A SIMPLE KINETIC APPROACH TO DERIVE THE ECOLOGICAL DOSE VALUE, ED(50), FOR THE ASSESSMENT OF CR(VI) TOXICITY TO SOIL BIOLOGICAL PROPERTIES

Three New Zealand soils of contrasting texture, organic matter content and cation exchange capacity (CEC) were amended with K2Cr2O7 solution s, spanning two concentration ranges, 0-5 mu mol Cr(VI) g(-1) soil and 0-50 mu mol Cr(VI) g(-1) soil. Samples were assayed for phosphatase, sulphatase and urease enzyme activities, and for basal respiration, mi crobial biomass C, dimethyl sulphoxide(DMSO)-reducing activity and den itrification, 3 and 60 d after amendment. Extractability of Cr(VI) fro m similarly amended samples was measured from 0 to 100 d. Cr(VI) prove d to be strongly inhibiting of most of the biological properties and i n most instances inhibition was explained by one or both of two simple Michaelis-Menten kinetic models. The first of these (Model 1) simulat ed fully competitive kinetics and the second (Model 2) simulated parti ally competitive kinetics. A single inhibition constant, similar to ED (50) as conceptualized in previous studies, could usually be calculate d for each property in each soil. The properties could be ranked in th e following order of decreasing sensitivity to Cr(VI): denitrification > DMSO-reduction > sulphatase activity approximate to biomass C > pho sphatase activity > urease activity > respiration. For the most sensit ive property, denitrification, ED(50) values range from 63 to 730 nmol Cr g(-1) soil. Soil mineral surface area, organic matter content and CEC influenced the sensitivity of properties between soils. Although t he extent of inhibition often diminished with time, the differences we re generally much smaller than the observed decline in extractability of Cr(VI), indicating that a persistent, long-term inhibition, outlast ing the Cr(VI) itself, had occurred.