Symbiotic N2 fixation (SNF) and transfer of fixed N from clover to grass in a grass-white clover mixture was measured during two years using 15N isotope dilution techniques. White clover derived from 87 to 99% of its N from the atmosphere. The total SNF ranged from 150 to 277 kg N ha-1 yr-1, which corresponded to 34-45 mg N fixed per gram of white clover dry weight. The transfer of fixed N from clover to grass corresponded to approximately 10 kg N ha-1 yr-1
