 Nitrogen – crucial to all forms of life. It's an essential plant nutrient, with a major role in crop productivity. Yet nitrogen is often unavailable in the soils for the crops. As a result, the world's farmers apply up to 112 million tons of nitrogen fertilizers each year, fertilizers with an estimated cost of 41 billion US dollars. While nitrogen is the key component of all agricultural production systems, if used inappropriately, it can pollute waters, lead to greenhouse gas emissions, and further accelerate climate change. The ability to trace nitrogen as it moves through agricultural ecosystems is therefore essential to maximise its use while protecting the environment. Nitrogen 15, a stable isotope of nitrogen, is such an ideal tracer. It can be easily measured in soils, plants, water and air, both in the laboratory and in nature. The joint FAO IAEA Division of Nuclear Techniques in Food and Agriculture assists member states globally in the use of Nitrogen 15, introducing methods to trace the fate of nitrogen fertilizers in soils, plants and the environment to ensure that agricultural practices are environmentally sustainable. Quantify the efficiency with which plants use the applied nitrogen fertilizer so that farmers can minimise the quantity they apply. Determine how much nitrogen, legume crops capture from the atmosphere, but then enhances soil fertility and reduces the need for chemical fertilizers. Only through improved agricultural management practices will it be possible to reap the full benefits of this valuable nutrient resource, curtail its negative impact on the environment and ensure global food security.