Sources and methods of use of organic nitrogen (N) in the southern Great Plains (SGP) need testing to find alternatives to increasingly expensive inorganic fertilizer. We examined the function of grass pea ( Lathyrus sativus L.), a cool-season pulse, as a preplant N source for continuous, no-till winter wheat ( Triticum aestivum L.). The study was conducted in central Oklahoma (35degrees40?N, 98degrees00?W, elevation 414 masl) from 2004 to 2008, on three replicate blocks of four experimental plots (6 by 10 m). Inoculated grass pea seed ('AC-Greenfix') was sown during late summer fallow (mid-August) in one randomly chosen plot per block (75 kg ha -1, 60-cm rows; 75% germination). Three additional plots per block mimicked summer fallow with 0 (control), 40, or 80 kg N ha -1 inorganic fertilizer applied. All treatments were repeated on the same plots throughout the study. Samples were collected from grass pea plots at flowering in early October to define aboveground biomass and analyzed for N concentration and digestibility. Aboveground biomass of grass pea was shredded with a flail mower and left on the soil surface, fertilizer treatments were applied, and wheat ('Jagger') was sown (100 kg ha -1, 20-cm rows). Aboveground wheat biomass was collected at three growth stages (elongation, flowering, physiological maturity) and analyzed for N concentration. Grass pea aboveground biomass contained enough N to meet the needs of wheat at planting in only 1 yr. Wheat biomass and amounts of N in wheat aboveground biomass in response to grass pea were intermediate between the 0 and 40 kg applied N ha -1, as was yield and N accumulated in wheat grain. Therefore, grass pea was not effective as a preplant source of N for continuous no-till winter wheat in the SGP. Additional research is required to define factors that limit the function of grass pea as a source of N for continuous no-till winter wheat and its potential function in other crop rotations.
