Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/10/2004
Publication Date: 2/28/2005
Citation: Baker, J.M., Griffis, T.J. 2005. Examining strategies to improve the carbon balance of corn/soybean agriculture using eddy covariance and mass balance techniques. Agricultural and Forest Meteorology. 128:163-177. Interpretive Summary: It has become generally accepted that increasing atmospheric concentrations of certain gases, particularly carbon dioxide, may cause the earth's climate to change, with potentially adverse consequences. Agriculture may be able to play a role in reducing this threat by storing carbon in the soil. If farmers can develop practices that do this, it may help the United States meet its stated goals of reducing net emissions of carbon dioxide. However, to this point there have been no effective, accurate methods for measuring the difference in carbon storage between different agricultural systems. We have developed a method in which we continuously measure the carbon dioxide exchange between a farm field and the atmosphere in two adjacent fields. The fields have the same soil type and are subjected to the same weather conditions. Both are in corn/soybean rotation. One of the fields is farmed conventionally, with fall chisel/disk tillage. In the other field we used two practices that have been proposed to store carbon: reduced tillage and a spring (oats) cover crop prior to soybean planting. By continuously comparing the carbon exchange rates from the two fields we were able to discern the effects of these two practices on the carbon balance of each field. We found that reduced tillage in the fall did indeed result in less carbon dioxide loss, but there appeared to be no benefit of a spring cover crop. The oats did capture carbon through photosynthesis, but all of it was lost by respiration in the weeks after the crop was killed and the beans were planted. The oats crop residue also had a slight inhibitory effect on early season growth of the soybeans. For the entire year, the overall carbon balance of the two fields was virtually identical. This methodology will be useful for scientists and others in determining the true impact of various production strategies on the net sotrage of release of carbon from farm fields.
Technical Abstract: There has been much interest in the potential role that agricultural practices might play in sequestering carbon to help offset rising atmospheric CO2 concentrations. A number of management methods that might increase soil C levels have been suggested, but there is little if any available supporting quantitative data that are necessary to properly support recommendations or policy changes. Changes are too spatially variable and too small relative to background levels to be detected within a reasonable period of time by soil sampling and analysis. As an alternative we have used paired, long-term micrometeorological measurements of contrasting management systems in immediately adjacent fields to discern the impact of two specific practices, reduced tillage and a spring cover crop, on the C balance of the soybean portion of a corn/soybean rotation, the dominant cropping system in much of the midwestern United States. One of the two fields was farmed conventionally, with fall chisel/disk tillage and soybean planting in late May. In the alternative field, we used reduced tillage (strip till) in the fall, and a spring oats cover crop that was planted in early April, then killed with a herbicide shortly after soybean planting in late May. Both fields have the same soil type, and were instrumented in the same way, with a sonic anemometer and open-path infrared gas analyzer. Cumulative eddy covariance measurements of CO2 exchange showed that reduced tillage resulted in lower soil respiration rates in the fall, but the difference did not persist through the following spring. The spring oats cover crop did fix additional C, but it was rapidly respired after the oats were killed, and the surface crop residue slowed the initial development of the subsequent soybean crop. Yields for the two fields were similar, but slightly higher for the conventional field. Overall, there was almost no difference in cumulative NEE for the two fields, and both showed a net loss of C for the year, presumably representing respiration of C remaining in corn residue left from the previous year. We conclude that neither of the management practices tested showed much potential for improving the NEE of soybeans in a corn/soybean rotation.