- Nitrous Oxide (N2O) emissions from ag make up over 75% of the U.S.'s total N2O emissions.
- N2O is found in ag primarily from fertilizer application and management of solid waste from animals.
- N2O has 296 times more impact on the climate than CO2.
As anyone who deals with government knows, a key issue is how terms are defined. So what is the definition used for ag lands as it pertains to this issue? The term used is "Agricultural Soil Management" (IPCC Source Category 4D), and the EPA defines this term this way: "Only direct emissions from agricultural lands (i.e., croplands and grasslands), along with emissions from PRP manure."
PRP is defined as "the deposition of manure on soils by animals on pasture, range, and paddock (PRP) (i.e., by animals whose manure is not managed)."
"Agricultural soils are responsible for the majority of U.S. N2O emissions. Estimated emissions from this source in 2003 were 253.5 Tg CO2 Eq. (818 Gg N2O)." (p. 19 in pdf)
So I want to know if this: do coventional ag practices have more or less N2O than organic or sustainable ag practices (e.g. low-till), and by how much?
As a starting point comes this quote from the EPA's Climate Change Inventory Report:
"Heavy utilization of synthetic nitrogen fertilizers in crop production typically results in significantly more N2O emissions from agricultural soils than that occurring from less intensive, low-tillage techniques."
But a few pages later in the report comes this:
"N2O emissions cannot be partitioned into the contribution of N2O from different N inputs (e.g., N2O emissions from synthetic fertilizer applications cannot be distinguished from those emissions resulting from manure applications). Therefore, it was not possible to separate out these individual contributors to N2O flux, as is suggested in the IPCC Guidelines." (pdf p.21)
To further refine the definitions, there are major crops and non-major crops.
- Major cropping systems are "corn, soybean, wheat, alfalfa hay, other hay, sorghum, and cotton" and "represent approximately 90 percent of total cropped land in the United States."
- Non-major crop types "include fruits, nuts, and vegetables, which account for approximately 5 percent of U.S. N fertilizer use (TFI 2000) and other crops not simulated by DAYCENT (barley, oats, tobacco, sugar cane, sugar beets, sunflower, millet, peanuts, etc.) which account for approximately 10 percent of total U.S. fertilizer use."
Source for quotes: EPA, "US Emissions Inventory 2005: Inventory of U.S. Greenhouse Gas Emissions and Sinks,1990-2003", Chapter 6 Agriculture, pp. 195-227.