ARS | Publication request: Do mitigation strategies reduce global warming potential in the northern U.S. Corn Belt?

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 28, 2011
Publication Date: August 8, 2011
Citation: Johnson, J.M., Archer, D.W., Weyers, S.L., Barbour, N.W. 2011. Do mitigation strategies reduce global warming potential in the northern U.S. Corn Belt? Journal of Environmental Quality. 40:1551-1559.

Interpretive Summary: A gas that changes the amount of energy retained in or lost from the earth's atmosphere is called a greenhouse gas. Carbon dioxide, methane and nitrous oxide are such gases. Global warming potential (GWP) is the impact of a greenhouse gas relative to carbon dioxide. This allows direct comparison among greenhouse gases. Nitrous oxide is about 300 times and methane is more than 20 times more potent as a greenhouse gas compared to carbon dioxide. During photosynthesis, plants change carbon dioxide into plant material. Most of the carbon in the plant material is changed back into carbon dioxide, but a small amount may be stored for a long time as soil organic matter. Storing carbon as soil organic matter is good for the soil and is one way to help reduce greenhouse gas in the atmosphere. Tillage, fertilization, temperature and moisture can affect the release of these gases from soil. Some soil organisms release methane and others consume and break down methane. We measured greenhouse gas emission and change in soil carbon among three farming systems to see if cropping systems can be managed to reduce their GWP. One system had a corn and soybean rotation, was intensively tilled annually, and had high fertilizer inputs. A second system rotated four crops (corn, soybean, wheat, and alfalfa), was tilled only twice in four years, and received fertilizer as needed. The third system differed from the second only in that it was not fertilized. We measured how much carbon was stored in the soil and how much of each of carbon dioxide, methane and nitrous oxide were released over three years. Best management practices kept the emission of nitrous oxide as low as that observed when nitrogen fertilizer was withheld. Reducing tillage and adding more crops to the rotation increased soil carbon near the surface. These systems had similar GWPs. This work provides information on the overall impact of management on greenhouse gas emission. It benefits producers, scientists, and policymakers by providing guidance toward developing agricultural systems that reduce global warming risks without sacrificing productivity or profitability.

Technical Abstract: Agriculture is both an anthropogenic source of CO2, CH4, and N2O, and a sink for CO2 and CH4. Management can impact agriculture's net global warming potential (GWP) by changing source and/or sink. This study compared GWP among three crop management systems: business as usual (BAU), optimum greenhouse gas benefit (OGGB), and maximum C sequestration (MAXC). The BAU scenario was a two-year corn (Zea mays L.)-soybean (Glycine max L. [Merr.]) rotation (C-S) with conventional tillage (moldboard or chisel plow) and added N-P-K-fertilizer. The OGGB scenario was a four-year C-S-wheat (Triticum aestivum L.)/alfalfa (Medicago aestivum L.)-alfalfa rotation (C-S-W-A) with strip tillage (mole-knife) and no N-P-K fertilizer. The MAXC scenario was a four-year C-S-W-A rotation with strip tillage (mole-knife) with N-P-K fertilizer. The objective of this study was to compare GWP among these three contrasting crop management systems in Mollisol soils in west central Minnesota. Biomass production, GWP of N2O and CH4, and change in SOC and total C were measured. The only measurable accumulation of SOC was in the surface 0.05 m of the MAXC and OGGB. Integrating over a 0.6 m profile, there were no significant changes in SOC in MAXC or OGGB, but a measurable loss in the BAU scenario. Flux of N2O was similar among all scenarios. Applying N fertilizer in a split application and based on spring soil test resulted in N2O emission comparable to withholding N fertilizer. All scenarios were small CH4 sinks with no difference among scenarios. Overall, these management systems had similar GWP.