As nitrogen (N) levels increase in terrestrial systems, predicting their effects on diversity is crucial. While there are many studies that have documented that N-enrichment influences local (alpha) diversity in plant communities, much less is known about how it affects regional (beta) diversity. We hypothesized that N-enrichment would affect the extent of beta-diversity by reducing the variability among local communities as well as increasing the rate at which these communities converged. We tested our hypothesis in 135 1m2 experimental California annual grassland communities by varying the initial relative abundance of native and non-native annual grasses, forbs, and N-fixing forb species through seed addition across 3 soil N levels (low, ambient, high). We measured species abundances over two growing seasons during establishment, peak biomass, flowering, and seed production. By using communities that were initially distinct from each other, we were able to determine community structure and the rate of convergence across a soil N gradient.
Communities became more similar to each other over time in all N environments. The rate of convergence was fastest in communities in high N conditions. Communities in the low N environments had the slowest rate of convergence. By the end of two growing seasons, the high N and ambient communities had converged along a similar trajectory. We conclude that soil N environments can significantly impact the rate of convergence. However, most communities still converge along the same trajectory, resulting in a smaller long-term effect on beta-diversity in annual grassland communities. As rates of N enrichment continue to increase, determining the rate of convergence and effects on diversity may aid in the restoration of native ecosystems.