Carbon Sequestration in Agricultural Systems

Concerns about the impact of rising atmospheric CO₂ concentrations on global climate have inspired efforts to identify ways in which CO₂ can be removed from the atmosphere, or sequestered.  In this project we are examining various farming strategies that have been proposed to increase carbon storage in soils as organic matter.  These include reduced tillage and the use of cover crops.  The approach that we are using involves continuous measurement of surface/atmosphere CO₂ exchange in neighboring fields with the same soil type, but differing farming practices.  CO₂ exchange and water use are measured by eddy covariance, using a sonic anemometer and open-path infrared gas analyzer.  One field is managed in conventional fashion for southern Minnesota, which means a corn/soybean rotation, with fall tillage consisting of chisel plowing.  In the alternative field, we use a form of reduced fall tillage known as strip-till, in which most residue is left on the surface, except for a narrow zone in each row.  In the spring the seed is planted into the tilled zone.  The other practice that we are testing in the alternative field is the use of a cover crop following corn and before soybean. In the initial biennium we used spring oats as the cover crop.  The results showed (J.M Baker & T.J. Griffis, 2004) showed that a) reduced tillage had little influence on soil respiration and b) while spring oats resulted in increased carbon fixation, it was rapidly lost following soybean planting, so that the alternative field had the same biennial net ecosystem exchange (NEE) as the conventional field.  In the second biennium we have used a different cover crop, winter rye.  Initial analysis of the data suggests that this is a more promising approach to increase carbon sequestration.  Rye, since it is planted in the fall, produces much more biomass than spring oats prior to soybean planting.  In addition it produces more lignin, particularly as it nears maturity, and lignin is more resistant to decomposition than other plant compounds.  Winter cover crops have other advantages as well, protecting the soil surface against erosion, and taking up excess soil nitrate that might otherwise be lost to ground water.  However, their use in corn/soybean systems in Minnesota is challenging, primarily due to the narrow time frame for establishment, following fall corn harvest and prior to soil freezing. To address this, we are testing aerial seeding, where a helicopter is used to seed rye into standing corn in late summer so the rye can germinate and begin growth earlier, prior to corn harvest. For more information, contact Dr. John Baker, or to view current data click here.