Elevated CO2 and Nitrogen Effects on Soil CO2 Flux from a Pasture upon Return to Cultivation.

Soil CO₂ efflux patterns associated with converting pastures back to row crop production remain understudied in the Southeastern U.S. A 10-year study of bahiagrass (Paspalum notatum Flüggé) response to elevated CO₂ was conducted using open top field chambers on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). Plants were subjected to ambient or elevated (ambient plus 200 ppm) CO₂ and grown under managed ([(NH₄)₂SO₄] at 90 kg ha^-1 3x yr^-1) and unmanaged conditions (no added N), both of which are common in the Southeast. At study termination, soil CO₂ flux was continuously monitored (automated carbon efflux system or ACES) following glyphosate applications and tillage to document CO₂ loss associated with pasture conversion to row crop production. Concurrent measures of the herbicide termination process were documented with an active light sensor (Greenseeker® meter). Following the initial herbicide application, managed plots showed higher vigor which declined and became similar to unmanaged plots after 3 weeks; there was no CO₂ effect during this period. After the second herbicide application, no GreenSeeker differences were detected, and monitoring was discontinued one week later. Cumulative soil CO₂ flux was higher under elevated CO₂ only for the week prior to the first herbicide application and for the period between the second herbicide application and the tillage event. The only N effect occurred during the second week after tillage (coinciding with rainfall) where daily CO₂ flux was higher in managed plots. For the entire sampling period, total cumulative CO₂ loss was not affected by either N or CO₂ level. These findings suggest that conversion of pasture to row crop systems will not be greatly impacted by N management or atmospheric CO₂ level.