Nutrient Management Specialist
Department of Dairy and Animal Science
Penn State University
What are greenhouse gases?
Naturally occurring greenhouse gases consist of water vapor (H2O), carbon dioxide (CO2)), methane (CH4), nitrous oxide (N2O) and ozone (O3). Gases produced from industrial activities include chlorofluorocarbons and hydrochlorofluorocarbons.
What are the primary sources of naturally occurring greenhouse gases?
Carbon dioxide makes up 84.6% of all emissions. The major sources of CO2 emissions are fossil fuel combustion, iron and steel production, cement manufacturing and municipal solid waste combustion. In the United States in 2004, fuel combustion accounted for 95% of CO2 emissions.
Methane makes up 7.9% of all emissions. The major sources include landfills, natural gas systems, enteric fermentation (dairy and beef cattle primarily), and coal mining.
Nitrous oxide makes up 5.5% of all emissions. The major sources include agricultural soil management and mobile combustion.
How does agriculture contribute to greenhouse gases?
Agriculture contributes approximately 6-7% of the total U. S. greenhouse gas emissions. Methane from enteric fermentation represents 20% and manure management 7% of the total CH4 emitted. Ruminants (beef, dairy, goats, sheep) are the main contributors to enteric fermentation. Because of the ruminant animal’s unique digestive system that includes a microbial fermentation vat, “the rumen”, methane is produced as a byproduct which is exhaled or belched by the animal. Dietary strategies that focus on forage quality and management practices that improve animal growth and production can reduce the amount of methane produced. From 1990 to 2004, emissions from enteric fermentation have decreased due mostly to reduced beef and dairy cattle numbers.
Manure management can produce both CH4 and N2O. Methane is produced when manure decomposes without oxygen present. Nitrous oxide is produced as part of the nitrogen cycle in livestock manure and contributes very little to emissions. There are manure management systems, i.e. digesters, which can utilize the methane to generate power not only on the farm but off farm as well. As farm technology advances, more options will be available to producers to harness methane.
Agricultural soils contribute the majority of N2O emissions in the U.S. Nitrous oxide is produced naturally in soils via a microbial process. Direct and indirect pathways for N2O emissions include commercial fertilizer application, managed application of livestock and poultry waste, manure deposition from grazing animals, sewage sludge, nitrogen fixing crops and retention of crop residues. There are year to year fluctuations in emissions due to variation in weather, use of synthetic fertilizer and crop production.
Should agriculture be concerned?
All industries, including agriculture should be concerned about how our day to day activities affect the environment. Resources are being committed to research the development of on-farm practices that can help minimize not only greenhouse gases, but ammonia and fine particulates. How agriculture contributes to greenhouse gas emissions should be kept in perspective relative to other industries and how they contribute to the big picture, not only nationally but globally.
Reference: INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS AND SINKS: 1990-2004. April 2006. USEPA #430-R-06-002