Global Warming Potential of Delayed-Flood Rice Associated With Methane From a Silt-Loam Soil As Affected By Previous Crop and Cultivar.

Arkansas is the largest rice (Oryza sativa L.) producer in the United States; however, research assessing methane (GWP 25 times carbon dioxide) is limited. Currently, the EPA uses a methane  emission factor of 5333 kg CO₂-eq ha^-1 regardless of management practices.The current study investigated the influence of previous crop [soybean (Glycine max L.) or rice] and cultivar (standard stature, semi-dwarf, or hybrid) on methane production.  Research was conducted in 2012 on a silt-loam soil using the chamber method. Sampling occurred weekly from flooding until flood release and every other day for 1 week post-flood release. Grain was harvested using a research-grade combine. Methane fluxes differed between previous crops over time throughout the season among cultivars (P = 0.01). Fluxes from flooding to prior to heading were lower when rice followed soybean. Hybrid yield was greater (10.0 Mg ha^-1) than the standard-stature(8.6 Mg ha^-1), but the semi-dwarf (9.3 Mg ha^-1) yield did not differ from either other cultivar. Season-long area- and yield-based emissions differed based on cultivar or previous crop (P< 0.05). Rice following rice (5733 kg CO₂-eq ha^-1) had 1.5 times greater global warming potential (GWP) as compared to rice following soybean (3933 kg CO₂-eq ha^-1). Semi-dwarf (5200 kg CO₂-eq ha^-1) and standard-stature (5800 kg CO₂-eq ha^-1) cultivar GWPs were 1.5 and 1.7 times greater than the hybrid, respectively. Yield-based emissions indicated the semi-dwarf [563 kg CO₂-eq (Mg grain^-1)] and the standard-stature [687 kg CO₂-eq (Mg grain^-1)] GWP’s were 1.6 and 2.0 times greater than the hybrid [195 kg CO₂-eq (Mg grain^-1)]. Results indicated hybrid rice’s GWP was lower than the other cultivars, particularly when yield-scaled. These results indicate that the EPA emission factor would have overestimated the GWP when rice was grown following soybean, which accounts for 60% of Arkansas production and when hybrid rice was grown (~ 50%).