Urea Fertilizer Decreases N2o Emissions Compared to Anhydrous Ammonia in Corn Cropping Systems in Minnesota.

Quantifying nitrous oxide (N₂O) emissions from corn and soybean fields under different fertilizer regimes is essential to developing national inventories of greenhouse gas (GHG) emissions.  The objective of this study was to compare N₂O emissions in plots managed for more than 15 yr under continuous corn (C/C) versus corn-soybean (C/S) rotation that were fertilized during the corn-phase with either anhydrous ammonia (AA) or urea (U).  Over three growing seasons, N₂O emissions from C/C were nearly identical to C/S.  However, in both systems, N₂O emissions with AA were twice the emissions with U.  After accounting for N₂O emissions during the soybean-phase, it was estimated that a shift from C/S to C/C would result in an increase in emissions of 0.78 kg N ha^-1 (equivalent to 0.11 Mg CO₂-C ha^-1) when AA was used, compared to an increase of only 0.21 kg N ha^-1 (0.03 Mg CO₂-C ha^-1) when U was used as fertilizer.  In light of global trends toward increased use of U, these results have the positive implication that fertilizer-induced N₂O emissions may decline in future, at least per unit of applied N. While soil carbon dioxide emissions were 20% higher under C/C, crop residue from the prior year did not affect soil inorganic N or dissolved organic carbon (DOC) levels during the subsequent season, although DOC levels with AA were higher than with U.  We also compared different flux-calculation schemes, including a new method for correcting chamber-induced errors, and found that selection of a calculation method altered seasonal emissions estimates by as much as 35%.