Effects of soil nitrogen enrichment on prairie plant communities native to Lake Michigan sand dunes
Nitrogen enrichment, caused by atmospheric deposition, has been reported to reduce the richness and diversity of plant species on nitrogen-limited soil. The sand prairies in the southern shore of Lake Michigan are on such soil and receive approximately 14-23kg of ammonium nitrate per hectare as wet deposition each year. In this study, we hypothesized that the enrichment of nitrogen in soil would decrease the diversity and richness of the species that are native to the Lake Michigan sand dunes. We tested this hypothesis by conducting an experiment that compares germination, biomass production, and community composition of ten native plant species under varying degrees of nitrogen enrichment in soil. Germination success and biomass production were determined under five different treatment levels of ammonium nitrate, ranging from 0 to135 kg ha-1 over a year in a greenhouse. In the field, we monitored for three growing seasons (2010-2012) the trajectory of plant community development that was established from the seeding of ten species in 20 plots using randomized block design; 5 plots for each of 4 treatment levels of ammonium nitrate at the rates of 0, 30, 60, and 120 kg ha-1 yr-1.
Seven of the ten species exhibited increased germination success at the ambient level (15 kg ha-1 yr-1). However, the germination has decreased drastically at the higher treatment levels in all species. This reduction was particularly severe for the forb species such as Lupinus perennis, Mornarda punctata and Asclepias tuberosa. Six of the ten species appeared to increase biomass production and to allocate biomass to aboveground shoots with the ammonium nitrate treatments at or above 60 kg ha-1 yr-1. Only four species have established in the field plots. Our DCA (Detrended Correspondence Analysis) revealed a clear divergence of the vegetation in the control plots from the rest of the plots that received ammonium nitrate. All plots were dominated by grass species such as Schizachyrium scoparium and Panicum virgatum in 2010 and 2011. In 2012, L. perennis and M. punctata increased their dominance substantially in the control plots while S. scoparium maintained its dominance in the other plots that received ammonium nitrate. Our results suggest that the addition of nitrogen to soil favors grass, suppresses forb species, and reduces the diversity and richness of the native species in the sand dunes of Lake Michigan.