Bioavailability of allelochemicals as affected by companion compounds in soil matrices.

Multicompound allelochemical interactions were studied using Centaurea maculosa as a model source to understand how the bioavailability of complex allelochemical mixtures is modified in soil-microbial systems. Litter decomposition of C. maculosa in sandy loam soil yielded five phenolic acids, namely, hydroxybenzoic, vanillic, protocatechuic, p-coumaric, and ferulic acids. The degradation studies were conducted by exogenous application of catechin, the primary allelochemical exuded by C. maculosa, and the phenolic acid cosolutes in a sandy loam and silt loam soil. Compared to a single-solute system, in a multisolute system the persistence of individual allelochemicals was significantly increased in both soils. Oxidation and sorption were primarily involved in the disappearance of allelochemicals. Mass spectrometric data showed that catechin rapidly underwent polymerization to form procyanidin dimer both in soil and in bioassay medium, resulting in reduced persistence and phytotoxicity. Hence, catechin phytotoxicity could occur only under conditions that would inhibit these condensation reactions. This study clearly demonstrates that various soil mechanisms including competitive sorption and preferential degradation would increase the persistence of allelochemical mixtures in a soil matrix.