Growth Responses to Ammonium-N, Nitrate-N and Defoliation in Agropyron dasystachyum from the Canadian Mixed Grassland

-Responses to defoliation, nitrogen form (NH4 or NO3) and supplemental nitrogen (0-225 mg kg-' dry soil weight) were studied in Agropyron dasystachyum (Hook.) Scribn., a dominant grass in Canadian mixed prairie. Plants were grown in the greenhouse in soil containing nitrapyrin to retard nitrification of ammonium. Shoot, root and crown biomasses increased with nitrogen supply; root biomass decreased at the highest nitrogen levels. Defoliation reduced all biomass components, including cumulative biomass (calculated by adding biomass removed in the clipping treatment to regrowth), but increased shoot/ root ratios. Biomass allocation to shoots increased when nitrogen was supplied as NH4, but remained constant when supplied as NO3. Defoliation reduced tillering, but only in plants receiving additional nitrogen. Defoliated plants were able to compensate fully for losses in leaf area in part through increases in specific leaf area (cm2 g-'). Defoliation and high levels of NH4 independently shifted plant growth pattern toward enhanced production of photosynthetic tissues.