Plant polyphenolics (tannins) enter soil by leaching, decomposition, and root exudation where they may move with water, sorb to soil, or be decomposed by microbes.  Polyphenolics appear to have both positive and negative consequences for soil microorganisms, with reported roles as carbon sources, as determinants of nitrogen mineralization, and as inhibitors of microbial growth via toxicity.  The compounds loosely grouped as “tannins” are defined by their polyphenolic character and their ability to precipitate protein but this simple functional definition obscures the diversity of tannins that contributes to their distinctive chemical attributes and biological activities.  We hypothesized that determining how individual, defined polyphenolics affect soil microbial growth would provide us with better insights into the ecological roles played by this major group of natural products.  We used purified polyphenolics, representing different structural subclasses of tannins, as substrates for soil microbial growth using forest soil collected at the Ecology Research Center, Oxford OH.  We measured the respiratory response using the Microresp© 96 well plate system, following the guidelines in the kit for soil preparation and amounts of substrate.  The tannins were added as aqueous solutions to replicate samples of the soil in the 96 well plate.  The samples were incubated for a total of 200 h at room temperature, with CO₂ determined every 24 h.  

Results/Conclusions

Pure polyphenolics such as pentagalloyl glucose and epigallocatechin gallate were respired more slowly than simple carbon substrates such as glucose.  Unlike glucose, the phenolics were only partially oxidized to CO₂.  Each compound we evaluated was degraded at a unique rate.  We found the Microresp plate system to be well suited for rapidly comparing soil microbial response to many different polyphenolics because it required a small soil sample and low concentrations of substrate.  Because the soil samples used in the Microresp are quite small, we were able to use even tannins that we had limited quantities of, and we were able to dissolve the required amount of tannin water at neutral pH.   Determining specific rates and extent of decomposition for purified, defined polyphenolics will allow us to identify structure-activity relationships.  From those relationships we will develop a predictive model for microbial responses to tannins based on their chemical properties.  W have found that the  Microresp system is an effective method for comparing soil microbial response to various compounds.