Nitrogen Tracers in Nitrogen Cycle Studies—Past Use and Future Needs

Abstract Nitrogen research is directed toward two main objectives, food and fiber production and environmental control. To achieve N balance in productive ecosystems, better quantitative estimates of N transformation rates are needed. Nitrogen tracers are indispensable for making many of these estimates. Either 15 N‐depleted or 15 N‐enriched materials can be used. The use of 15 N‐depleted materials is limited to studies where dilution form other N is less than 2,000‐fold, but these materials are potentially available in ton amounts. Use of variations in natural 15 N abundance may be useful in observing qualitative relationships among N cycle processes over large areas or extremely long time periods. Such use is questionable for obtaining quantitative information for short‐term N transformation processes. Obvious information gaps are quantitative data on atmospheric N 2 fixation and denitrification in cropped field soils and N transformation data for many other ecosystems. A program for computer data retrieval and correlation is outlined.