Climate change is becoming most serious concern to the whole world. Fossil fuel burning and land use changes have emitted and are continuing to emit increasing quantities of green house gases into the Earth’s atmosphere.
These green house gases include carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) etc. Due to anthropogenic activities a rise in these gases has caused a rise in temperature of Earth’s atmosphere which led to the green house effect, resulting in climate change.
Over the last century, atmospheric concentration of CO2 increased from a pre-industrial value of 278 parts per million (ppm) to 379 ppm in 2005, and the average global temperature increased by 0.74C. According to the scientists, this is the largest and fastest warming trend that they have been able to discern in the history of the earth.
The major impacts and threats of global warming are wide spread. Increasing ocean temperatures causing thermal expansion of the oceans and in a combination with melt water from land based ice is causing sea level rise. Sea levels increased during the 20th century by 0.17 meter.
As a result of global warming the type, frequency and intensity of extream events, such as tropical cyclones, floods, droughts and heavy precipitation events are expected to rise even with relatively small average temperature increases.
Changes in rain fall pattern are likely to lead to severe water shortages or flooding. Melting of glaciers can cause flooding and soil erosion. Increasing sea levels mean greater risk of storm surge, inundation and wave damage to coastline, particularly in small island states and countries with low line deltas.
Rising temperatures will cause shifts in crop growing seasons which affects food security and changes in the distribution of disease vectors putting more people at risk from diseases such as malaria and dengue fever.
Temperature increases will potentially increase rates of extinction for many habitats and species such as coral reefs, bareal forests and Mediterranean and mountain habitats. Thus climate change will have wide ranging effects on environment and on socio-economic and related factors, including water resources, agriculture and food security, human health, terrestrial ecosystems and biodiversity and costal zones. Therefore it is an urgent need to combat with climate change.
Mitigation of Climate Change
Many options and technologies are being generated to tackle the situation. Out of which soil carbon sequestration has been emerging as a plausible strategy. Soil is the largest carbon reservoir of the terrestrial carbon cycle and holds the greatest mitigation potential, provided it is linked to the intensification of carbon capture through biomass production.
The strategy is to increase soil carbon density, improved depth distribution of SOC by encapsulating it with in stable micro-aggregates so that carbon is protected from microbial processes or as recalcitrant carbon with long turn over time. In this context, managing agro-ecosystems is an important strategy for carbon sequestration.
The sink capacity of soil organic matter for atmospheric CO2 can be greatly enhanced when degraded soils and ecosystems are restored; marginal agricultural soils are converted to a restorative land use or replanted to perennial vegetation. Soil and crop management practices that enhance soil carbon pool include conservation tillage, cover crops, nutrients and irrigation management, restoring degraded soil and pasture management.
Soil carbon sequestration is the process of transforming carbon dioxide from the atmosphere into the soil through crop residues and other organic solids and in a form which is not immediately reemitted. The basic concept of carbon sequestration in terrestrial ecosystem is to remove carbon from the atmosphere by various means and storing it in the soil.
Carbon sequestration on land occurs in standing biomass (eg Lumber), living biomass in soil (perennial roots and microorganisms), recalcitrant organic matter in surface soil (eg. Humus) and inorganic carbon in subsoil (eg. Carbonates). Sequestration of SOC from plant biomass is a key sequestration pathway in agriculture which offers an offset strategy for green house gas emissions.
It is also important at the farm level to build soil fertility, protect soil from compaction and nurture soil biodiversity. The off farm benefits of soil carbon sequestration include the protection of streams, lakes and rivers from sediment, nutrients and run off from agricultural fields as well as enhanced wild life habitat.
The SOC content in most cultivated soils is generally less than 5 g/kg as compared to 15 to 20 g/kg in uncultivated soils. Low SOC content in is attributed to ploughing, removal of crop residue and other bio-solids and mining of soil fertility.
In general soil carbon content increases with increase in clay content and rain fall and decreases with increase in mean annual temperature. Due to low clay contents, the SOC content is especially low in alluvial soils of the Indo-Gangatic Plains, coarse textured soils of southern India.
The prevalent low levels of SOC content are attributed to soil-mining practices of excessive tillage, imbalance in fertilizer use, little or no crop residues returned to the soil, and sever soil degradation. The total soil carbon pool also comprises the soil inorganic carbon which is generally high in calcareous soils of arid and semi-arid regions.
