Use of Nitrogen-15 Isotopic Techniques to Estimate Nitrogen Cycling from a Mustard Cover Crop to Potatoes

Farmers in the Pacific Northwest are using cover crops such as white and brown mustards (Brassica hirta) in rotation with potatoes (Solanum tuberosum L.) because it reduces potential wind erosion and serves as a biocontrol method for a number of plant pathogens. However, there is no information about the N cycling from the mustard cover crop to potato. We used a 15N isotopic crop residue exchange technique to assess this N cycling potential in situ. We conducted two 15N field studies from 2001 to 2004 in the Columbia Basin in eastern Washington on a Quincy sandy (Xeric Torripsamments) soil containing 4 g kg^-1 soil organic matter under center pivot sprinkler irrigation to assess the fate and N cycling from a mustard cover crop to potato. The established rotation at the site was a winter wheat (Triticum aestivum L.)-sweet corn (Zea mayes L.)-sweet corn-potato rotation. The aboveground mustard cover crop recovered 34 to 51% of the 56 kg 15N fertilizer applied. The total aboveground biomass 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. This study shows that the mustard cover crop can provide 30 to 40 kg N ha^-1 toward the N requirement of a subsequent potato crop.