Assessment of Bioavailable Nickel in Soil Using Nickel-Depleted Urease
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Nickel (Ni) is a required element but may become toxic to plants, animals and humans if normal levels are exceeded. To assess the health and environmental effects of Ni, the bioavailable, not total, concentration of Ni in soil must be accurately measured. Urease has an absolute requirement for Ni to function, and this was used to develop a method to assess Ni bioavailability in soil. Bacteria with urease activity were enriched from Spinks sandy soil by growing the cells in Luria Broth medium. This created a culture with high amount of potential urease activity but with low actual activity because of Ni limitations. The bacterial culture was inoculated into a test soil that is composed of acid-washed Spinks sandy soil as a carrier soil and the soil with unknown Ni bioavailability, and was equilibrated with bioavailable Ni in soil for four hours. The urease response to the bioavailable Ni was measured using steam distillation. Resaults showed that urease activity was positively correlated to Ni bioavailability. The following equation could be used to calculate Ni bioavailability from urease activity when Ni bioavailability was in the range from 0 to 0.06 mM: Ni Bioavailability (mM) = Urease Activity (μg/30 ml culture/2 h)/k/f where k is equal to 15646 μg/30 ml culture/2 h/mM, and f is the percentage of unknown soil in the test soil. The effects of soil pH and soil organic matter content on Ni bioavailability were evaluated through urease bioassay. Results showed that a change in soil total C content from 0.009% to 12.7% decreased Ni bioavailability by 50%, and a change in soil pH from 4.15 to 9.94 decreased Ni bioavailability by 70%. Urease bioassay is recommended when measuring Ni bioavailability in soil because it is accurate, simple, sensitive, and can be used in most laboratories.
This was an oral presentation at the 2006 annual ASA/SSSA/CSSA meetings in Indianapolis, IN.
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