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

Abstract

Quantifying N2O 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 N2O 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 NH3 (AA) or urea (U). During three growing seasons, N2O emissions from corn following corn were nearly identical to corn following soybean. In both systems, however, 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 annual emissions of 0.78 kg N ha−1 (equivalent to 0.11 Mg CO2–C ha−1) when AA was used, compared with only 0.21 kg N ha−1 (0.03 Mg CO2–C ha−1) with U. In light of trends toward increased use of U, these results suggest that fertilizer‐induced soil N2O 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 CO2 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 N2O emissions estimates by as much as 35%.