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
|
