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Carbon-Cost and
    Energy Balance

Background
Agriculture has the potential to sequester or store carbon in soil, which can mitigate the global warming effect of rising atmospheric CO2 concentration from the burning of fossil fuels. Agriculture, however, is an energy intensive industry and a lot of fossil fuel is consumed in the production of grain. In calculating net carbon sequestration we must consider fossil fuel “carbon costs” of producing the crop as well as the global warming potential of other radiatively active gases (e.g., nitrous oxide) that result from agricultural activity.

Project Goals
We are keeping a detailed inventory of fossil fuel consumed in the production of irrigated and rainfed corn and soybean in the Carbon Sequestration Project. In addition, the emissions of other important radiatively active gases (nitrous oxide and methane) are measured to determine the total intrinsic “C-costs” of crop production outside of the net ecosystem exchange of CO2. Another important aspect of the carbon budget is the se of exported grain for renewable fuels such as corn grain ethanol. In this case we have factored in the use of corn grain for ethanol and the value of this biofuel in offsetting fossil fuel use in the transportation sector.Progress: The dominant source of fossil fuel emissions in the production of corn is nitrogen fertilizer but the consumption of energy in the drying of corn grain and irrigation are also high. Total energy consumed by rainfed corn is 1/2 that of irrigated corn. Soybean fossil fuel emissions are 1/3 of those associated with corn.

  Source   Irrigated Maize   Rainfed Maize
      kg C ha-1   %   kg C ha-1   %  
  Nitrogen   149.9   31.3   109.5   45.4  
  Irrigation   127.0   26.6   -   -  
  Fertigation   4.4   0.9   -   -  
  Drying   100.1   20.3   64.8   26.9  
  Machinery   38.4   8.0   24.8   10.3  
  Embodied   16.2   3.4   8.2   3.4  
  Seed   31.9   6.7   23.6   9.8  
  Herbicide   10.4   2.2   10.4   4.3  
  Insecticide   0.04   0   0.03   0  
  Total   478       241      

Table 1. Typical CO2- C emissions attributed to fossil fuel inputs to irrigated and rainfed corn.
Depreciable inputs represent the amortized fossil fuel emissions associated with the
manufacture of equipment such as pivots, combines and tractors.

Trace gas emissions of nitrous oxide (N2O) and methane (CH4 ) are an important component of the greenhouse gas emission form agriculturally managed soils. The emissions we have measured are roughly equivalent to those form fossil fuel consumption on farm.

  Trace Gas Year 1   Year 2   Year 3  
  Emissions 2001-2002   2002-2003   2003-2004  
    ................ g CO2- C equivalent m-2 ................  
    Site 1: Irrigated Continuous Corn  
    Corn   Corn   Corn  
  N2O 41   29   53  
  CH4  3     -4   1  
    Site 2: Irrigated Corn-Soybean Rotation  
    Corn   Soybean   Corn  
  N2O 56   21   48  
  CH4  -1     -4   -14  
    Site 3: Rainfed Corn-Soybean Rotation  
    Corn   Soybean   Corn  
  N2O 41   20   50  
  CH4  -5     -5   -1  

Table 2. Annual trace gas emissions of nitrous oxide (N2O) and methane
(CH4 ) expressed as g CO2- C equivalent m-2.

The overall net ecosystem exchange of CO2- C for both corn and soybean is very high, however a large amount of C is exported form the field in grain harvest. When fossil fuel costs and trace gas emissions are factored into the budget, these systems are neutral to negative in C-sequestration.

    Site 1   Site 2   Site 3
Component   (Irrigated Corn)   (Irrigated Corn-   (Rainfed Corn-
    3-yr Average   Soybean)   Soybean)
        Avg Yr2 & Yr3   Avg Yr2 & Yr3
    .......................................... g Carbon m-2 ..........................................
Annual NEE   441   262   190
Grain C Removed in Harvest   -498   -361   -225
Annual NEE + Grain C   -57   -99   -35
             
N2O Flux (CO2- C Equiv) (3)   -41   -35   -35
CH4 Flux (CO2- C Equiv) (4)   0   9   2
Intrinsic C Costs (from fossil fuels used in production) (5)   -54   -33   -15
Annual NEE + Grain C + (3) + (4) + (5)   -152   -158   -83

Table 3. Average annual agroecosystem carbon budget. Annual NEE is the net annual
exchange of CO2-C between the soil/crop and atmosphere and represents C fixed in
photosynthesis minus C lost as ecosystem respiration. A negative number indicates
a net annual emission of CO2- C to the atmosphere.

Since corn ethanol is a biofuel, it has the potential to offset fossil fuel consumption in the transporation sector. We examined the overall impact of using corn-ethanol to offset fossil fuel use on the C-budget of these systems. Even though irrigated corn consumes twice the energy of rainfed corn, the resultant increase in grain for ethanol nearly offsets this energy use and the output:input ratio of energy production:consumption is >1. Ethanol therefore has the potential to contribute to C-sequestration.

Corn Ethanol Energy Balance: Irrigated vs. Rainfed Corn
  System   Outputs*   Inputs**   Net Balance
      ................................. GJ ha-1 .................................
  Irrigated   142   111   31 (1 : 3 : 1)
  Rainfed    92     68   24 (1 : 4 : 1)
 *Energy contained in ethanol produced from grain, and energy value of ethanol co-products.
**Energy required for field production inputs, drying, transport, and processing to ethanol.

Table 4.  Increases in the energy efficiency of modern ethanol plants, conversion efficiency of
grain to ethanol and improved farm input efficiency all contribute to a positive output /input energy ratio.
Ratio under “net balance” is the ratio of net energy output to energy input in production of corn ethanol.

If we factor in the emissions of CO2- C in the production of corn and ethanol against the fossil fuel offset value of this biofuel, it mitigates the trace gas emissions of N2O. Corn ethanol would also be a verifiable C source to be traded on the carbon market.

    Site 1   Site 2   Site 3
Component   (Irrigated Corn)   (Irrigated Corn-   (Rainfed Corn-
    3-yr Average   Soybean)   Soybean)
        Avg Yr2 & Yr3   Avg Yr2 & Yr3
    ......................... Average Annual Emissions (g CO2- C m-2) .........................
Outputs            
   Ethanol   257   140   77
   Co-Products   45   24   13
   Total Outputs   302   164   90
Inputs            
   Intrinsic C-Cost   -54   -33   -15
   Ethanol Conversion   -197   -107   -59
   Total Outputs   -251   -140   -74
Net Balance   +51   +24   +16

Table 5. Net greenhouse gas emissions associated with processing corn for ethanol production.
Outputs for ethanol and co-products represent the offset of fossil fuel emissions.

Staff
Dan Walters Professor, Department of Agronomy and Horticulture

Daniel Ginting Research Assistant Professor, Department of Agronomy and Horticulture

Ken Cassman Professor, Department of Agronomy and Horticulture

Achim Dobermann is professor of soil science and nutrient management at the University of Nebraska-Lincoln. From 1992 to 2000, he was a soil scientist at the International Rice Research Institute and led a multi-national research program on developing new concepts and tools for site-specific nutrient management in irrigated rice systems. Prof. Dobermann conducts research on nutrient cycling, soil variability, geospatial and crop modeling, soil greenhouse gas emissions, and approaches for site-specific nutrient management in major cereal production systems of Asia and North America. He has published two books and more than 60 papers in international scientific journals.

Shashi Verma Professor, School of Natural Resources, UNL

Mark Schroeder


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