Overview

Greenhouse Gas Emissions globally from all human activity are leading to changes in the Earth’s climate system; these changes are already observed and will continue at an unknown degree as long as the concentration of long-lived GHG in the atmosphere continues to increase [4]. Agriculture will need to adapt to these changes in climate and weather patterns, and can also be part of the solution in reducing GHG emissions through climate-smart agriculture practices.

Energy use results in emissions of greenhouse gases (GHGs) from combustion of fossil fuels. Other agricultural activities also contribute to GHG emissions, including gaseous losses of synthetic and organic nitrogen fertilizers as nitrous oxide (N₂O) emissions from the burning of crop residues in the field, and methane (CH₄) emissions from fields flooded for rice production.

The U.S. government produces an annual report of GHG emissions from all sectors [28]. Total GHG emissions from crop cultivation—including methane from rice cultivation and nitrous oxide emissions from soils—represented roughly 60 percent of emissions from the U.S. agriculture sector; livestock was the remaining largest source. Agriculture as a whole contributed 7.7 percent of total national GHG emissions in 2015. These estimates do not include energy use on farms, which is accounted for in other sectors in the U.S. inventory report. At the same time, conservation agriculture practices can contribute to reducing the overall U.S. net emissions by storing, or sequestering, carbon in soils [12]. Improving soil carbon is also a key strategy for enhancing soil health [15], which can also enhance the land’s resilience to extreme weather events. Soil carbon sequestration is also an important factor in reducing the net GHG emissions from crop production. While this indicator does not include this soil carbon sink, we explore what is known about national trends in our assessment of Soil Carbon.

SECTION 1:

RESULTS

Results for the Greenhouse Gas Emissions Indicator for the ten crops are illustrated and described in the figures below. Energy Use is presented in the amount of carbon dioxide equivalent (CO2e) in pounds per unit of crop production. Thus, in addition to responding to changes in greenhouse gas emissions, this indicator is sensitive to trends in crop yield. Additional information can be seen in the crop specific sections of the report.

The trends described below are the result of many different environmental, social, and policy drivers and represent innumerable individual producer decisions regarding management. Our intention is to provide an overview of how these forces and decisions, in aggregate, have influenced the resource use efficiency of U.S. commodity agriculture over the long term. Where the data that were used in the indicator calculations can explain changes over time, some interpretation is provided. However, we do not attempt a thorough, geographically specific interpretation that would be needed to fully understand some of these trends or provide a robust statistical assessment. Thus, while linear trend lines are provided to illustrate whether there is a consistent directional change over time, this should not be considered a measure of the statistical significance of trends.

Greenhouse Gas Emissions Indicator by Crop

SECTION 2:

METHODOLOGY

© Steve Werblow

The Greenhouse Gas Emissions indicator shares the same boundaries of calculations as the Energy Use indicator, and utilizes much of the same data. In addition to translating the energy use into emissions based on fuel type, the U.S. EPA inventory of emissions [28] is used to provide estimates of methane and nitrous oxide emissions. The Greenhouse Gas Emissions indicator does not account for soil carbon stocks or fluxes, as those are not currently included in the corresponding field-scale metric. We consider national-level trends in soil carbon in Part Two of this report, and are currently considering revisions to the field-scale metric to allow for incorporation of soil carbon in the total GHG balance. 

The data sources used are similar to the Energy Use indicator but transformed using further assumptions regarding fuel type and supplemented with data from the U.S. EPA inventory. The inventory values used are for nitrous oxide from soils, methane from flooded rice fields, and residue burning. These are calculated with complex models and come with an additional level of uncertainty. Thus the uncertainty for the Greenhouse Gas Emissions indicator should be considered to be slightly higher than that for the Energy Use indicator.

Emissions from Energy Use

Energy use, as described in detail in the previous section, is converted to emissions by considering the source of energy (fuel type), and the resulting emissions are reported as pounds of carbon dioxide (CO₂) equivalent (CO₂e). CO₂e is a common measure for assessing total GHG emissions that accounts for the relative strength of the Global Warming Potential (GWP) of different GHGs. Thus, CO₂e provides a method to combine emissions of CO₂ with emissions of methane and nitrous oxide in a common unit for comparison. A factor of 22.3 pounds CO₂ emitted per gallon of diesel combusted was used.

The carbon emissions due to equipment operation for alternative tillage systems were taken from West and Marland [25] (Table 1.4).

Table 1.4: Emissions from machinery operations from West and Marland (2002) [25].

