We examined the impact of nitrogen addition on early successional dynamics and plant species diversity in replicate plots of an old-field succession treatment in south-western Michigan, USA, for 7 years following abandonment from agriculture.
Nitrogen addition significantly increased above-ground plant biomass, but had no effect on the displacement or persistence of different life-history groups. Annual species made up a similar proportion of community biomass and decreased at equivalent rates in control and nitrogen-addition microplots. Perennial species biomass increased at similar rates in the control and nitrogen-addition microplots.
Annual species dominated initially, but declined in abundance over the first 4 years and were replaced by biennials and then herbaceous perennials. The resulting high numbers of species losses and gains were not affected by nitrogen addition.
Nitrogen addition did not affect mean species density or the Shannon index of diversity. There was also no temporal change in the similarity of species composition between control and nitrogen-addition microplots, indicating that nitrogen addition did not cause divergence over time in community composition. In year 7, similar species dominated the nitrogen-addition and control microplots, but there were differences in species composition and abundance of the subordinate species in each community.
Only in year 6 of this study was there a significant negative relationship between community biomass and the Shannon diversity index. There was no significant relationship between community biomass and species density in any of the first 7 years following abandonment. Thus, the community response to increased biomass due to nitrogen addition was expressed primarily through changes in community structure, not richness.
The effects of nitrogen addition on the early successional dynamics, species composition and consequently diversity of a plant community may depend on initial site conditions. The response to nitrogen enrichment may be slower in sites such as ours with fertile soils that become dominated by herbaceous perennial dicots.