Enrichment of soil by nitrogen from atmospheric deposition causes changes in plant communities.  While a majority of studies on this subject have addressed decline of biological diversity and invasion of exotic species, little has been studied on the responses of native plant species and the dynamics of their assemblage.  In this study, we hypothesized that soil nitrogen enrichment may cause native species to alter their biomass allocations in nitrogen-limited soils of Lake Michigan sand dunes.  Little bluestem (Schizachyrium scoparium) and Canadian rye (Elymus canadensis), common grass species in the sand dunes as well as North America prairies, were selected to test our hypothesis.  Schizachyrium and Elymus stands were established in 20 cylindrical plots (5 control and 5 N-enriched plots for each species; each plot made of a PVC cylinder with 30 cm diameter buried to 30 cm depth).  Each N-enriched plot received one gram of ammonium nitrate in every two weeks for 10 times from May to September 2007.

Results/Conclusions There was no statistically significant difference in the total biomass production between the two species in response to the nitrogen addition.  However, they exhibited opposite ways of biomass allocation.  Schizachyrium increased its proportional allocation to aboveground biomass from 33% (control) to 53% (N-enriched), and the belowground biomass allocation of Elymus increased from 42% (control) to 57% (nitrogen-enriched).  With the nitrogen addition, the aboveground biomass was higher in the Schizachyrium (274 g m^-2) than the Elymus (185 g m^-2) plots, and the opposite was true for the belowground biomass (Schizachyrium 128 g m^-2; Elymus 252 g m^-2).  Based on these results, we hypothesize that these shifts in biomass allocation may determine the relative dominance of the two species if they compete to each other.  Schizachyrium may establish a preemptive dominance with its robust aboveground biomass.  Meanwhile, the larger belowground investment may be advantageous for resource acquisition (e.g., water and minerals) and rhizome reproduction of Elymus.