Pine wood and barley straw biochar amendments to Kettering and Cameroon sandy silt loam soils (15, 30, or 150？mg biochar g？1 soil) caused significant reductions (up to 80%, ) in concentrations of substrate and extractable product in soil dehydrogenase and phosphomonoesterase enzyme assays. Likely this was caused by increased solid-phase sorption of the chemicals in the presence of the biochars under assay conditions. The relationship between assay chemical sorption and biochar concentration depended on the chemical, soil type, biochar type, and their interactions; hence, no uniform correction factor could be derived. This biochar impact on assay constituents will limit the identification of genuine biochar effects on soil enzymes. It is recommended that the assumption of saturating substrate concentrations be checked and that product standards be matrix-matched when conducting enzyme assays with biochar-amended soil. 1. Introduction Soil extracellular and intracellular enzymes are the catalysts of organic matter decomposition. Understanding the effect of biochar on the activity of these key enzymes has been identified as a research priority [1]. Soil enzyme activity is typically quantified using assays of the potential activity, where artificial substrates are added at saturating concentrations and undergo enzyme-catalysed transformation to form coloured or fluorescent products [2]. As biochars have been shown to possess a high sorptive affinity for organic chemicals (e.g., [3]), this raises the question of whether biochar might sorb both (i) the organic chemicals added as artificial substrates and (ii) the products in soil enzyme assays. Some studies have considered this possibility [4–6] but the effect of biochar on the behaviour of both the products and substrates used in enzyme assays has not yet been systematically or explicitly quantified under assay conditions. This information is needed if the effects of biochar on soil function are to be correctly identified. Therefore, we examined the effect of biochar addition to soil on (a) the concentration of assay substrate under assay conditions and (b) the extractability of assay product for two enzyme assays that are commonly used to characterize soil quality: the p-nitrophenyl phosphate (pNPP)-based phosphatase assay [7] and the iodonitrotetrazolium chloride (INT)-based dehydrogenase assay [8, 9]. 2. Materials and Methods 2.1. Soil and Biochar Experiments were conducted using two soil types (Kettering sandy silt loam (total C = 1.8%, pH 7.3) and Cameroon sandy silt loam (total C = 1.3%, pH 5.4)) and
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