Evaluating Nitrogen Heterogeneity in the Albany Pine Bush

Ecological theory predicts that heterogeneous environments should support more species. In such habitats, species are able to partition resources such as space, sunlight, water, and nutrients to prevent niche overlap and competition. This study at the Albany Pine Bush assessed soil nitrogen heterogeneity by analyzing the soil nitrogen content at close and far (1 m away) distances from nitrogen-fixing wild Lupinus (Lupine) plants. In addition, we analyzed foliar δ¹⁵N in Schizachyrium scoparium (Little Bluestem) leaves at the same close and far distances from Lupinus. We used these two measures to quantify the degree of heterogeneity and the spatial impact of a native nitrogen-fixer on its surrounding vegetation. We performed these analyses in a native-dominated pine barren and also a site that had been previously invaded by another nitrogen-fixing species, Robinia pseudoacacia (black locust), and subsequently restored. Paired t-tests of soil percent nitrogen in close versus far distances from Lupinus showed they were not significantly different from one another in either the barren or the restored sites. δ¹⁵N values were significantly higher close to Lupinus than far, but only in the restored site. There was no difference in the means of δ¹⁵N values for Schizachyrium scoparium tissue near and far from Lupinus. Both sites were homogenous in nitrogen and δ¹⁵N values, contrary to the pattern that we had hypothesized. Like other barren ecosystems, the Albany Pine Bush is an overall nutrient-poor environment made up of sandy soils, and this may explain why we did not observe soil heterogeneity. In such nutrient-poor habitats, nitrogen limitation may be strong enough to prevent the creation of “islands of fertility” near nitrogen-fixing species. Additionally, it is possible that the nitrogen that is put into the system by nitrogen-fixing species gets exported deep into the soils and thus was not quantified in our study.