Assessment of the 1 M NH4NO3 extraction protocol to identify mobile forms of Cd in soils

Extraction of soils by leaching 20 g in 50 ml of unbuffered 1 M NH4NO3 for 2 h has become the standard German protocol (DIN 19730) to estimate mobile and potentially hazardous forms of trace elements. This protocol was examin ed using soil controls of chernozem and podzol character collected in the C anadian Prairies and Ontario. It was found that this procedure was not robu st with respect to change in sample weight per unit volume of reagent or to contact time. A 3- to 15-fold increase in Cd extracted was produced by dec reasing sample weight from 10 g to 1 g in 50 mi of reagent. This change in extractable-Cd was more pronounced with increasing pH of the soil, ranging from 5.2 to 8.1 amongst the control samples. Leach period was less signific ant in determining results but longer time (1-3 h) generally produced lower extractable-Cd, suggesting readsorption. Lack of stability of Cd in this r eagent was further demonstrated by spiking experiments where Cd was added a t the beginning of the leach. Recoveries of Cd at 50 and 200 ppb were below 20% for soils of pH 6.1 and 6.9. Such severe adsorption of Cd was not enco untered with another unbuffered reagent, 1 M NH4Cl, operating at a similar initial pH of 5. Cadmium levels extracted from soils by NH4Cl and 0.1 M Na4 P2O7 were comparable and could be used to predict uptake in durum wheat on the Canadian Prairies, whereas data by the 1 M NH4NO3 leach were generally near the detection limit of 0.1 ppb (ng g(-1)) Cd. Readsorption of Cd and l ack of equilibrium in the 1 M NH4NO3 extraction could lead to underestimati on of plant-available Cd in neutral and alkaline soils. (C) 1998 Elsevier S cience B.V. All rights reserved.