Empirical data on methane (CH4) and nitrous oxide (N2O) emission are needed for management systems from many regions of the United States to evaluate mitigation strategies. The primary objectives of this study were to assess and compare crop productivity, CH4 andN(2)O flux, and yield-scaled emissions between a conventionally and an organically managed system. All phases of a corn (Zea mays L.)-soybean [Glycine max L. (Merr.)]-wheat (Triticum aestivum L.) over alfalfa (Medicago sativa L.)-alfalfa rotation were present each year. Both systems emitted about 4.2 kg N2O-N ha(-1) yr(-1) including growing and nongrowing season emissions, which cumulatively represents 4.74 and 9.26% of 267 kg synthetic-N and 136 kg manure-N applied, respectively. The equivalent of 0.84% of the 78 kg urea-N and 0.76% of the 136 kg manure-N were emitted as N2O ha(-1) within 30-d of fertilizer application in the conventionally managed system and organically managed system, respectively. Following the application of starter fertilizer to the conventionally managed corn, the equivalent of 3.45% of the 11 kg starter N was emitted within 30 d. The largest spring-thaw N2O flux was measured in the conventionally managed system following alfalfa, which had been killed the previous fall. Yield-scaled N2O+CH4 emission (Mg CO2 equivalents Mg-1 yield) was 1.6- to 5-times greater in the organically managed system, which had lower yield but similar emission compared to the conventionally managed system. Thus, viability of organic systems to mitigate greenhouse gas (GHG) emission may be compromised when crop productivity is reduced. Study results highlight the importance of assessing emission and crop production when evaluating GHG mitigation strategies.
