1. Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon EX20 2SB,
2. NERC Radiocarbon Laboratory, Scottish Enterprise Technology Park, East Kilbride G75 0QF,
3. Environmental and Analytical Section, School of Chemistry, University of Bristol, Bristol BS8 1RJ, and
4. Institute of Terrestrial Ecology, Merlewood Research Station, Grange‐over‐Sands LA11 6JU, Cumbria, UK