Comparison of Isothermal Microcalorimetry Versus Respirometry Assays to Evaluate Short- and Long-term Impact of Microplastics on Soil Microbial Activity

Soil microbial activity is a key indicator of soil health, traditionally assessed using methods like enzymatic activity, community-level physiological profiling (CLPP), microbial biomass, and soil respiration. This study introduces isothermal microcalorimetry assessed with the CalScreener™ system as a novel tool for evaluating soil activity and validates its use in assessing the impact of anthropogenic pollutants like microplastics. Soil samples were exposed to microplastic particles from conventional and biodegradable plastic mulches, one of the main sources of microplastic pollution in agricultural soils. Microbial activity was assessed after short-term (10 days) and long-term (12 months) microplastic exposure using respirometry, isothermal microcalorimetry, and microbial CLPP. Isothermal microcalorimetry's effectiveness to assess microbial activity was validated by observing distinct thermograms between active and autoclaved soil, and differences between treatments under basal and glucose-amended conditions. Both respirometry and microcalorimetry showed similar results, revealing higher basal activity in soils with biodegradable microplastics after long-term exposure, compared to conventional microplastics and unamended soil. Isothermal microcalorimetry offers advantages over traditional methods, including shorter assessment periods and the need for smaller soil sample amounts. While CLPP did not detect significant differences in overall soil activity among treatments, it may be a useful technique for characterizing microbial functional traits. This study provides, for the first time, insights into the use of isothermal microcalorimetry as a novel methodological approach to evaluate the potential impact of microplastics on soil biological activity.