The three tillage systems are consistent with the definitions used by the CTIC and USDA’s ARMS data: conventional till, reduced till, and no-till. CTIC provides data over time of the percentage of each crop under the different tillage practices. The CTIC values are provided for corn, soybeans, wheat, and cotton [50]. USDA ARMS data are used for rice; conventional till is assumed for potatoes with the assumption of little or no change in tillage practices (and thus tillage energy and emissions) for potatoes over time [52]. Conventional till uses the most energy for machinery, and hence produces the largest carbon emissions of the three practices. No-till uses the least amount of energy, and hence produces the least amount of carbon emissions. For crops not included explicitly in West and Marland [25], the same adjustments as were made for Energy Use, described above, were used.

The analysis in this report assumes that these emissions factors have not changed over time. According to researchers at the Nebraska Tractor Test Laboratory [57], the focus of agricultural engine research and development has been to reduce emissions from farm equipment. While the specific impact of this assumption is not known, the likely impact of improvements in energy efficiency and associated emissions from farm equipment over time would be reduced emissions. That trend is not captured in the results reported here. Changes in the emissions from machinery therefore come only from changing tillage practices over time. Efficiency gains due to increased adoption of no-till and reduced-till practices are captured using the CTIC [50] and ARMS [46] data for the share of each crop under each tillage system.

Emissions from the pumping and distribution of irrigation water are estimated from the energy calculation. Given the prevalence of electric pumps used in irrigation, the improvements in emissions from the national grid are taken into consideration with regard to irrigation. The emissions from grain drying, crop storage (potatoes), and transport are likewise calculated in a consistent manner with the energy used for these activities. The amounts of fuel energy combusted and electricity consumed are used to estimate GHG emissions. Propane is assumed as the fuel used for drying, while diesel is assumed as the fuel used for transport. Electricity values are assumed as average emissions from the national grid including improvements in emissions over time.

Emissions Embedded in Chemicals and Fertilizers Applied

USDA’s Agricultural Chemical Usage report provided data on chemical usage and fertilizer use for all crops [47]. These product application rates were interpolated between reference years on a rate-per-acre basis. Emissions factors for product-embodied CO₂ were taken from the GREET model version 1.8d [48] for fertilizer and from Audsley [49] for crop protection products. These emission factors were adjusted to account for efficiency changes over time for natural gas to ammonia fertilizer conversion (for nitrogen fertilizer), and for emissions changes on the electric grid over time (for crop protection products). The electric grid correction factor was chosen for crop protection products because of the high relative importance of electric power in their production [49].

The embedded GHGs in seed is estimated in the same manner as for energy—as a fraction of the total GHGs to produce the crop, using the same adjustment factors described in the section on Energy Use.

Nitrous Oxide Emissions from Soils

Nitrous oxide is a GHG with a global warming potential (GWP) of 296 times that of CO₂ [58]. Nitrous oxide released from soil microbial activity in association with fertilizer nitrogen application is an important source of emissions. However, the range of estimates for nitrous oxide as a percent of nitrogen applied is very wide depending on the source of nitrogen, the method of application, and the soil conditions at the time of application. A literature review by Snyder et al. [59] found that nitrous oxide emissions as a percent of nitrogen applied can range from near zero to nearly 20 percent of applied nitrogen lost as nitrous oxide. Bouwman et al. [60] report a global mean of 0.9 percent of nitrogen from fertilizer is released from soil as nitrous oxide, while a recent paper by Shcherbak et al. [61] found that while the nitrogen fertilizer application rate remains the best single indicator of nitrous oxide emissions, it is still imprecise and does not follow linear trends.

For the purposes of this analysis we use a single factor, consistent with a Tier 1 approach as recommended by the Intergovernmental Panel on Climate Change (IPCC) [58], to estimate nitrous oxide emissions from fertilizer applications at a national average scale. The applied nitrogen from synthetic fertilizer and manure is multiplied by 1.4 percent to estimate the nitrogen that is emitted as nitrous oxide. This 1.4 percent factor accounts for emissions from all sources, both direct and indirect. The IPCC assumes that 1 percent of applied nitrogen fertilizer (uncertainty range of 0.3–3.0 percent) is lost from direct emissions of nitrous oxide at the field level due to nitrification/denitrification. This assumption is based on scientific publications that report losses for specific crops and cropping systems [62]. Indirect nitrous oxide emissions result from denitrification of volatilized ammonia (NH₃) deposited elsewhere, from nitrate (NO₃) lost to leaching and runoff as the nitrogen cascades through other ecosystems after leaving the field. The IPCC assessment protocol assumes that volatilization losses represent 10 percent of applied nitrogen, and that nitrous oxide-nitrogen emissions for these losses are 1 percent of this amount; leaching losses are assumed to be 30 percent of applied nitrogen, and nitrous oxide-nitrogen emissions are 0.75 percent of that amount [62]. Therefore, the IPCC default value for total direct and indirect nitrous oxide emissions represents about 1.4 percent of the applied nitrogen from fertilizer.

