.The response of nitric oxide (NO) emissions to nitrogen (N) input in agricultural ecosystems is generally assumed to be linear, but the availability of inorganic soil N exceeding crop N demands may cause a threshold response of NO. We conducted a four-rotation field measurement with discrepant N fertilizer rates during the wheat season of the rice-wheat rotation in the Taihu Lake region in China to test the hypothesis. Across all years, a nonlinear model well characterized the response of cumulative NO emissions to N fertilizer rates. The direct emission factors of NO induced by fertilizer also increased nonlinearly with increasing N doses, with a mean of 1.06% (ranging from 0.54% to 1.49%). NO emissions were low (