Isotope studies on the nitrogen chain : proceedings of the Symposium on the Use of Isotopes in Studies of Nitrogen Metabolism in the Soil-Plant-Animal System jointly organized by the International Atomic Energy Agency and the Food and Agriculture Organization of the United Nations in Co-Operation with the Joint Commission on Applied Radioactivity (ICSU) and held in Vienna, 28 August-1 September 1967. (Proceedings series, 1968)