Allelopathic agents used in this study and some of their degradation products.
Rates of transformation and/or disappearance of phenolic acids in soil solutions have also been determined under a variety of circumstances and in
various soils In general, there is a rapid initial transformation (e.g., loss of the carboxylic acid group) of phenolic acids. For example, 90% of the carboxylic acid carbon of p-hydroxybenzoic acid, syringic acid, and vanillic acid was lost within 1 week.22 Losses of other side chain carbons or ring carbons, however, took
considerably longer, on the order of weeks to months. '
In addition, there are a number of other factors besides chemical structure that determine rates;17 for example, transformation of p-coumaric acid was delayed in the presence of glucose, phenylalanine, and p-hydroxybenzoic acid, but not methionine, in Cecil Bt - horizon soils.31 The delays suggested preferential utilization of carbon sources by soil microorganisms. Additional evidence for differential carbon utilization in soils has also been provided by Martin and Haider29
and Haider et al.,23 who observed that mineralization of 14C-labeled ring carbon of glucose was more rapid than from phenolic acids, and by Sugai and Schimel,42 who observed in taiga soils from a series of successional stages that glucose, p-
hydroxybenzoic acid and salicylic acid were processed very differently by soil microorganisms. More than twice as much glucose was converted to biomass than either of the phenolic acids, and although both phenolic acids were metabolized,
only p-hydroxybenzoic acid was assimilated by the microbes. Finally, any environmental factors (e.g., temperature, pH, moisture, nutrition) that influence the activity of soil microorganisms or make phenolic acids less available to microbes (e.g., increased sorption by soil particles) will influence the transformation rates of free phenolic acids. To provide some insight into how a range of soil physicochemical and biotic factors may influence the rates of transformation of phenolic acids, I will compare the rates in an open and a closed system.
Steady state rates of transformation, as measured by the disappearance of p-coumaric acid after adjustment for soil fixation, in a continuous flow Cecil Ap -horizon soil (pH 5.0; typic Kanhapludult, clayey, kaolinitic, thermic) system (i.e., open system) supplied with a range of nutrient concentrations (0 to 50% Hoagland's solution) and 187 pg/h (53.4 pg/mL, 3.5 mL/h) of p-coumaric acid for 72 h at room temperature ranged from 0.035 to 0.076 picomoles/CFU (colony forming units) of p-coumaric acid utilizing bacteria/h, with a mean + standard error of 0.047 + 0.006.3 Bacterial populations that could utilize p-coumaric acid ranged from 1.07 x 105 to 4.00 x 105 CFU/g soil depending on nutrient levels supplied to the system. These microbial communities, we suspect, were derived from quiescent and dormant bacteria, since laboratory stored air-dried soil was used for this study. For these calculations it was assumed that all the CFU determined after the 72 h treatment represented active p-coumaric acid utilizing bacteria. The initial p-coumaric acid utilizing bacterial populations for the air-dried soil were 0.64 x 105 CFU/g soil. The impact of other soil microorganisms on phenolic acid transformation were not determined.
In test tubes (i.e., closed system; unpublished data) containing 1 g air-dried autoclaved Cecil Ap - horizon soil (pH 5.0), 82 pg p-coumaric acid, Hoagland's solution (all solutions adjusted to pH 5.0), and soil extract for inoculum (total of 1.5 ml) the average linear transformation rates for p-coumaric acid over 48 hr, once microbial utilization was evident, were 3.6 x 10-4 + 1.7 x 10-4 picomole/CFU of p-coumaric acid utilizing bacteria/h, about 130 times slower than what was observed for the mean utilization in the steady-state continuous flow system. The CFU of p-coumaric acid utilizing bacteria/g soil in the test tube system averaged 1.46 x 108 over the 48 h interval. Initial CFU of p-coumaric acid utilizing bacterial populations/g soil 24 hr after addition of inoculum were 105 + 15. Utilization of p-coumaric acid by microbes in the test tubes was determined by 0.25 M EDTA (pH 7.0) extractions at 6 h intervals and HPLC analyses.2 CFU for bacteria that utilized p-coumaric acid as a sole carbon source were also determined at 6 h intervals by the plate-dilution frequency technique24 utilizing 0.5 mM p-coumaric acid as the sole carbon source in a basal medium.3,7
Differences in rates of transformation and/or utilization between the two systems are possibly due to a) constant input vs. single input of p-coumaric acid and nutrient solution, b) aerobic (open system) vs. more anaerobic (closed system) conditions, c) little chance for accumulation of transformation products and/or toxic microbial byproducts (constant flushing of system) vs. potential build up of transformation products and/or toxic microbial byproducts (closed system), d) different microbial communities both in terms of species (air-dried soil vs. autoclaved-inoculated soil) and numbers (105 vs. 108), and e) input of p-coumaric acid (53 pg/mL/h or 187 pg/h vs. 58 pg/mL one time addition) added to different amounts of soil (60 g of soil for the flow-through system vs. 1 g of soil for the test tube system).
Given the bacterial populations that utilized p-coumaric acid as a sole carbon source and the physicochemical (e.g., constant temperature, adequate nutrition and moisture) and biotic conditions of these two laboratory systems, utilization of p-coumaric acid ranged from 0.6 to 5.0 pg/g soil/h for the open systems and 8.6 pg/g soil/h for the closed system. The pg values for the open system represent steady-state rates as modified by nutrition, while the pg values for the closed system represent maximum rates. Whether such rates ever occur in field soils is not known, since the physicochemical and biotic environments of field soils are so different from those of laboratory systems. Laboratory soil systems provide potential rates of utilizations, but until field rates are determined the importance of microbial activity in phenolic acid depletion from soil solutions will not be known.
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