Nitrogen Fixation by Irrigated Berseem Clover versus Soil Nitrogen Supply

Abstract

AbstractNitrogen fixation was estimated by the difference method for two cultivars of berseem clover (Trifohum alexandrinum L.), using annual ryegrass (Lolium multiflorum Lam.) as the control in field trials from 1983 to 1988. Five or six cuttings were obtained each year under supplemental irrigation, and dry matter production and uptake of nitrogen into harvested forage were measured.Estimates of nitrogen fixation by berseem clover ranged from 272 to 400 kg N/ha depending on year and cultivar, while plant N derived from soil (based on N uptake by the ryegrass) ranged from 50 to 235 kg N/ha. Graphs of nitrogen fixed versus available soil nitrogen had a reverse linear‐plateau shape with a plateau at lower soil nitrogen levels that broke sharply in a linear decline as available soil N surpassed a minimum threshold. Such a two‐phase response suggests that nitrogen fixation by the clover was limited by soil N supply above this threshold and by other limiting factors at low soil N levels, perhaps genetic traits controlling carbon assimilation or environmental limitations other than nitrogen. Seasonal nitrogen process patterns showed abundant nitrogen fixation by the clover after it had depleted most of the soil nitrogen supply.