Activated carbon is a commonly used component in allelopathy studies. Recently, there has been evidence that activated carbon may confound results in these experiments by directly affecting plant performance.  Moreover, most studies do not include activated carbon in isolation and with allelopathic plants when testing for allelopathic effects.  Our questions were: (1) Does activated carbon confound competitive effects?  (2) Does activated carbon directly alter toxicity in the tissues of an allelopathic plant?  Six target species were grown in a factorial experiment with and without activated carbon or a native allelopathic species (Solidago canadensis) in a greenhouse experiment.  These target species composed of grasses, herbaceous perennials and biennials represented native and non-native species.  The species were: C3 grasses - Danthonia spicata (N) vs. Poa pratensis (NN), herbaceous perennials - Eupatorium rugosum (N) vs. Chrysanthemum leucanthemum (NN) and herbaceous biennials - Aster pilosus (N) vs. Daucus carota (NN).  Target species aboveground biomass was harvested and weighed as well as S. canadensis above and belowground biomass.  Analysis of variance was used to assess the relationship between growth and activated carbon.  The effect of activated carbon on S. canadensis toxicity was assessed with laboratory germination trials, with and without activated carbon.

Results/Conclusions

Generally, there were growth differences among treatments for all target species.  Activated carbon increased aboveground biomass for all target species in the absence of S. canadensis.  However, in combination with S. canadensis, activated carbon reduced growth in half of the species.  This result was opposite of the expectation that activated carbon should mitigate allelopathic activity contributed by S. canadensis and improve growth in susceptible plants.  Activated carbon also increased S. canadensis growth in aboveground tissues, but slightly decreased growth in belowground tissues.  Laboratory bioassays revealed that activated carbon increased toxicity for a model organism (radish) in S. canadensis rhizome extracts, but not for leaves.  These results suggest that using activated carbon to tease apart competitive outcomes may not be an appropriate method for allelopathy studies as it may directly affect toxicity and enhance competitive effects.