Ag Emissions and GWP, part 2

I am trying to uncover more Global Warming Potential (GWP) impacts from our food system. Some segments of agriculture are already tracked by the EPA (discussed in Part 1 of this chain): enteric fermentation in domestic livestock, livestock manure management, rice cultivation, agricultural soil management, and field burning of agricultural residues. There are some gaps that need addressing, and would best be aligned with other research like Heller/Keoleian's report and the resulting breakdown of energy use by segment: ag production (EPA), transport, processing, packaging, food retail, restaurants, home refrigeration, and then waste disposal (but not in Heller report).

Let's start with what we know. The US DOE's Energy Information Administration's (EIA) Emissions of Greenhouse Gases in the United States 2005 report details Carbon Dioxide (CO2), Methane (CH4), and Nitrous Oxide (N2O) emissions as well as other GHG gases.

Methane
CH4 is produced as part of normal digestive processes in animals(1). It is also a byproduct of landfills and decomposition, and landfills have already been tapped for this energy source. The EIA methane report shows that from what we measured in 2005 for anthropogenic methane emissions we know:

• 
• Methane has a GWP rating of 23.
  
• Total U.S. Methane Emissions were 26.6 million metric tons
• 611.9 million metric tons CO2 equivalent (CO2e)
• Agriculture released 173.4 million metric tons CO2e
• 28.3% of total emissions
  

Of note: "emissions increases from enteric fermentation and animal waste management more than offset small decreases in emissions from rice cultivation and crop residue burning. Of total estimated methane emissions from agricultural activities, 93 percent (170.9 MMTCO₂e) results from livestock management, of which 68 percent (115.6MMTCO₂e) can be traced to enteric fermentation in ruminant animals and the remainder (55.3 MMTCO₂e) to anaerobic decomposition of livestock wastes. A small portion of U.S. agricultural methane emissions result from crop residue burning and wetland rice cultivation."(2)

We need to calculate the other ag and food emissions from other data sets.


Nitrous Oxide
N2O is found in ag primarily from fertilizer application and management of solid waste from animals . If applied properly as a fertilizer, nitrogen is taken up by the plants, but "Indirect emissions from nitrogen fertilization result from adding excess nitrogen to the soil, which in turn enriches ground and surface waters, such as rivers and streams, and results in emissions of nitrous oxide."(3)

What we do know:

• Nitrous oxide has a GWP rating of 296.
• Total U.S. N2O emissions for 2005 were 1.2 million metric tons
• 366.56 million metric tons CO2e
• Agriculture (what is measured) released 279.9 million metric tons CO2e
• 76.4% of total emissions
  

Of note: "Nitrogen fertilization of agricultural soils accounted for 78 percent of U.S. agricultural emissions of nitrous oxide in 2005. Nearly all the remaining agricultural emissions (22 percent) can be traced to the management of the solid waste of domesticated animals."(4)

This means that nitrogen fertilization adds 218.3 million metric tons of nitrous oxide emissions, which is 60% of total U.S. nitrous oxide emissions from this one act. Now, nitrogen is a critical component to plant fertilization. The question is: how do we make that fertilizer, and are we applying efficiently?

Another question that arises is: why the increase in nitrous oxide in the last couple years? Is our land yielding less and therefore requiring more fertilizer?

Sources:
(1) http://epa.gov/climatechange/emissions/downloads06/06Agriculture.pdf
(2) ibid.
(3) http://www.eia.doe.gov/oiaf/1605/ggrpt/nitrous.html
(4) ibid.