The allelopathic properties of decomposing buckwheat residues are not directly related to phenolic compounds in soil

Previous studies have shown that residues of common buckwheat roots (BRR) (but not entire common buckwheat plants, BPR) in the soil inhibited the growth of barnyardgrass (Echinochloa crus-galli L.). The objective of the study was to determine how both the residues affect the content of free phenolics, their esters and glycosides in the soil. The aqueous extracts were used to analyse of unbound phenolic compounds, while those bound to the soil were extracted with sodium citrate. Moreover, an in vitro test was used to assess the allelopathic effect of phenolic compounds present in the soil against barnyardgrass. Among the analysed phenolic compounds after 7 days of BPR and BRR decomposition, only ortho-, meta- and para-coumaric acids and apigenin were found in measurable amounts in the soil. The concentrations of free phenolic compounds were very low. Much higher contents occurred for the esters of these compounds, while no glycosides were found. The contents of phenolic compounds bound to soil were many times higher than unbound ones. The 37-day decomposition period resulted in an increase in bound phenolics, while the content of unbound changed slightly. Overall, the levels of phenolic compounds in the soil with the BRR-amended soil and no-buckwheat residue control were low, and significantly higher in the soil with BPR. An in vitro test showed that m-, p-coumaric acids and apigenin added to growth medium at a concentration higher than in the soil did not affect barnyardgrass shoot growth. Since the levels of phenolic compounds in the soil containing BRR and control soil were low and similar, phenolic compounds cannot be directly responsible for the allelopathic properties caused by the presence of BRR.