Since we entered into a new century, one of the biggest problem that we are facing is of global warming. Global warming, as we all know, is the increase in average temperature of the earth’s surface. It is most prominently caused by the emissions of harmful greenhouse gases(GHGs). These emissions have been released by fossil fuels combustion, powerplants, deforestation and agricultural burning practices. These emissions have led the global temperature so high that it becomes unbearable today. The emissions increases the temperature of earth by increasing the concentration of CO2 in the atmosphere. Therefore, there is an urgent need to reduce the concentration of CO2 in the atmosphere. One way of doing this is to capture the carbon from atmosphere and store for long term in the soil. This method provides an additional benefit that it increases agricultural productivity as well. This method is more commonly called as Carbon Sequestration.
So, what’s Carbon Sequestration exactly? Let’s understand that.
Carbon Sequestration is basically a process of removal of CO2 from atmosphere and the long term storage of carbon. It is one of the method used in order to defer global warming. Carbon predominantly exist as plant biomass, soil organic matter and as the gas carbon dioxide(CO2)in air and water. Carbon is stored in oceans, soils, vegetation (especially forests) and geological formations. It is estimated that soils contain carbon three times more than the amount stored in living plants and animals. 75% of the carbon found on earth exist in the soil and hence soils play a very important role in balancing the global carbon cycle. It is one way to slow down the process of accumulation of greenhouse gases and thereby play an eminent role in mitigating climatic change.
Within the last few decades the amount of carbon present in the atmosphere has significantly increased. With the carbon present in excessive amount in atmosphere, it has led to the problem of global warming. Increase in carbon level in atmosphere has a direct correlation with increase in temperature. Therefore, it has now become an urgency to follow some ways to reduce the carbon level in atmosphere. One way of doing is the global storage of carbon in soils. This is one of the best method to reduce the amount of carbon in the atmosphere. Moreover, it results in the enhancement of agricultural production. It improves soil fertility, soil structure and soil water storage capacity.
Sequestration of industrially produced CO2 is done using subsurface saline aquifers, reservoirs, ocean water, aging oil fields or any other carbon sinks.
Removal of CO2 from the atmosphere is done through biological, chemical and physical processes. Biological process involves the restoration of wetland, creation of new bogs, replanting of trees on croplands and pasture lands.
Sequestration of carbon in soil is known as soil carbon sequestration. It is a long term storage of carbon in soil and takes approximately 25 years to bring about a significant increase in the carbon level of the soil.
HOW IS CARBON SEQUESTERED IN SOIL?
The process of carbon sequestration starts with the process of photosynthesis. Plants absorb carbon from air and return some of it to atmosphere through respiration. The carbon content that is present in the plants is either eaten up by the animals or added to the soil as litter when plants finally decompose. One of the most common way the carbon is absorbed in the soil is as soil organic matter(SOM). SOM is an organic matter of the soil consisting predominantly of plant and animal residues at different stages of decomposition. Interstingly, Carbon in soils can be stored for millennia. The factors such as climatic conditions, natural vegetations, soil texture and drainage affect the quantity and the extent of time the carbon is stored.
SOIL CARBON SEQUESTRATION
The process of storage of carbon in soil is known as soil carbon sequestration. In this process, CO2 is removed from the atmosphere and stored in the carbon pool. Carbon is stored in the plants in the form of organic matter through the process of photosynthesis. Soil carbon sequestration in arid and semi-arid regions occur from conversion of carbon dioxide from air found in soil into inorganic forms. Carbon sequestration capacity of the soil depends on number of processes such as rainfall infiltration, soil erosion, deposition of sediment and soil temperature.
The large scale conversion of natural ecosystems to agricultural use has resulted in reductions of SOC levels . Depletion of soil organic carbon lead to an opportunity to store carbon in the soil through the adoption of various agricultural practices. There are certain factors that affect the amount and length of time carbon is stored in soil. Climatic conditions, natural vegetation, soil texture and drainage are all such factors. Soil carbon sequestration has now been considered as a strategy for mitigating climatic change.
The long term conversion of forestland into cropland has caused a reduction in the soil carbon content. However, there is a major potential for increasing soil carbon through the restoration of degraded soils, soil conservation practices and by better land and soil management practices.
Agriculture soils are the largest reservoirs of carbon and therefore hold potential for expanded carbon sequestration, which in turn, will mitigate the increasing atmospheric concentration of carbon dioxide. It is estimated that soils have the capacity to sequester 20 Pg C in 25 years. Degraded soils in drylands have less than 0.5% of soil organic carbon whereas by good sustainable land management the SOC is increased to 2-3%.