While sophisticated models exist to more closely estimate nitrous oxide emissions on a field scale [28], these models do not provide estimates over time that are crop specific. Rather, they simulate multi-year cropping systems over thousands of fields and use complex aggregation and weighting to derive a total estimate at the national level. The U.S. EPA inventory modeling is discussed in more detail in Part Two of this report. Field to Market continues to explore emissions factors for nitrous oxide that would provide appropriate variation based on nutrient management and cropping system, but in the current report for a national average, the 1.4 percent factor is applied. We recognize that this is likely an overestimate of nitrogen losses from well-managed, high-yielding systems.

Data on mean annual nitrogen applied from fertilizer and manure application were taken from USDA’s ARMS data [46], which include tons applied and manure source by crop over time. Data are not reported for all years; therefore, non-reported years are interpolated on a rate-per-acre basis and held constant prior to the data beginning and after the last data point. Management factors such as split application on nitrogen as well as application method and timing can have significant impacts on the ultimate emissions level from applied nitrogen. The approach we have taken does not capture these differences or their potential to have changed over time. This approach also does not account for background soil nitrous oxide emissions that occur in cropping systems without nitrogen fertilizer applications (e.g., soybeans and other nitrogen-fixing leguminous crops).

To convert the emissions from applied nitrogen into CO₂e, we have accounted for the ratio of the molecular weight of nitrous oxide to nitrogen (44/28) and the CO₂e factor for nitrous oxide (296). Using these factors, 100 pounds of applied nitrogen results in emissions of 651 pounds CO₂e. Example: Emissions from 100 pounds applied nitrogen = 100 X 1.4 percent X (44/28) X 296 = 651 pounds CO₂e.

Emissions from Field Burning and Residue Removal

Emissions from field burning of surface residue are a relatively small share of total emissions from agricultural production; however, in cases where residue is burnt, the impact can be significant. Field-burning emissions are not calculated for potatoes, sugar beets, or peanuts, which typically have no surface residue that would warrant burning. Additionally, little or no field burning is performed for soybeans or cotton. Levels of residue burning are taken directly from the EPA reporting of GHGs from agriculture [28]. The quantity of surface residue available to be burned is calculated as a proportion of the crops’ yield; crop-specific factors are available for every crop. The final calculation determines the amount of methane and nitrous oxide released into the atmosphere. The release of CO₂ is not counted, as it is expected to be released over time via decomposition and is thus considered part of the natural annual uptake and emission of CO₂ from plant growth rather than an anthropogenic emission. Among the crops in our analysis, burning of rice residue is the most prevalent, with 10 percent of acres burnt [28]. Emissions from residue burning account for about 0.5 percent of total emissions for rice.

Among the crops in this analysis, wheat is the only crop for which a measurable share of the acres has residue removed following the primary crop harvest. Removing the residue from an annual crop field reduces the GHG impact by reducing the CO₂ emissions from residue breakdown on the field. A value of 0.21 pound nitrogen from residue per bushel of grain harvested times the amount of acres harvested for straw of wheat harvested is thus subtracted from the indicator. According to USDA ERS [35], straw is removed from 13 percent of all wheat acres with an assumed 50 percent of the surface residue being removed. At the national level, wheat straw removal reduces GHG emissions for the crop by between 0.5 and 0.75 percent. The same assumed fractions were also used to calculate residue burning and removal for barley.

Methane Emissions from Flooded Rice

Methane emissions are the result of anaerobic conditions that occur in fields that need to be flooded for continuous periods of time during the growing season in order to produce a rice crop. Emissions for rice are based on the levels reported in the U.S. EPA’s annual inventory of GHG emissions [28]. A recent change in methodology for the U.S. EPA report resulted in a significant change in nationally estimated methane emissions from rice production. In the latest report [28], a detailed process-based model was applied to simulate rice production on mineral soils, replacing a simpler accounting approach. While the trend of emissions remains similar to that reported in 2012, the absolute amount of GHG emissions from rice is higher in the 2016 report due to the revision in the U.S. EPA methodology.

U.S. inventory data were scaled to a per-planted-acre basis for the period 1990 through 2010. Years prior to 1990 were set to the 1990 level, while years after 2010 were held constant at the 2010 level, again on a per-planted-acre basis. Consistent with U.S. EPA’s reporting of the data, methane emissions have trended lower over time on both a per-acre and per-unit-of-production basis. It should be noted that methane emissions from other crops due to flood irrigation are considered to be insignificant due to the relatively limited number of acres flooded and the short duration of flooding.