Effects of Microbes from Coal-Related Commercial Humic Substances on Hydroponic Crop Cultivation: A Microbiological View for Agronomical Use of Humic Substances

Here, coal-related humic substances (HSs) were examined to confirm whether sterilization treatments induce their inferior ability to stimulate lettuce in hydroponic cultivations. Interestingly, a drastic reduction in both lettuce biomass and microbial colony-forming units of the crop culture solutions was observed when the autoclaved HSs were treated. Some microbial genera (i.e., Bacillus and Aspergillus) identifiable in the bare HS-treated hydroponic systems were able to be isolated by direct inoculation of bare HS powders on conventional microbial nutrients, supporting that flourishing microbes in the hydroponic cultivations derive from bare HSs-treated. Moreover, coincubation of some isolated bacterial and fungal strains (i.e., Bacillus and Aspergillus genera) from HSs with lettuce resulted in a significant increase in plant biomass and enhanced resistance to NaCl-related abiotic stresses. Microbial volatile organic compounds renowned for plant stimulation were detected by using solid-phase microextraction coupled with gas chromatography-mass spectrometry. It was finally confirmed that the isolates are capable of utilizing carbon substrates such as pectin and tween 20 or 40, which are relevant to those of microbes isolated from peat and leonardite (i.e., HS extraction sources). Overall, our results suggest that microbiological factors could be considered when commercial coal-related HSs are applied in hydroponic crop cultivations.