THE ROLE OF AGRICULTURE IN

NITROUS OXIDE EMISSIONS

 

Nitrous oxide (N₂O), the third most abundant greenhouse gas, is closely tied to production agriculture. Agriculture activities accounted for 76 percent of N₂O emissions in the U.S. in 2005, and 86 percent globally, according to the U.S. Energy Information Administration.

 

These activities include the application of organic and inorganic fertilizers, planting nitrogen-fixing crops, burning plant residue, managing manure, and adding nitrogen to water sources from field runoff, according to the U.S. Environmental Protection Agency.

 

Emissions from agriculture (Tg CO2 Eq.)
Year	1990	1995	2000	2001	2002	2003	2004	2005
N2O	375.9	362.7	386.9	399.2	376.2	359.9	348.7	375.1
Agriculture soil management	366.9	353.4	376.8	389.0	366.1	350.2	338.8	365.1
Manure management	8.6	9.0	9.6	9.8	9.7	9.3	9.4	9.5
Field burning of agriculture residue	0.4	0.4	0.5	0.5	0.4	0.4	0.5	0.5

Source: http://epa.gov/climatechange/emissions/downloads06/07Agriculture.pdf

 

Much attention has been targeted toward carbon dioxide, which is the most abundant greenhouse gas in the atmosphere; however, N₂O is about 300 times more powerful due to its duration in the atmosphere and its heat-absorbing capabilities.

 

Each greenhouse gas is given a carbon equivalent, or a value of global warming potential. Carbon dioxide is the baseline of this scale with a value of one. Methane has a value of 21, and N₂O has a value of 310, making it much more effective at warming the atmosphere.

 

Nitrogen naturally enters the soil through the activities of nitrogen-fixing bacteria (and blue-green algae), and the deposition of animal wastes. Even if there were no human activity, these processes would occur and convert inert atmospheric nitrogen into forms that are essential to plant and animal life. Nitrogen-fixing bacteria absorb atmospheric nitrogen and transform it into ammonia (NH3). Some nitrogen-fixing bacteria are free-living in soil and water, while others are symbiotic with certain plants, fungi, and other organisms. The NH3 produced by symbiotic bacteria is rapidly converted into certain amino acids within the plants. Once this nitrogen is incorporated into plant tissue, it then cycles back into the soil as organic residue. As organic matter is decomposed, the nitrogen is transformed into ammonium (NH4) and simple organic compounds. The NH4 then goes through the microbial-driven processes of nitrification and denitrification.

 

Nitrification and denitrification are driven by the activity of microorganisms in soils. Nitrification is the aerobic microbial oxidation of ammonium (NH4) to nitrate (NO3). In a well-aerated soil, most of the ammonium in the soil is converted into nitrate. Denitrification is the anaerobic microbial reduction of nitrate to nitrogen gas (N2). Nitrous oxide is a gaseous intermediate product in the reaction sequence of denitrification, which leaks from microbial cells into the soil and then into the atmosphere. Nitrous oxide is also produced during nitrification, although by a less well-understood mechanism.

 

Nitrogen also enters the soil as a direct result of human activities, and this has a significant indirect effect on nitrous oxide emissions. Nitrogen additions through human activities are done mainly through organic and inorganic fertilization. Both sources of nitrogen fertilizer lead to increased levels of mineral and organic nitrogen in the soil, which lead to higher rates of nitrification and possibly denitrification. Also, the planting of nitrogen-fixing crops such as soybean and alfalfa is a human activity that increases the natural process of nitrogen fixation in the soil. Burning crop residues also contributes to N₂O emissions due to the incomplete combustion of agricultural waste, but occurs on a much smaller scale than fertilizer applications.

 

Studies are underway to understand how agricultural and land-use management practices can affect N₂O emissions, but this has not yet been extensively researched. Manure management, for example, plays a factor in nitrous oxide emissions, but methane has been researched more intensively than N₂O on this subject. Research has shown that inorganic nitrogen application practices can have a significant effect on N₂O emissions, but more work is needed on this topic.

 

An indirect source of N₂O emissions is from water leaching and runoff from agricultural fields. Applied nitrogen fertilizer and manure that is not absorbed by crops may leach through the soil or wash into surface waters. There, part of the nitrogen is converted into N₂O through denitrification.

 

Some believe that water runoff is a major contribution to N₂O emissions, but little research has been done to determine that.

 

Nitrous oxide emissions vary greatly from region to region, depending on agricultural production and waste management practices, climate, soil type, and transportation factors. For example, temperate, intensive agricultural areas such as the Midwestern U.S. will produce much more nitrous oxide than other areas because of the quantities of nitrogen that are added to the soil.

 

In summary, potential nitrous oxide mitigation practices in agriculture include:

 

* Improved nitrogen fertilizer efficiency

* Improved nitrogen placement, timing, and rate

* Reduced soil erosion

 

-- Katie Starzec, CASMGS Communications, Kansas State University

kstarzec@ksu.edu

 

 

http://ohioline.osu.edu/aex-fact/0463.html