Effect of Inoculum Standardization on Community Level Physiological Profiles Of Compost Samples

Abstract

The potential of Community Level Physiological Profiles (CLPPs) as a compost maturity test is significant, given the sensitivity and the simplicity of the assay. Nevertheless, application of the technique to compost requires more methodological work because of the fast changes in physicochemical and microbiological properties and the dependence of the rate of color development on the inoculum density. The aim of this work was to investigate the reliability of a new approach to standardize inoculum density and the effects of different data interpretation and storage conditions on CLPPs of compost samples with different degree of stabilization. Compost samples (fresh, air dried, frozen and air dried-rewetted) with different degree of stabilization were extracted with a two-batch procedure and then inoculated on Biolog Ecoplates. Standardization of inoculum density was performed according to microbial biomass content determined by substrate induced respiration (SIR) on fresh samples. Microplates were incubated at 30°C in the dark and optical density (OD) was measured at 592 nm every 8 hours for 7 days. Principal component analysis (PCA) was performed on various data sets: OD values at fixed average well color development (AWCD), kinetic parameters and area under the OD curve as a function of time. PCA of fresh samples showed that classification and ordination of samples according to their age were satisfactory with fixed AWCD and area under the curve data sets, suggesting the reliability of SIR for the standardization of inoculum density. These results support the most universally applicable approach to data collection, implying approximate standardization of inoculum density and continuous plate reading, but, at the same time, underline the importance of the data set construction for PCA analysis. Ordination patterns obtained from air dried samples showed a good resemblance with those obtained from fresh ones, suggesting that analyses aimed to evaluate the degree of compost maturity could be successfully performed on samples with low water content.