There is growing interest in the use of annual legumes as green manure crops to replace conventional summerfallow in the spring wheat production systems of western Canada. A series of field experiments was established at three sites in western Canada (Lethbridge, Swift Current, and Saskatoon) in each of two seasons to quantify the N contribution of green manure to subsequent crops and organic matter reserves. 15N-labelled plant material from two annual legume species [Tangier flatpea (Lathyrus tingitanus 'Tinga') and lentil (Lens culinaris 'Indianhead')], as well as 15N-labelled ammonium sulfate, was applied to field microplots in midsummer. The following spring, an additional ammonium sulfate treatment was established and all plots were seeded to spring wheat (Triticum aestivum 'Leader' or 'Katepwa'). On average, the wheat crop recovered 14% of the green manure N compared with 36% of the fertilizer N. Conversely, the relative contribution of the green manure to the organic N pool in the surface soil layer was approximately twice that of inorganic fertilizer. This residual organic N was relatively recalcitrant to further mineralization, as was evident from minimal uptake of applied N in the second year after application. These results suggest that annual legumes can be a significant source of N to subsequent crops in the rotation, provided that N yields are sufficient. The primary advantage of green manure production, however, may be the long-term replenishment of stable organic N reserves in the soil. Key words: N mineralization, organic matter, 15N, annual legumes, green manure, lentil, pea