Technical Abstract: 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-year study near Culbertson, MT on a Williams loam (fine-loamy, mixed Typic Argiboroll) determined GM impacts on soil-N fertility, soil organic carbon (SOC), water use, yield, and water productivity of spring wheat (Triticum aestivum L.). Lentil (Lens culinaris Medikus cv. Indianhead) was grown in rotation with non-fertilized spring wheat and killed by mechanical (ML) or chemical (CL) methods. Inorganic N fertilizer treatments were annually cropped wheat (AW), and wheat-fallow rotations (FR) with mechanical (MF) or chemical (CF) fallow management. Wheat yield during the first five years of the study was 33% less on GM than FR partly due to lower soil nitrate. During the latter six years wheat yield differed by 2%, due in part to 26% greater spring soil nitrate (0-0.6 m) on GM than FR. A soil nitrogen mass balance approach for the latter six years showed a 2.2-fold N-cycling advantage of GM over FR. Water use during non-wheat periods was similar in GM and FR when lentil was killed at full bloom, but when grown to lower pod set GM used 20% more water than FR. After 11 yr SOC in the surface 15 cm declined 8%, 10%, and 9% with GM, FR, and AW management, respectively. Green manure, with proper management, maintains water productivity, offsets fertilizer N needs after about three cropping cycles, and reduces SOC depletion compared with traditional wheat-fallow rotations.