Species mixture changes the content of phenolic acids in the soil of Eucalyptus plantations

Accumulation of phenolic acids in monocropping plantations led to declining soil fertility and reduced productivity due to allelopathic effects. The effects of mixed versus mono-specific plantations on the seasonal turnover of phenolic acids in soil remains unclear. This study explored in plantations of Eucalyptus grandis x Eucalyptus urophylla and Erythrophleum fordii Oliv. (mixed) versus E. grandis x E. urophylla only (pure) the effect of species mixture on the phenolic acid content in the soil. We analyzed both total phenolic acid content as well as six phenolic acids separately and our measurements were conducted over four seasons and in an entire year. We expected that the mixed plantation would exhibit lower accumulation of both total and individual phenolic acids in soil versus the pure plantation. Contrary to our hypothesis, the total content of phenolic acids in the mixed plantation was significantly (p < 0.001) higher than that in the pure Eucalyptus plantation in all seasons. We also observed a significant effect of stand type (mixed or pure), season, and interaction between stand type and season on total soil phenolic acid content (p < 0.001). This may cause an enhanced allelopathic effect of phenolic acids in the soil of the mixed stand. In terms of the individual phenolic acids, the mixed plantation resulted in significantly higher (p < 0.001) p-hydroxybenzoic acid and vanillic acid contents in all seasons, as compared with the pure Eucalyptus plantation. Species mixture appears to reduce the content of ferulic acid, coumaric acid, benzoic acid, and salicylic acid in most seasons. A significant effect (p < 0.001) of stand type, season, and their interaction on the content of all individual phenolic acids in the soil, with the exception of benzoic acid, was observed. This study thus strongly suggests that both total and individual phenolic acids should be studied to evaluate the possible allelopathic effects in forest ecosystems and verify that species mixture can also increase phenolic acid concentration.