Urea Decreases Nitrous Oxide Emissions Compared with Anhydrous Ammonia in a Minnesota Corn Cropping System

Quantifying N 2 O emissions from corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] fields under different fertilizer regimes is essential to developing national inventories of greenhouse gas emissions. The objective of this study was to compare N 2 O emissions in plots managed for more than 15 yr under continuous corn (C/C) vs. a corn–soybean (C/S) rotation that were fertilized during the corn phase with either anhydrous NH 3 (AA) or urea (U). During three growing seasons, N 2 O emissions from corn following corn were nearly identical to corn following soybean. In both systems, however, N 2 O emissions with AA were twice the emissions with U. After accounting for N 2 O emissions during the soybean phase, it was estimated that a shift from C/S to C/C would result in an increase in annual emissions of 0.78 kg N ha −1 (equivalent to 0.11 Mg CO 2 –C ha −1 ) when AA was used, compared with only 0.21 kg N ha −1 (0.03 Mg CO 2 –C ha −1 ) with U. In light of trends toward increased use of U, these results suggest that fertilizer‐induced soil N 2 O emissions may decline in the future, at least per unit of applied N, although further study is needed in different soils and cropping systems. While soil CO 2 emissions were 20% higher under C/C, crop residue from the prior year did not affect soil inorganic N or dissolved organic C during the subsequent season. 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 N 2 O emissions estimates by as much as 35%.