BENEFITS OF SOIL CARBON SEQUESTRATION
• Help mitigate climate change
• Increases soil biodiversity
• Increases soil fertility
• Increase soil water storage capacity
• Enhances food production
• Decreased nutrient loss
• Reduction in soil erosion
• Increased water conservation
• Greater crop production
• Improvement in soil structure
SOIL CARBON SEQUESTRATION IN INDIA
India posses total area of land of 330 million hectares (Mha). Out of which 162 Mha consists of cultivated land, 70 Mha of forest and woodland, 10 Mha of pasture land, 9 Mha of permanent crops and rest for other land uses.
The soil organic pool is estimated to be at 22 Pg to 30cm depth and 65 Pg to 150cm depth where as the soil inorganic pool is estimated to be at 196 Pg to 1m depth. The SOC concentration is more in uncultivated land than in cultivated land. SOC concentration in cultivated land is less than 5g/kg where as in uncultivated land it is 15-20g/kg.
The Potential of carbon sequestration in India is estimated at 8-10 Tg C/y for restoration of degraded soils, 6 to 8 Tg C/y for erosion control,5 to 7 Tg C/y for adoption of RMPs and 20 to 24 Tg C/y for secondary carbonates. Therefore, the total soil carbon sequestration potential is 39 to 49 Tg C/y.
1. SOILS OF INDIA AND THEIR SOIL CARBON POOL
India is known to have various types of soils having varying characterstics. They are also characterized by the wide range of SOC concentration. The SOC increases with increase in clay content, rainfall and decrease with rise in mean annual temperature.
The data in the above table shows that the SOC concentration of most of the soils is less than 10g/kg. The low SOC concentration is related to the low clay content in the soil. Therefore, low SOC concentration is seen in the alluvial soils of Indo-Gangetic Plains, coarse textured soils of south india and in soils of arid zones of northwestern India.
However, the decreased levels of SOC concentrations are due to increased practice of tillage, imbalance in usage of fertilizers and increased soil degradation.
The SOC pool in Indian soil accounts for 2.4% of the world pool for 1m depth and 2.7% to 2m depth.
Apart from occurring in organic form, carbon also occur in inorganic form in soil, commonly called as SIC. It is high in calcareous soils. They are found in arid and semi-arid regions. The total soil inorganic pool in Indian soil is estimated to be at 195 Pg to 1m depth where as in world soils, it is estimated to be at 722.5 Pg to 1m depth.
2. SOIL DEGRADATION AND SOIL ORGANIC CARBON POOL
The major cause of decline in soil organic carbon pool in degraded soils is that there is a significant amount of reduction in biomass productivity and less amount of crop residues are returned to the soil. An example of this would be the salt affected soils of Haryana, West Bengal and Andhra Pradesh. At the surface of 0 to 10 cm layer, the SOC concentration was lower than 5g/kg.
Soil erosion is also one factor that depletes SOC pool severly. The soil organic carbon is commonly reduced due to surface runoff and high speed wind because it is present in the vicinity of soil surface and it has low density. The loss of SOC by erosion and runoff are high on gentle slopes.
The above table estimates about the degradation of soil by different processes in Indian soils.
3. SOIL QUALITY AND SOIL ORGANIC CARBON CONCENTRATION
Soil organic matter is an essential component of soil for the formation of both micro and macro-aggregates. Degree of aggregation and its stability is directly dependent on SOC concentration. Soils having high aggregation have high SOC concentration due to which the soil may have high water holding capacity, low soil erosion and low loss of plant nutrients into the groundwater. Use of efficiency of fertilizers, irrigation etc are high in the soils with the high SOC concentration. Infact, the soils having high soil organic carbon concentration have also high biomass productivity.
4. TECHNOLOGICAL OPTIONS FOR SOIL CARBON SEQUESTRATION
The main motive is to increase soil organic carbon density and the distribution of SOC in subsoil. It can be increased by increasing carbon input into the soil and reducing losses by soil erosion. The distribution of SOC in sub soil surface can be attained by planting deep rooted species having high below ground biomass production.
The strategy to increase carbon content in soil can be achieved through a wide range of land use and soil management practices.
Infact, the eroded soils can be restored by enhancing biomass production. Similarly, restoring the salt affected soils can increase SOC pool. When sodic soil is planted to perennials, a significant increase in SOC pool is observed. The SOC increased from 10 Mg/ha to 30-45 Mg/ha over an 8 year period of planting tree species.
The above table shows the potential of SOC sequestration for restoring soils prone to different degradation process. It is clearly evident from the table that the potential of SOC sequestration is 2.6-3.9 Tg C/y for restoring the soils susceptible to water erosion,0.4-0.6 Tg C/y for wind erosion and so on. The total potential for restoring degraded soils in India is 7.2-9.8 Tg C/y.
Potential of Soil Carbon Sequestration in India
Soil organic matter is basically a reservoir of stored carbon that cycles between the earth and the atmosphere. It is mostly composed of humus which is stabilized by clay particles, while the rest being less stable composing of decaying plant and animal matter. It has been estimated that soil holds twice as much carbon as is in atmospheric carbon dioxide (CO2), so there is potential for a lot of carbon to lose from soil. On the other side, soil with the suitable properties can remove CO2 from the atmosphere and sequester it.
