Weed residues can impact nitrogen (N) cycling in agro-ecosystems that primarily utilize POST weed control. Quantifying this potential N source or sink may influence weed control and fertilization practices. A laboratory experiment measured the rate and quantity of N release from common lambsquarters, common ragweed, and giant foxtail. Weeds were grown in the field at four N rates (0, 67, 134, or 202 kg N ha−1) and collected at two weed heights (10 or 20 cm) to give a range of residue chemical composition. Residue chemical composition parameters of carbon : N (C : N) ratio and total N, nitrate-N, acid detergent fiber, and neutral detergent fiber concentration were measured and correlated with N release. Nitrogen release from weed residue mixed with soil was determined over a 12-wk period. Nitrogen was released from all weed residues at 12 wk. Prior to 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 : N ratio. Weed residue with a C : N ratio of < 19 (weeds grown with N application and 10-cm weeds) released 25 to 45% total N concentration within 2 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) initially immobilized N and may not contribute N within the growing season.