The timing of nitrogen (N) fertilizer application may influence germination, emergence, and competitiveness of weeds. Research was conducted to determine the influence of total inorganic soil N (N_it) on the germination, emergence, and growth of five weed species. In a greenhouse experiment, seed of five weed species were exposed to four levels of N, and seed germination was measured. In the field, urea ammonium nitrate (UAN 28%) was applied at multiple rates at three spring timings, and N_it, weed emergence, and growth were measured for 21 to 35 d after application (DAA). Germination of the four dicotyledonous and single grass species was not stimulated by 450 ppmw of N compared with the untreated control. In the field, N_it of 112 or 168 kg N ha⁻¹, measured at 7 and 21 DAA, was always greater than N_it in the untreated control. The duration of the available N pulse in the upper 8 cm of soil was dependent on N application rate and timing. At 8 to 16 cm of soil depth, N_it was greater when 168 kg N ha⁻¹ was applied compared with no N at 21 and 35 DAA in 2004. Emergence of common lambsquarters increased as N application rate increased for each application date in 2003, but not in 2004. Emergence of ladysthumb increased with N application rate for the April 15, 2003, date; emergence of giant foxtail increased with N application rate for the April 6, 2004, date. Weed biomass was always greater when 168 kg N ha⁻¹ was applied compared with no N, and at four of six N application dates, when 112 kg N ha⁻¹ was applied. This research shows that spring N fertilizer applications increase N_it and weed growth, but the influence of N on weed emergence is dependent on the weed species, seed source, and environmental conditions.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; ladysthumb, Polygonum persicaria L. POLPE.