The above table describes the potential to store carbon in soil through various processes. By the restoration of degraded soils, approximately 10 Tg of carbon can be stored in soil per year. Agriculture intensification can help in storing carbon having potential of 5.5-6.7 Tg C/y. Moreover, soil can also store carbon in inorganic form as well such as CaCO3.
The Total potential of soil carbon sequestration in India is 39.3 to 49.3 Tg C/y (44.3±7.1) .
Storing organic matter is very important in soil. It plays an eminent role in modulating the Earth’s climate and improves soil physical and chemical properties for supporting plant life, including the crops for agriculture. Therefore, understanding the best conditions and practices for stabilizing the carbon in soil is important. For example, C. Vogel and coworkers found that it is not only the amount of clay in soil but the type of clay minerals (ones having rough surfaces rather than smooth) that influences sequestration potential of soil organic carbon (SOC).
In general, if organic matter in the ground is kept undisturbed, it will stick around. Physical disruption, such as tillage (overturning of soil) and erosion, promotes the oxidation of organic matter through microbial respiration, which sends CO2 to the atmosphere. Waterlogged soils, such as wetland and peatlands, prevents the process of oxidation and may therefore contain up to 20 % organic matter, making the restoration of wetlands as one of the effective means of carbon sequestration. Similarly, reforestation and restoration of grassland ecosystems are effective for sequestering carbon in Indian soils.
HOW MUCH CARBON CAN SOIL STORE?
Organic content of the soil is the organic matter found in the living organisms. Increasing the organic content of soil is one method to reduce the concentration of carbon dioxide in atmosphere, thereby mitigating the problem of global warming.
When OC of soil is increased, it benefits to many beneficial chemical, physical and biological processes of soil ecosystem.
The amount of carbon that the soil can store is dependent on its soil type. More the percentage of clay in soil, more is the amount of carbon the soil can hold.
INPUTS AND LOSSES OF ORGANIC CARBON
The amount of carbon that is actually stored in soil is directly proportional to the difference of inputs and losses of carbon in soil.
The main inputs of OC in soil are crop residues, roots of the plants and animal manure.
Losses of OC in soil is primarily due to decomposition by microorganism, erosion of the soil surface and offtake of plants and animal productions. In the process of decomposition microorganisms convert soil organic matter into carbon dioxide(CO2). Loss of OC by soil erosion also have an impact on SOC levels.
Figure 2: The influence of soil type, climate and management factors on the storage of organic carbon (OC).
The capacity of soil to store carbon depends on soil type. Primarily, it depends on the amount of clay material in the soil. Basically, the amount of organic content that is actually stored in soil increases with increase in clay content. The clay particles present in the soil forms a protecting layer on organic matter so that they remain protected from decomposition. Particles of organic matter gets adsorbed on clay particles and then clay particles forms a coating on them as well. This prevents microorganisms to come in contact with organic matter.
It determines the attainable storage of OC in soil. In dryland regions, rainfall is one factor that influences the plant productivity and hence the inputs of SOC in soil. Soils that receive high amount of rainfall has great attainable storage of OC in soil.
Management plays a very important role in determining the actual potential for storing of carbon in soil. Some of the practices that increases the OC of soil are:
• Increased plant growth increases SOC in soil. For example- providing optimum nutrition and increasing water-use efficiency.
• Plants grown for longer period of time increases OC to soil.
• Improving the soil structure increases the SOC in soil by reducing the losses of OC from soil by the process of decomposition and soil erosion. For example- Retention of stubble and maintaining ground cover.
Some of the management techniques that help in carbon sink in soil are:
• Conservation tillage- It includes practicing of mulch tillage which minimizes the manipulation of soil. It reduces soil erosion, increases water-use efficiency and increases carbon content in soil. It has been estimated that conservation tillage provides a high amount of potential to sequester a significant amount of carbon dioxide.
• Cover Cropping- Cover cropping is the method of using of crops such as clover and small grains for the purpose of protection. Moreover, it also improves the soil between the periods of successive crop production. Cover cropping also enhances soil structure and adds organic matter to soil.
• Crop Rotation- Crop rotation is a method in which crops are grown successively on the same area of land. By bringing the variance in the type of crops grown, soil organic matter of the soil can be increased.
R.LAL. (2015, April 30). researchgate. Retrieved july 6, 2018, from researchgate: https://www.researchgate.net/publication/226328545_Soil_Carbon_Sequestration_in_India
www.soilquality.org.au. (2018, july 1). Retrieved july 7, 2018, from soil quality: http://www.soilquality.org.au/factsheets/how-much-carbon-can-soil-store