Nonlinear response of N₂O flux to incremental fertilizer addition in a continuous maize (Zea mays L.) cropping system

The relationship between nitrous oxide (N 2 O) flux and N availability in agricultural ecosystems is usually assumed to be linear, with the same proportion of nitrogen lost as N 2 O regardless of input level. We conducted a 3‐year, high‐resolution N fertilizer response study in southwest Michigan USA to test the hypothesis that N 2 O fluxes increase mainly in response to N additions that exceed crop N needs. We added urea ammonium nitrate or granular urea at nine levels (0–292 kg N ha −1 ) to four replicate plots of continuous maize. We measured N 2 O fluxes and available soil N biweekly following fertilization and grain yields at the end of the growing season. From 2001 to 2003 N 2 O fluxes were moderately low (ca. 20 g N 2 O‐N ha −1 day −1 ) at levels of N addition to 101 kg N ha −1 , where grain yields were maximized, after which fluxes more than doubled (to >50 g N 2 O‐N ha −1 day −1 ). This threshold N 2 O response to N fertilization suggests that agricultural N 2 O fluxes could be reduced with no or little yield penalty by reducing N fertilizer inputs to levels that just satisfy crop needs.