**6. Conclusion**

The use of nitrogen-fixing legume-based leys, whether they are used for grazing, conservation or mulched to build soil fertility, is the basis of most organic systems. Their use is enshrined in the organic standards, which require the inclusion of legumes in rotations. The wider benefits of legumes, particularly in providing food for pollinators, are also increasingly being recognized. Globally, the amount of carbon di oxide respired from the root systems of N2-fixing legumes could be comparable to, or higher than, the carbon di oxide generated during nitrogen-fertilizer production. However, the carbon di oxide respired from the nodulated roots of legumes originated from the atmosphere via photosynthesis, so any of the carbon di oxide that was not subsequently recaptured by the plant and eventually escaped from the legume canopy to the atmosphere would essentially be carbon neutral. By contrast, all the carbon di oxide released during the synthesis of fertilizer nitrogen would be derived from fossil energy and represents a net contribution to atmospheric concentrations of carbon di oxide.
