Soils and the conditional allelopathic effects of a tropical invader

• Allelopathic effects of exotic species are conditional.
• Variation in the allelopathic effects of Prosopis juliflora are explored.
• P. juliflora allelopathic effects were highly conditional.
• Our results, the straightforward demonstration of conditionality, are novel.

Allelopathy may contribute to the formation of mono-dominant stands of exotic species, but the effects of allelochemicals can be highly conditional. We explored variation in the production of phenolics in leaves, accumulation of phenolics in soils, and the inhibitory effects of soils under an aggressive invader Prosopis juliflora across a range of invaded sites and potential mechanisms by which soils alter the effects of P. juliflora leaf litter. For eight sites in Northwest India we compared the concentration of total phenolics and the seedling growth of Brassica campestris in soils from beneath P. juliflora to that in soils collected away from P. juliflora canopies. We then explored these effects in detail in soils from two sites that differed substantially in texture by germinating seeds of B. campestris in these soils amended with P. juliflora macerated leaf leachate. Finally, we tested the effects of l-tryptophan in soils from these two sites on the seedling growth of B. campestris. Across all sites soil beneath P. juliflora contained higher levels of total phenolics and suppressed the growth of B. campestris than soil that was not under P. juliflora. We observed much variation among P. juliflora-invaded sites in the total phenolic levels of soils and the degree to which they suppressed B. campestris and the concentration of phenolics in soils significantly correlated with the root length of B. campestris when grown in these soils. Soil from two sites amended with P. juliflora macerated leachate suppressed seedling growth of B. campestris, with the effect being higher in sandy soil than sandy loam soil. In soil amended with leachate the strong suppression of B. campestris corresponded with much higher total phenolic and l-tryptophan concentrations. However, in other tests l-tryptophan did not affect B. campestris. Our results indicate that the allelopathic effects of P. juliflora can be highly conditional and that variation in soil texture might contribute to this conditionality.