Nitrogen Recycling by Nonleguminous Winter Cover Crops to Reduce Leaching in Potato Rotations

Overwinter N leaching increases the potential for NO 3 contamination of ground water under irrigated desert soils. Our objectives were to (i) identify winter cover crops for recovering N and (ii) determine the N availability following green manure cover crops relative to N uptake by a succeeding potato ( Solanum tuberosum L.) crop. A 2‐yr field study was conducted on commercial fields on Quincy loamy sand (mixed mesic Xeric Torripsamments). Cover crops were seeded following sweet corn ( Zea mays L.) as fall‐incorporated sudangrass ( Sorghum bicolor L., ‘Sordan’); fall‐ and spring‐incorporated white mustard ( Brassica hirta Moench, ‘Martegena’); and spring‐incorporated wheat ( Triticum aestivum L., ‘Stevens’), rapeseed ( B. napus L., ‘Jupiter’), and rye ( Secale cereale L.). Potato (‘Russet Burbank’) was planted 3 to 5 wk after spring incorporation of green manures. All cover crops except sudangrass accumulated 112 to 142 kg N ha −1 when planted in August at Plymouth, WA, but N uptake was decreased by >50% when planted in September at Quincy, WA. Overwintering cover crops lowered soil mineral N at 0 to 180 cm by 155 kg N ha −1 compared with bare fallow at Plymouth. Fall‐incorporated mustard released greater inorganic N over the winter compared with spring incorporation. Soil NH 4 and NO 3 increased following overwintering cover crops by potato planting, providing timely increases in N availability to the potato crop. Winter cover crops can improve N cycling and reduce the amount of N below the root zone in potato‐based rotations.