Simple kinetic approach to determine the toxicity of As[V] to soil biological properties

Three New Zealand soils of contrasting texture, organic matter content and CEC were amended with Na2HAsO4(.)7H2O solutions, spanning the concentration range, 0-50 micromol As[V] g(-1) soil. Samples were assayed for phosphatase, sulphatase and urease enzyme activities and for basal respiration, microbial biomass C (by substrate-induced respiration, SIR), dimethyl sulphoxide (DMSO)-reducing activity and denitrification, 3 and 60 d after amendment. Only phosphatase, sulphatase and DMSO-reducing activities were consistently inhibited by As[V], the remaining properties were generally unaffected or were stimulated. When inhibition occurred, it could in most instances be explained by one or both of two simple Michaelis Menten kinetic models. The first of these (model 1) described fully competitive kinetics and the second (model 2) described partially competitive kinetics. A single inhibition constant, similar to ED(50) (ecological dose) as conceptualised in previous studies, could be calculated. In comparison with heavy metals, As[V] was not a potent inhibitor of soil biochemical properties, with ED(50) values ranging from 2.18-556 micromol As g(-1) soil (0.163-41.7 g kg(-1)). Generally, phosphatase was the most sensitive property, probably due to the structural similarity of phosphate and arsenate. Basal respiration and denitrification were the most activated properties, the former increasing linearly with increasing As[V] concentration. Soil textural characteristics influenced the sensitivity of properties between the different soils; the coarsely textured sandy soil was both the most biochemically sensitive to and the least sorptive of As[V]. For one soil only there was a consistent effect of time since amendment, with diminished inhibition or enhanced activation at 60 d compared with 3 d.