Response of native plants to elevated soil nitrogen in the sand dunes of Lake Michigan, USA

- Ambient N-deposition promotes seed germination.
- Enhanced germination leads to reduction or exhaustion of viable seeds in soil.
- Elevated soil-N favors grass over forb species.
- Grass dominance reduces plant species diversity.

Atmospheric deposition of nitrogen (N) has been reported to reduce the diversity of plant species in N-limited soil. The sand dunes along the southern shore of Lake Michigan have been N-limited historically but receive substantial amount of N from the atmosphere currently. We investigated the effect of N-addition to soil on seed germination rates and species composition of nine native plants. In the greenhouse, the ambient addition (15 kg N ha− 1 yr− 1) enhanced the germination of Elymus canadensis and Echinacea purpurea, while the 9 × ambient N addition reduced seed germination of Dalea purpurea and Elymus. However, regardless the results from greenhouse, these species exhibited no or poor emergence in the field plots. Panicum virgatum was the only species whose germination was promoted significantly by the ambient addition in the greenhouse and also emerged in the field plots in the early years (2010 − 2012), but its dominance diminished during 2013-2014. The decline of Panicum suggests a failure of establishment and reproduction after germination. Such failure has likely led to reduction or exhaustion of viable seeds in soil. Meanwhile, the species that showed relatively low germination rates (< 20%) in the greenhouse, such as Lupinus perennis, Monarda punctata and Schizachyrium scoparium, established substantially in the field plots. Also in the field plots, soil with elevated-N (3 ×, 5 × and 9 × ambient) appeared to favor grass over forb species. Dominance of grass over forb species has led to a reduction in species diversity and divergence of species composition between the ambient and elevated-N plots.