Carbon is present in the soil in two forms-as soil organic carbon (SOC) and soil inorganic carbon (SIC). It is of great significance as it determines the ecosystem and agro-ecosystem functions and influences the soil fertility, water holding capacity etc. It has a major role to play in controlling the levels of green house gases (GHGs) with special reference to Carbon Dioxide.

Carbon sequestration is catching up globally to dump the excess carbon and to lock it up in deep geological sinks from where it can not find its way to the atmosphere. The process is known as carbon capture and storage (CCS). The process involves capturing carbon dioxide at large and stationery points and then transporting it to the sink and injecting the gas in to deep structures from where it can not be reemitted. Of late the technique of carbon sequestration is catching up. It involves transferring carbon dioxide from atmosphere in to soil through crop residues and other solid residues.

The soil scientists claim that the soils can be a major sink for carbon. Lead author of a paper on the utility of soils as carbon sinks, T. Bhattacharyya of National Bureau of Soil Survey and Landuse Planning, Nagpur in one of his papers in Current Science (August 2008) says "Among the other known sources to enhance the CCS, the role of soils as an important natural resource, in capturing and storing carbon has not been adequately explained. The soils capture and store both organic (through photosynthesis of plants and then to soils as decomposed plant materials and roots) and inorganic carbon (through the formation of pedogenic calcium carbonate)".

There is a basic ignorance about soils in India. The soil is like a precious carpet spread over the land. Soil formation is a very complex natural process and it takes 1000 years at least to form. Sand, silt and clay the main constituents of the soil are enriched by a variety of chemical compounds in a very slow and gradual process. The soil is further enriched by humus formed as a result of decaying roots, stems and leaves and dead organisms.

Nature is at work everywhere and this very complex process of soil formation goes on right under our feet through our lives and under our bodies after we are dead, without our knowledge. 

Carbon capture and storage in soil though has immense potential, yet it is a complicated process. A prior knowledge about the spatial distribution of SOC and SIC and also bulk density (BD) is essential. Bhattacharyya and his co-workers have used agro-climatic zones (ACZs), bioclimatic systems (BCS) of India and agro-eco-sub regions (AESRs) maps as base maps for identifying areas for Carbon sequestration in soils.

They evaluated in detail the total carbon stock for four different soil depths, i.e. from 0 to 0.3m; 0-0.5m; 0-1.0m and 0-15m from all the 15 ACZs of the country including the Andaman-Nicobar Islands. It is in fact a monumental database to establish and prioritise areas for CCS in the soils.
Bhattacharyya and his team mates after an exhaustive study have recommended sequestration of atmospheric carbon dioxide in the form of SIC, for example soil generated carbonate and using it to enrich the soil organic carbon of the drier regions of the country. 

Carbon sequestration through biomass has been suggested by Divy Ninad Koul and Pankaj Pawar of Uttar Banga Krishi Viswavidyalaya, Cooch Behar. In a paper published in the September, 08 issue of Current Science they claim that it is a cheap and viable option. They found that landuse has to do a lot with carbon sequestration. For example, in the terai region of west Bengal it was observed that while fallow land and agriculture land sequester 5.86% and 4.73% carbon respectively, compared to natural forest of Shorea robusta. The agroforestry systems like Tea Gardens and agri-horticulture contributed 24.24% and 9.09% carbon respectively. The agri-horticulture system provides sustenance to the farmer sequesters carbon and also helps to earn carbon credits. 

Koul and Pawar postulate that tree plus crop based system of agriforestry can be further increased using improved planting materials of perennial components. They observed that a plantation of trees of Delbergia sissoo and T. arjuna contributed 31.59% and23.92% carbon respectively, compared to natural forest of Shorea robusta. Despite their capacity to sequester more carbon the tree plantations or natural forests can not cover larger areas. Demand for the land for human use is more. Therefore, agroforestry is one viable option. 

Carbon storage in biomass or soil varies in climatic regions. For example, average carbon storage by agroforestry has been estimated to be 9, 21 and 50 Mg Carbon per hectare in semiarid, sub-humid and humid regions respectively. Diverse agroforestry practices also enhance the percentage of carbon sequestered. Home gardens are one of the ideal ways to capture carbon and closet it. The type of trees and planned density of plantations also augments the carbon capture say Koul and Pawar.

In this direction improved clones of poplar trees in the Terai region of Uttarakhand yielded 50 meter cube wood per hectare per year in a short span of eight years. Similarly Eucalytptus clones planted in Andhra Pradesh in comparatively dry and un-irrigated conditions produced 25-25 meter cube per hectare per year of wood. Both these examples point towards sequestration of more carbon per unit area per unit time.

Role of nitrogen fixing trees in agroforestry system will help in improving the production of trees and their sustainability. They can also be used for reclaiming the degraded soils. However, the researchers caution that ruminant based agro-silvo-pastoral systems and rice-paddy agro-silviculture systems are often a source of methane. Such plantations can do more harm than being advantageous for CCS. 

A note by the Soil Science Society of America on Carbon sequestration says that worldwide soil organic carbon in the top one meter of soil comprises about 3/4th of the earth's terrestrial carbon; nevertheless there is tremendous potential to sequester additional carbon in soil. An improved management of crop, grazing and forest lands is estimated to potentially offset 30,000 to 60,000 million metric tons of carbon released by fossil fuel combustion over the next 50 years.

The issue needs the attention of all the soil scientists, agronomists and foresters. What to plant, ho much to plant where for better cost benefit ratio and also better carbon credits needs to be researched upon. May be the day is not far when the soil will become a grave for the carbon that your car pumps in to the atmosphere!