As the major building block on Earth, carbon is a foundation for life. Plant biomass and soil organic matter contain carbon and it can be found as the gas carbon dioxide in the atmosphere and dissolved in seawater which stores most of the Earth's carbon. However, soils contain approximately 75% of the terrestrial carbon pool (ESA 2000). Agriculture covers 33% of the world's land and though agricultural practices are a large perpetrator for greenhouse gas emissions, its soils can play a major role in maintaining a balanced global carbon cycle.
Each plant will use carbon in a self sustaining way, transforming it into carbohydrates, cellulose and other sugars before converting them back into carbon dioxide. Soil organisms transform and cycle nutrients, decomposing them through the environment and liberating carbon and nutrients. Crop farming removes plant products in the environment and from natural cycles before decomposition can occur, removing essential elements (carbon, nitrogen, phosphorus etc.). Soil disturbance and increased rates of decomposition result in carbon release to the atmosphere along with increased soil erosion and leaching of soil nutrients, further reducing the potential to act as greenhouse gas sinks.
When farming methods increase the organic carbon content of the soil, it captures more carbon dioxide than it emits which means more carbon dioxide is removed from the atmosphere and is stored in the soil. This process is called carbon sequestration or 'soil sink' when referring to agricultural soil that accumulates carbon (Forge 2001). Supporting soil conservation in farming has initiated financial and environmental incentives to trial developments in creating carbon sinks for agricultural soils. The carbon sequestration potential depends on soil’s capacity to store and accumulate humus formed from the organic materials in the soil environment and its ability to resist microbial decomposition. The sequestration potential of a soil depends on the vegetation it supports, its mineralogical composition, the depth of the solum, soil drainage, the availability of water and air, and the temperature of the soil environment.
Farm management is key to the amount and length of time carbon is stored and this means using practices that increase the carbon content in the soil. Improved soil and water quality, decreased nutrient loss, reduced soil erosion, increased water conservation, conservation tillage (leaving crop residues on the soil surface), cover cropping (using crops such as clover to protect soil between crop production) and crop rotation (diversifying the sequence of crops on the same land) are known management techniques that help soils store carbon (ESA 2000). They are also soil conservation techniques and greater crop production may result from increasing the amount of carbon stored in agricultural soils.
There is high potential for carbon sequestration by organic farms. Niggli et al. (2009) estimated that the global average sequestration potential of organic croplands of 0.9-2.4 Gt CO2 per year, which is equivalent to an average of about 200 to 400 kg carbon per hectare and year for all croplands. This estimate is supported by sufficient scientific evidence that organic agriculture can sequester more carbon than conventional farming practices or inhibit the carbon release. There are some constraints for carbon storage on organic farms such as the need for a combination of methods to obtain maximum storage, the capacity for farmers to adopt and maintain practices and the ability for farmers to monitor carbon stocks.
Carbon sequestration of agricultural land has a potential in mitigating carbon dioxide emissions as well as other greenhouse gases and can help soil conservation in disturbed ecosystems. Yet it is not a permanent solution as there is much to learn about carbon sequestration. After a number of years soil carbon reserves will reach a new balance and new practices must be maintained to prevent the carbon dioxide from being re-released. Research into this area aims to find ways that increase the storage of carbon in soil. This also includes examining the underlying mechanisms controlling soil structure in relation with carbon storage and the relationships with carbon storage between biodiversity and atmospheric carbon dioxide levels. Currently farmers can include soil sinks as part of a broader strategy aimed at reducing their greenhouse gas emissions.
References
1. Ecological Society of America (ESA). 2000. Carbon sequestration in soils. http://www.esa.org/education_diversity/pdfDocs/carbonsequestrationinsoils.pdf
2. Forge, F. 2001. Carbon sequestration by argicultural soil. http://publications.gc.ca/Collection-R/LoPBdP/BP/prb0038-e.htm
3. Niggli, U., Fliessbach, A., Hepperly, P. and Scialabba, N. 2009. Low Greenhouse Gas Agriculture: Mitigation and Adaptation Potential of Sustainable Farming Systems. FAO, April 2009, Rev. 2 – 2009.
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