Teasing apart plant community responses to N enrichment: The role of competition and soil microbes

Background/Question/Methods

Human activities have increased nitrogen (N) availability dramatically in systems throughout the globe. Elevated N increases productivity and alters plant community composition, often leading to diversity loss; however a mechanistic understanding of what causes these responses and whether causal factors are consistent across systems is lacking. We use greenhouse mesocosm experiments to test the relative importance of competition and soil microbial communities in driving plant community responses to N enrichment. We manipulated N fertilization, microbial community (soil inoculum from long-term control or fertilized field plots), and competitive environment (planting density) in all factorial combinations. We performed experiments in three different systems: tallgrass prairie, alpine tundra, and desert grassland. We expect system-specific responses to treatments in our greenhouse experiments: fertilizer and competitive environment will reduce diversity most in the tallgrass prairie which has the highest productivity response to N enrichment in the field, and N microbial inoculum will reduce diversity in the alpine and desert systems because of the importance of plant-microbe symbioses in stressful environments. We also hypothesize that synergistic effects will occur: soil inoculum from fertilized conditions will strengthen the N effect on species compositional change. 

Results/Conclusions

The relative importance of fertilization, competitive environment, and microbial community on plant community structure was system specific: fertilization and competition reduced diversity in tallgrass prairie and alpine tundra mesocosms, while microbes had a much larger negative effect in desert grassland mesocosms. Despite weak effects of soil microbes on diversity in two of the three systems, ordination showed that the microbial community strongly affected species composition in all systems. Contrary to our hypothesis, N inoculum did not exacerbate species compositional changes with fertilization: fertilization and inoculum additively affected composition in tallgrass prairie and alpine tundra, and only inoculum affected composition in desert grassland. While other experiments have likewise demonstrated plant community-level consequences of microbial composition, this is the first to show that microbial communities can have equal or greater effects on plant community composition than do competition and elevated nutrients per se, suggesting that microbial change plays an important role in plant community responses to N deposition. Our results also indicate that mechanisms of diversity response to N enrichment are system-specific, thus managing for biodiversity with global change may call for different strategies across system types.