Nitrogen Fixation by Red Clover (Trifolium pratenseL.) Grown in Mixtures with Timothy (Phleum pratenseL.) at Different Levels of Nitrogen Fertilization

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Abstract
In a field study in south-western (Særheim) and northern Norway (Våg⊘nes) the methods of 15N isotope dilution and N-difference were used to determine symbiotical N2 fixation in red clover grown in a mixture with timothy. Where no N fertilizer was used 210 kg N ha−1 was fixed at Særheim and 128 kg at Våg⊘nes, as estimated by isotope dilution. Application of inorganic N strongly depressed the yield of fixed N, mainly due to a decrease in the proportion of clover in the sward. The percentage of N derived from symbiosis was less influenced by the N fertilization. Estimates of fixed N yield by the two methods were significantly correlated. The amounts of fixed N estimated by the N-difference method were, however, significantly higher than the N yields determined by the isotope dilution technique.