition estimated at 150 Gt which leaves a vacuum that could be filled if the lost C can be recaptured back and stored in the soil [12].

The amount of carbon emitted annually into the atmosphere is estimated at 8.7 Gt C while only 3.8 Gt/year is found in the atmosphere at a given time [4]. This leaves an unaccounted balance of 4.9 Gt C/year that is believed to have been sequestered on terrestrial systems (oceans, forests, soils, etc.). The realization that the terrestrial systems (including soil) have the capacity to sequester this difference (4.9 Gt C/year) has generated interest in the potential of these systems to sequester and store carbon in long-lived pools thereby preventing its accumulation in the atmosphere [3, 4, 13–15]. Just like the way the soil sequesters and stores, organic carbon, thereby reducing the amount in the atmosphere, it can equally release carbon (through CO2 ) into the atmosphere and raise the concentration of carbon dioxide [12].

Over the last few decades, the soil has lost considerable quantity of carbon as a result of anthropogenic activities such as deforestation and agricultural activities. Managed ecosystems such as agriculture are believed to have already lost 30–55% of their original soil organic carbon stock since conversion [7]. The lost productivity of agricultural and degraded lands together offers an opportunity for recovering 50–60% of the original carbon content through adoption of carbon sequestration strategies [13]. This situation creates an opportunity for the replenishment of the lost carbon stock through adoption of deliberate strategies and policies of carbon sequestration. This may likely reduce the amount of CO2 in the atmosphere.

#### **3.1. Mechanisms of carbon capture and sequestration**

Soil carbon is originally derived from the CO2 assimilated by plants through photosynthesis and converted to simple sugars and eventually returned to the soil as soil organic matter. Photosynthesis is the process where plants produces organic compounds such as carbohydrate by using solar energy to convert CO2 and water into organic compounds such as 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 generally believed to be 45% of the GPP [16].

According to the Soil Science Society of America, it is the storage of carbon in a stable solid

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

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

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

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]. The direct

7

into soil inorganic compounds such as calcium

Carbon Sequestration in Soils: The Opportunities and Challenges

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

form in the soil as a result of direct and indirect fixation of atmospheric CO<sup>2</sup>

fixation involves natural conversion of CO<sup>2</sup>

have the capacity of sequestering 0.4–0.8 Pg [21].

contain up to 704 Pg C. This is a very significant amount.

soil organisms [23, 24].

**4.2. Carbon stock in forest soils**

20% of anthropogenic emission [29].

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 pools in the soils [17].
