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. 7H(2)O solutions, spanning the concentratio n range, 0-50 mu mol As[V] g(-1) soil. Samples were assayed for phosphatase , sulphatase and urease enzyme activities and for basal respiration, microb ial biomass C (by substrate-induced respiration, SIR), dimethyl sulphoxide (DMSO)-reducing activity and denitrification, 3 and 60 d after amendment. O nly phosphatase, sulphatase and DMSO-reducing activities were consistently inhibited by As[V], the remaining properties were generally unaffected or w ere stimulated. When inhibition occurred, it could in most instances be exp lained by one or both of two simple Michaelis Menten kinetic models. The fi rst of these (model 1) described fully competitive kinetics and the second (model 2) described partially competitive kinetics. A single inhibition con stant, similar to ED50 (ecological dose) as conceptualised in previous stud ies, could be calculated. In comparison with heavy metals, As[V] was not a potent inhibitor of soil biochemical properties, with ED50 values ranging f rom 2.18-556 mu mol As g(-1) soil (0.163-41.7 g kg(-1)). Generally, phospha tase was the most sensitive property, probably due to the structural simila rity of phosphate and arsenate. Basal respiration and denitrification were the most activated properties, the former increasing linearly with increasi ng As[V] concentration. Soil textural characteristics influenced the sensit ivity of properties between the different soils; the coarsely textured sand y 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 ame ndment, with diminished inhibition or enhanced activation at 60 d compared with 3 d. (C) 1999 Elsevier Science Ltd. All rights reserved.