Excessive N fertilizer use leads to enhanced nitrous oxide (N 2O) emissions from cotton ( Gossypium hirsutum L.) production systems. The objective of the study was to quantify nitrous oxide emissions from the ridges within a furrow-irrigated field during the growth of a cotton crop that had been fertilized with urea at 0, 120, 200, or 320 kg N ha -1. No measurements were taken from the furrows; we assumed similar N 2O emissions from the furrows in this system. The N 2O emissions increased exponentially with N fertilizer rate. Over the cotton-growing season, N 2O emissions totalled 0.51, 0.95, 0.78, and 10.62 kg N 2O-N ha -1, for the four respective N fertilizer rates. The cotton phase of the cotton-faba bean ( Vicia faba L.)-fallow rotation was the main contributor to the total N 2O emission. Over this 2-yr rotation, emissions totalled 1.23, 1.65, 1.44, and 11.48 kg N 2O-N ha -1. However, <0.35% of the N fertilizer applied was emitted as N 2O for the complete rotation where the economic optimal N fertilizer rate for the cotton crop was not exceeded. More than 3.5% of the N fertilizer was emitted as N 2O where 320 kg N ha -1 was applied, which was estimated to represent about 11 kg N ha -1. These data indicate that supra-optimal N fertilizer applications increase the net emissions of N 2O from the ridges in high-yielding furrow-irrigated cropping systems. The N 2O emissions could be decreased further by reducing or eliminating the time in fallow.
