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

Overwinter N leaching increases the potential for NO3 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 NH4 and NO3 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.