1. a Departamento de Solos , UFRGS , Caixa Postal 776, Porto Alegre , 90001–970 , RS , Brazil
2. b USDA, ARS , Appalachian Soil and Water Conservation Research Laboratory , Beckley , West Virginia , 25802–0867
3. c USDA, ARS, Environmental Chemistry Laboratory , Beltsville Agricultural Research Center , Beltsville , Maryland , 20705–2350