Weed shoot N concentration was not affected by N application timing-tillage intensity (P40:05) but was affected by N source in all years (Po0:05). The unfertilized control was always among treatments resulting in the lowest N concentration in all weeds (Fig. 1), indicating that N uptake by weeds increased with added N.T. arvenseand D. sophiashoot N concentrations were greater than those of B. tectorum (Po0:05), indicating that broadleaved, as well as grass weeds, potentially are serious competitors with crops for soil N