Nitrogen enrichment and plant communities.

Anthropogenic nitrogen (N) enrichment of many ecosystems throughout the globe has important ramifications for plant communities. Observational and experimental studies frequently find species richness declines with N enrichment, in concert with increasing primary production. Nitrogen enrichment also reorders species composition, including species turnover through gains and losses of species, changes in dominance and rarity, and shifts in the relative abundance of particular functional groups. Nitrogen has traditionally been considered the primary limiting nutrient for plant growth in terrestrial ecosystems, but recent synthetic work suggests that colimitation by phosphorus (P), water, and other resources is widespread, consistent with theoretical predictions. At the same time, disproportionate increases in ecosystem N input are expected to exacerbate limitation by P and other resources. Similarly, synthetic research has pointed out the important role of consumers and pathogens in determining plant community structure, especially with respect to shifting resource availability. We argue here that environmental and biotic contexts, including limitation by multiple resources, herbivores and pathogens, play important roles in our understanding of plant community responses to N enrichment.