Nitrogen is a component of the proteins and nucleic acids that are the building blocks of life. Of course, there is plenty of nitrogen in the biosphere but the amount that is available to support plant growth is often limited. This is especially so in intensive agriculture and world crop production is dependent on nitrogen fertiliser inputs. As a consequence, farmers rely on expert advice coupled with their own experience as to when and how much of which N fertiliser to add to the soil. This, in turn, becomes the purview of environmentalists and policy makers who are concerned about the broader impacts of fertiliser applications on the quality of the soil, water and atmosphere. For rational decision making by all these interested parties the rates and routes by which nitrogen is transformed must be measured accurately and the implications for plant yield and environmental health understood...