Proteinaceous and humic fluorescent components in chloroform-fumigated soil extracts: implication for microbial biomass estimation

Excitation-emission matrix fluorescence spectroscopy coupled with parallel factor analysis was employed for characterisation of chloroform fumigation-extractable soil organic matter, commonly used for soil microbial biomass estimation. This allowed, for the first time, to discriminate between humic-like (i.e. noncellular) and microbial protein-like, fumigation-extractable components, challenging the presumption that only microbial constituents contribute to the fumigation flush of C serving as a proxy measure for soil microbial C. A Vertisol was assayed under increasing K2SO4 extractant molarity (0-0.5 M), which allowed increasing organic matter extractability levels and the association of these increases with relative contributions from microbial versus humic sources. Expectedly, protein-like fluorescence was found negligible in the nonfumigated soil extracts while comprising the bulk of fluorescence of the material becoming K2SO4-extractable due to fumigation. Nevertheless, fumigation also led to an increase in extractable concentrations of humic-like components, showing that not only microbial constituents were fumigation-extractable. Humic-like fluorescence in the fumigation flush generally increased with decreasing K2SO4 molarity, being minimal at 0.25 M K2SO4. Considering also the preference for maximal flush of extractable soil organic matter, indicative of maximal fumigation efficiency, the use of 0.25 M K2SO4 seems preferable for extraction of microbial biomass with minimal interference from humic substances, for the investigated Vertisol. The presented working framework for assessment and alleviation of interference from humic substances in microbial biomass estimation is recommended to be applied specifically to any soil type before routine monitoring.