Non-additive effects of native-invasive tree litter mixtures enhance invasive species’ impacts on nutrient cycling during the growing season

Background/Question/Methods

Some of the most aggressive invasive plant species are able to promote their success through positive plant-soil feedbacks. One way they can do this is by producing high quality (low C:N) leaf litter, which can accelerate decomposition and enhance system nutrient availability. However, when leaf litters of differing quality decompose in a common environment, they can exhibit non-additive effects (NAE) on decomposition and N loss where the rate of decomposition differs from what would be expected based on each component litter independently. We hypothesized that litter mixtures containing invasive and native tree litter would experience synergistic NAE. To test this, we conducted a litter bag experiment using two-species litter mixtures from four invasive tree or shrub species (Acer ginnala, Elaeagnus umbellata, Lonicera maackii, and Morus alba) and four native tree species (Carya glabra, Cercis canadensis, Liriodendron tulipifera, and Quercus palustris) commonly found in Indiana, USA. To examine possible effects of evenness, bags containing mixed-species litter were filled at loading ratios of 10, 50, or 90 percent invasive species litter. Litter bags were collected after 90 or 365 days, and were measured for mass loss and nitrogen loss.

Results/Conclusions

We detected NAE on mass loss in all species pairings, although not at every loading ratio and the presence, sign, and strength of observed NAE varied over time. NAE on N loss were more frequently detected than NAE on mass loss, and were almost always antagonistic after 90 days and synergistic after 365 days of decomposition. The strength and sign of NAE were significantly correlated with relative differences between native and invasive species litter chemistry, where higher quality invasive species litter promoted stronger antagonistic NAE in both mass loss and N loss at 90 days, and stronger synergistic NAE at 365 days.  Effects of tissue chemistry were strongest in mixtures containing a majority of invasive species litter after 365 days of decomposition. The NAE on N loss we observed suggest that invasive species frequently stimulated increased N release from mixed litter during the growing season. Invasive plants with relatively nutrient-rich litter may therefore facilitate more positive plant-soil feedbacks than would be expected by enhancing N release from litter mixtures and promoting temporal synchrony between N availability and demand.