Long‐Term Lentil Green‐Manure Replacement for Fallow in the Semiarid Northern Great Plains

Summer fallow results in inefficient precipitation use and could potentially be replaced with a green manure (GM) crop that reduces fertilizer N application. A 12‐yr study on a Williams loam (fine‐loamy, mixed, superactive, frigid Typic Argiustoll) was used to determine GM impacts on soil‐N fertility, soil organic carbon (SOC), water use, yield, and water productivity (WP) of spring wheat (Triticum aestivumL.). Lentil (Lens culinarisMedikus cultivar Indianhead) was grown in rotation with nonfertilized spring wheat and killed either mechanically (GMMF) or chemically (GMCF). Inorganic N fertilizer treatments included annually cropped wheat (ACW), and wheat‐fallow rotations (WFR) with mechanical (MF) or chemical (CF) fallow management. Low soil nitrate during the first 5 yr of the study resulted in a 33% decrease (P< 0.05) in wheat yield in GM compared to WFR. During the latter 6 yr, wheat yield differed by 2%, due to 26% greater spring soil nitrate (0–0.6 m) in GM than WFR, and growing season change in nitrate N showed a 2.2‐fold N‐cycling advantage of GM over WFR. Water use during nonwheat periods was similar in GM and WFR when lentil was killed at full bloom, but when grown to lower pod set GM used 20% more water than WFR. After 11 yr, SOC in the surface 15 cm was 9.1% greater in treatments with chemical management than those with mechanical management. Green manure, with proper management, maintains WP and offsets fertilizer N needs after about three cropping cycles compared with traditional wheat–fallow rotations.