Relationship between plant phenolic acids released during soil mineralization and aggregate stabilization

Phenolic acids (PAs) from plant and microbial sources have been implicated as important components in a variety of soil processes, but little information is available on the decomposition rates of plant-derived PAs, synthesis of soil microbial PAs, incorporation of PAs into humic substances and stabilization of soil aggregate fractions. To obtain this information, a Webster soil (fine-loamy, mixed, superactive, mesic Typic Endoaquoll) was amended with seven plant residues (2% w/w) and incubated at 22 +/- 2 degrees C. Duplicate samples were extracted after incubation for 9, 29, and 84 d and analyzed for PA composition. The plant residues contained large amounts of ferulic [3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid] and coumaric acids [3-(4-hydroxyphenyl)-2-propenoic acid] (both C6-C3, phenylpropyl type), which decomposed according to pseudo first-order kinetics. Most of the remaining PAs showed little change in concentration after 9 d and only microbial synthesis of 4-hydroxybenzoic acid was noted during the study. Analysis of six purified soil microbial polymers isolated from pure culture confirmed that the tested microbial extracellular polymers contained benzoic and benzaldehyde PAs (C6-C1, phenylmethyl type). Evaluation of humic acids isolated from different residue-amended soil showed the majority of humic-PAs were C6-C3 PAs of plant origin. Increased mean weight diameter of soil aggregates was closely related to the increase in soil PA concentration and organic C content in the amended soil during incubation. The results suggest that plant PAs are extremely important in the soil C cycle, soil aggregation and in the formation of stable C, and measurement of soil ester-linked PA composition can provide an index of plant-derived C in soil.