Technical Abstract: It has been reported that some areas of the Pacific Northwest U.S. were among areas with the greatest potential for groundwater impacts from NO3-N leaching. Farmers in the Pacific Northwest are using cover crops such as white and brown mustards in rotation with potatoes because it reduces potential wind erosion and serves as a biocontrol practice. However there is no information on N cycling from cover crop mustard to potatoes. There is the need to assess the N cycling from the cover crop mustard to the potato. We used a new 15N isotopic crop residue exchange technique to assess this N cycling potential “in situ”. Two two-year 15N field research studies were conducted at the USDA-ARS Paterson, WA research site on a Quincy sandy (Xeric Torripsamments) soil containing 0.4% SOM under sprinkler irrigation. The established rotation at the site was a winter wheat (Triticum aestivum L.) - sweet corn (Zea mayes L.)- sweet corn – potato (Solanum tuberosum L.) rotation. We established two 15N field research plot studies from 2002 to 2004 to assess the potential of N cycling from cover crop mustard (Brassica hirta) to potato. The mustard cover crop recovered 34 to 52% of the 56 kg 15N fertilizer applied. The total yield and N uptake by the cover crop ranged from 4.6 to 7.5 Mg ha-1 and 92 to 142 kg N ha-1, respectively. About 29 % of the N in the cover crop was cycled and absorbed by the following potato crop. There is the potential to budget 30 to 40 kg N ha-1 in the cover crop as a source for N to the potato. The cover crop will contribute to reduce the N losses in irrigated potato systems, increase nitrogen use efficiency (NUE) and sequester N in the soil organic fraction for these sandy soil systems.