Anthropogenically metalliferous soils are widespread among the world’s novel ecosystems and urgently demand effective remediation and management. Where metal-contaminated soils cannot be capped with benign soil, management must take into account ecological consequences of plant metal uptake.
Plants with high leaf metal uptake have been hypothesized to increase local topsoil metal concentrations via decomposition of contaminated leaf litter, thereby poisoning neighboring plants. The idea of this form of positive plant-soil feedback, called elemental allelopathy, is nearly 30 years old, yet has rarely been tested.
We sought to test the strength of elemental allelopathy in a system where soils highly contaminated with zinc, lead, and cadmium from a zinc smelter lie underneath a recently applied compost layer. Logically, gray birch, a metal accumulating tree rapidly colonizing the site, could increase topsoil metal concentrations via its leaf litter, harming its neighbors. We measured metal concentrations of gray birch leaves, tested the impact of these leaves on the growth of four target species in the greenhouse, and monitored the performance of black oak and sugar maple seedlings planted in the field under gray birch and among the surrounding C4 grasses, controlling for aboveground competition by clearing aboveground vegetation in half of our plots.
We found no evidence supporting elemental allelopathy in our study system. Metal concentrations in gray birch leaves are indeed higher than most other species in the site, and higher than gray birch leaves from nearby uncontaminated sites. However, topsoil metal concentrations do not change with distance from birch trees.
None of four target species responded significantly to contaminated birch leaves crushed and mixed into their soil, in comparison to uncontaminated birch leaves or no leaves at all. One species showed similar growth inhibition from contaminated and uncontaminated birch leaves, suggesting species-specific, non-elemental allelopathy.
In the field, we found no evidence that birch-contaminated soils reduced seedling growth in a manner consistent with elemental allelopathy. Aboveground performance measurements suggested instead a potential mycorrhiza-mediated feedback, which we are currently pursuing further.
We conclude that elemental allelopathy is not operating through gray birch in our system, and we hypothesize that if it occurs, it does so only in highly specific conditions. We suggest further that elemental allelopathy is not a valid evolutionary explanation for the high rates of metal uptake found in many plant species. Other explanations for high metal uptake, including metal tolerance mechanisms or defenses against herbivory, seem more likely.