Electronic Nose Technology to Measure Soil Microbial Activity and Classify Soil Metabolic Status

Abstract

The electronic nose (E-nose) is a sensing technology that has been widely used to monitor environments in the last decade. In the present study, the capability of an E-nose, in combination with biochemical and microbiological techniques, of both detecting the microbial activity and estimating the metabolic status of soil ecosystems, was tested by measuring on one side respiration, enzyme activities and growth of bacteria in natural but simplified soil ecosystems over 23 days of incubation through traditional methodologies, and on the other side VOCs and/or gases evolution in their headspace during the same period through the E-nose. Both kinds of measurements, i.e. biochemical-microbiological and sensorial, succeeded in discriminating between inoculated and non-inoculated microcosms and in distinguishing different phases of bacterial growth and activity during the incubation. E-nose responses were highly and significantly correlated with all catalytic activities, respiration and different phases of bacterial growth. Hence, the E-nose was proved able to detect microbial activity in natural soil ecosystems. The metabolic status of these soil ecosystems was calculated on the base of some of the parameters previously tested in order to obtain a metabolic index (MI), ranging between 0 and 1. These MIs were then grouped into 3 classes, corresponding to a low, medium and high metabolic status. All the other measures were used as a multi-type data set in a partial least square discriminant analysis to generate a model capable of classifying the soil microcosms at different sampling dates according to the 3 classes of MIs. The resulting outcomes confirmed that the E-nose technology combined with both metabolic and biomass parameters can altogether represent reliable indicators of the metabolic status of soil ecosystems.