Calcareous soils are widely distributed covering 54% of the geographical area of India but especially occur in Rajasthan, Gujrat, Punjab, Haryana, Uttar Pradesh, Maharashtra, Karnatka, Tamil Nadue, Andhra Pradesh and parts of Madhya Pradesh and Bihar. Soil degradation, decline in soil quality with a reduction in biomass productivity and environment moderating capacity has severe impacts on the SOC pools.
In other words the low SOC pools in soils are partly due to the severe problem of soil degradation. The principal cause of decline in SOC pool in degraded soils is a reduction in biomass productivity and the low amount of crop residue and roots returned to the soil.
Therefore, there is a considerable potential of soil carbon sequestration in India due to its large land area and diverse ecoregions. Adoption of conservation effective measures that reduce erosion may lead to reduction of carbon emission from ecosystems.
Potential for carbon sequestration in India
Using ecoregions as the basis of extrapolation and the rates of soil carbon sequestration the total potential ranges from 12.7 to 16.5 Tg C/year. Included in this potential is also that of the restoration of degraded soils and ecosystems estimated at 7.2 to 9.8 Tg C/year.
Therefore potential of agricultural intensification for soil carbon sequestration is 5.5 to 6.7 Tg C/year. In addition, there is also a potential of soil inorganic carbon sequestration estimated at 21.8 to 25.6 Tg C/year. With reduction in erosion induces emission of soil carbon sequestration in India is 39.3 to 49.3 Tg C/year.
Strategies to enhance Carbon sequestration
A conservation agricultural system promotes soil carbon sequestration by tipping the balance in favour of carbon inputs relative to carbon outputs. Carbon sequestration can be achieved by maximizing carbon inputs and minimizing carbon output strategies as follows.
- Strategies for maximizing carbon input
- Selection of plant species, cultivar and variety
- Growth habit perennial or annual
- Rotation sequence
- Biomass energy crops
- Tillage: type and frequency
- Fertilization: rate, timing and placement
- Organic amendment
- Strategies for minimizing carbon loss from soil
- Reducing soil disturbance by less intensive tillage
- Controlling soil erosion
- Utilizing available soil water
- Promote optimum plant growth
- Reduces soil microbial activity
- Maintaining surface residue cover
- Increase plant water use and production
- More fungal dominance in soil
The soil carbon density can be enhanced by planting deep rooted species with high below ground biomass production. Use of crop residues and manure to enhance soil biodiversity especially earthworm activity is an important strategy for increasing `SOC content and soil quality.
Manuring and application of bio solids as crop residue or compost also enhance soil aggregation. Restoration of degraded soils and ecosystems erosion control and conversion of agriculturally marginal soils to a restorative land use are important options of soil carbon sequestration.
Restoring eroded soils can enhance biomass production and improve soil carbon content. Similarly restoration of salt affected soils can lead to a drastic increase in soil carbon pools. The overall strategy is to increase soil carbon density, distribution of soil carbon in the subsoil, aggregation and formation of secondary carbonates. Soil organic carbon is necessary to the formation of aggregates.
The degree of aggregation and the stability of aggregates is directly proportional to SOC content. Because of high aggregation, soils with high SOC content have high available water holding capacity, low susceptibility to soil erosion and have low losses of plant nutrients into ground water.
Soil organic matter is so valuable, it is referred as “Black Gold” enhanced soil carbon management i.e carbon sequestration is a win-win strategy. Agriculture wins with improved food and fibre production system and sustainability.
Society wins because of the enhanced environmental quality. Therefore the win-win scenario will increase productivity, improve soil quality and mitigate the green house effect. Therefore agricultural policies are needed to encourage farmers to improve soil quality by storing carbon that will lead to enhanced air, water quality and increased productivity as well as mitigating the green house effect. Soil carbon is most valuable resources and may serve as a “Second Crop” if global carbon trading system becomes a reality.
Corresponding author: [email protected]
S.K Singhal1, Mandira Barman2, Sarvendra Kumar2, Chobhe Kapil Atmaram2 and V.K. Sharma*1, Division of Soil Science & Agricultural Chemistry, ICAR- Indian Agricultural Research Institute, New Delhi-12
*1 Senior Scientist