1. a Tropical Research and Education Center , University of Florida , 18905 S.W. 280 Street, Homestead , FL , 33031 , U.S.A.
2. b Department of Crop and Soil Sciences , Michigan State University , East Lansing , MI , 48824 , U.S.A.
3. c University of California , Stable Isotope Facility, 122 Hunt Hall, One Shield Avenue, Davis , CA , 95616-8515 , U.S.A.