Agriculture allocates a considerable part to the world’s total greenhouse gas (GHG) emission. The sources that emit carbon dioxide (CO2), methane (CH4) and nitrous oxid (N2O) are manifold and include fertilizer application, livestock and manure management, rice cultivation or land clearing. GHG contribute to the warming of the planet which is linked to changes in patterns of water cycling.
Introduction
Agricultural GHG emission makes up to 13.5% of the worldwide GHG emission (data from 2004) which is about the same amount as for global transportation processes. Especially in developing countries, emissions related to agricultural systems are expected to rise by 58% until 2020 compared to 1990. Globally emission growth in agriculture is expected to increase by 38% in the respective timeframe. Most likely, the already observed increase in CH4 concentration is mainly due to agriculture expansion and burning of fossil fuels. The rising concentration of N2O is primarily due to agriculture.
Sources of emission
Fertilizer application
Fertilizers in agriculture are applied to add nutrients to the soil in order to create better growing conditions for crops. Compared to 1990 the application of fertilizer is expected to double in areas such as Africa, Latin America and the Middle East by 2020.
By adding synthetic nitrogen fertilizers to the fields or spraying manure and sewage sludge, higher amounts of nitrogen are directly or indirectly available to more organisms than the intended crop plants. Nitrogen is essential in many metabolic processes of ground living bacteria. Hence, these metabolic processes can be enhanced and conversion processes may produce and emit more metabolic products including N2O. N2O is the largest source of GHG emission within the agricultural sector and makes up to 38% of the global amount. Additionally, these metabolic processes can also favour the production of GHG such as CO2 and CH4.
Livestock and manure management
Digestive processes and enteric fermentation in livestock, especially in cattle and sheep, are a major source of CH4 emitted to the environment. Manure management means the ways of handling, storing and treating manure. When manure is broken down by bacteria under anaerobic conditions methane is released as metabolic product and can enter the environment. If conditions during storage change from aerobic to anaerobic, nitrous oxide is the primer break down product. When put out on the fields as fertilizer, manure will enter further GHG producing processes. As the global meat consumption is expected to rise in the future so will emissions from livestock and manure.
Rice cultivation
Under flooding rice fields produce methane due to anaerobic decomposition of organic matter. Methane production via rice cultivation is highest in China and South-East Asia. Rice plays a very important role in the nutrition of many Asian countries. Population trends assume rising population levels in these countries, resulting in a higher methane production.
Land clearing
In order to gain space for crop cultivation or livestock keeping formerly forested or savannah covered areas are deforested or burned. CO2 is not the only GHG which is emitted by burning. Burning savannahs or forests will also result in a higher emission of N2O and CH4. Till 2020 (compared to 1990) the amount of emitted N2O and CH4 through burning land will increase by around 15% in developed countries and more than 40% in developing countries, respectively.
GHG mitigation
Carbon sequestration
Carbon sequestration means the long-term storage of carbon (as CO2 or other forms of carbon), e.g. in the soil, in order to slow down the atmospheric accumulation of GHG. Enhancement and preservation of carbon sinks is vital and can be achieved by “best practice” crop land management. Approved actions include the reduction of bare fallow, restoration of degraded soils, improvement of pasture and grazing land, irrigation, crop and forage rotation as well as no-tillage practices. 90% of the technical potential to reduce GHG emission comes from carbon sequestration in the soil. However, soils cannot store an indefinite amount of CO2 and soil organic carbon may be re-released through fire or later tillage.
On-farm mitigation
During production processes GHG can be mitigated by using different schemes for livestock (feeding practices, dietary additives, animal breeds) and manure (storage, methane capturing methods) management. The addition of fertilizer may be reduced while in rice cultivation changes in the water management may lower the emission of CH4. Comprehensive analyses of the impact of mitigation options at farm level are important and can act as a guideline for the parties involved.
Bioenergy
Agricultural products, also in the form of left-overs or waste, may be used as source for bioenergy. The potential of biofuels to lower the emission of GHG is related to its production process meaning the way of manufacture and cultivation. However, bioenergy generated from agriculture may be of value to replace fossil fuels if trade-offs with food security and biodiversity are kept in mind.
References
Bumb BL & Baanante CA (1996): World trends in fertilizer use and projections to 2020. 2020 Brief #38, International Food Policy Research Institute, Washington, D.C. http://ageconsearch.umn.edu/bitstream/16353/1/br38.pdf [Access 2015-07-07]
Fundación Global Nature & Solagro (2014): Measures at farm level to reduce greenhouse gas emissions from EU agriculture. European Union, Bruxelles. http://www.europarl.europa.eu/RegData/etudes/note/join/2014/513997/IPOL-AGRI_NT(2014)513997_EN.pdf [Access 2015-07-07]
GTZ (2008): Climate change and agriculture – threats and opportunities. GTZ, Eschborn. http://ccsl.iccip.net/gtz_climatechange-agriculture.pdf [Access 2015-07-07]
IPCC (2007): Summary for policy makers. Climate Change 2007: Synthesis Report. Fourth Assessment Report of the Intergovernmental Panel for Climate Change. http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_full_report.pdf [Access 2015-07-07]
Smith, P., D. Martino, Z. Cai, D. Gwary, H. Janzen, P. Kumar, B. McCarl, S. Ogle, F. O’Mara, C. Rice, B. Scholes, O. Sirotenko (2007) Agriculture. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. https://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter8.pdf [Access 2015-07-07]
USEPA (2006): Global Mitigation of Non-CO2 Greenhouse Gases. Office of Atmospheric Programs, Washington D.C. http://www.epa.gov/climatechange/Downloads/EPAactivities/GM_Cover_TOC.pdf [Access 2015-07-07]
Further reading
Gerber, P.J., Steinfeld, H., Henderson, B., Mottet, A., Opio, C., Dijkman, J., Falcucci, A. & Tempio, G. (2013): Tackling climate change through livestock – A global assessment of emissions and mitigation opportunities. Food and Agriculture Organization of the United Nations (FAO), Rome. http://www.fao.org/docrep/018/i3437e/i3437e.pdf [Access 2015-07-07]
Global Research Alliance on Agricultural Greenhouse Gases: http://www.globalresearchalliance.org/ [Access 2015-07-07]
Greenhouse Gas Protocol (2014): GHG Protocol Agricultural Guidance - Interpreting the Corporate Accounting and Reporting Standard for the agricultural sector. Protocol Agricultural Guidance (April 26)_0.pdf http://www.ghgprotocol.org/files/ghgp/GHG Protocol Agricultural Guidance (April 26)_0.pdf [Access 2015-07-07]
IPCC (2007): Climate Change 2007: Mitigation of Climate Change. Chapter 8: Agriculture. https://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter8.pdf [Access 2015-07-07]
