Weed residues can impact nitrogen (N) cycling in agro-ecosystems thatprimarily utilize POST weed control. Quantifying this potential N source orsink may influence weed control and fertilization practices. A laboratoryexperiment measured the rate and quantity of N release from commonlambsquarters, common ragweed, and giant foxtail. Weeds were grown in thefield at four N rates (0, 67, 134, or 202 kg N ha−1) andcollected at two weed heights (10 or 20 cm) to give a range of residuechemical composition. Residue chemical composition parameters of carbon : N(C : N) ratio and total N, nitrate-N, acid detergent fiber, and neutraldetergent fiber concentration were measured and correlated with N release.Nitrogen release from weed residue mixed with soil was determined over a12-wk period. Nitrogen was released from all weed residues at 12 wk. Priorto 12 wk, N was immobilized by giant foxtail grown with no N application.Prior to 4 wk, N was immobilized by 20-cm weeds grown with no N application.Nitrogen release from weed residue was negatively correlated with C : Nratio. Weed residue with a C : N ratio of < 19 (weeds grown with Napplication and 10-cm weeds) released 25 to 45% total N concentration within2 wk and may contribute N within the growing season. Weed residue with a C :N ratio > 19 (giant foxtail and 20-cm weeds grown with no N) initiallyimmobilized N and may not contribute N within the growing season.