The15N methodology in estimating N2 fixation by vetch and pea grown in pure stand or in mixtures with oat
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Cereal-legume mixtures are frequently the best management decision for forage production instead of growing crops in pure stands. Nitrogen fertilization of cereal-legume mixtures is questionable since combined nitrogen could depress N2 fixation by legumes. The objectives of this study were (1) to examine the effect of N fertilization on N2 fixation by vetch and field peas in pure and in mixed stands with oats, and (2) to examine if there is any transfer of N from legumes to associated cereals. The field experiment was conducted for two growing seasons. The treatments were pure stands of vetch, pea and oats, and the mixtures of the two legumes with oats at the seeding ratios 90:10 and 75:25, fertilized with labelled15N at the rates of 15 and 90 kg N ha−1.
Nitrogen fertilization of 90 kg N ha−1 suppressed N2 fixation in both legumes grown in pure and in mixed stands. Crops grown in mixtures in many instances had lower atom %15N excess. Whether this was due to high N2 fixation in the case of legume and transfer in the case of oat or the differences were due to practical problems of the15N technique is not clearly shown by the results, so based on the literature the aspect is discussed as well as the precautions which should be considered in using the15N technique in such studies.
Key wordsfertilizer N legume-cereal mixtures 15N, N2 fixation N transfer pea vetch
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