Nutrient availability and invasive species affect rates of native species litter decomposition

Background/Question/Methods

North American forest ecosystems are in a dynamic state due to anthropogenic alterations such as climate change and the introduction of exotic species.  Climate change in the form of rising atmospheric carbon dioxide levels may change the pools and process rates of both carbon and nitrogen, with forests becoming increasingly nitrogen-limited. The invasion of nitrogen-fixing shrubs into forest understories could ameliorate this nutrient limitation. We examined the role of nutrient limitation in concert with the addition of an invasive species on the process rates of a relatively labile nutrient pool, the leaf litter from native Liquidambar styraciflua (sweetgum) plantations.  Previous research has shown that L. styraciflua litter grown under high CO2 and ambient CO2 decompose at the same rates, despite small differences in leaf chemistry. We were interested in addressing whether there are differences in decomposition rate due to 1)litter quality based on where litter was grown, 2)microbial community based on where litter was placed, or 3)the presence of the invasive species litter. We collected L. stryaciflua litter and litter of the invasive nitrogen-fixing Elaeagnus angustifolia that had grown under ambient conditions, under high CO2 conditions at the Oak Ridge National Laboratory Free Air Carbon Enrichment (ORNL FACE) site, and under high nitrogen fertilization at the ORNL Nitrogen Fertilization Enrichment (NFE) site. Litter bags, consisting of only native litter or of native and invasive litter mixed, were reciprocally placed under all nutrient conditions for a 12 month decomposition study.

Results/Conclusions

Initial analyses indicate that litter quality had a significant effect on decomposition rates, with L. stryraciflua litter grown at the nitrogen-fertilized sites decomposing most quickly at both the first and second removal dates. The effect of litter placement was only visible at the second removal date, at which point litter placed at the nitrogen fertilization site had decomposed at the fastest rate. There was no direct effect of mixing invasive E. angustifolia with the native litter on the rates of L. styraciflua decomposition, but there were significant interactions between this and the other factors tested. These results highlight the importance of using fully reciprocal experiments to address questions about how invasive species and nutrient availability interact to impact ecosystem processes.