Just as photosynthesis or chemosynthesis must reduce carbon dioxide in the atmosphere before it can be used in biological reactions, so must biological nitrogen change from elemental N 2 to a ‐3 level, as in ammonia, NH 3. In terrestrial and aquatic systems, reduced nitrogen is often a limiting nutrient for plant growth. About half of all global nitrogen fixation occurs industrially in a process that requires a metal catalyst at 500 atmospheres pressure and at 300° Celsius. In contrast, biological nitrogen fixation takes place in a much less extreme environment (about 25° and 1 atmosphere pressure) in the roots of leguminous plants or in bacteria, using enzymatic catalysts. Eventually it returns to the atmosphere as N 2. The overall process is referred to as the nitrogen cycle.