Changes in plant community composition, not diversity, during a decade of nitrogen and phosphorus additions drive above‐ground productivity in a tallgrass prairie

Summary Nutrient additions typically increase terrestrial ecosystem productivity, reduce plant diversity and alter plant community composition; however, the effects of P additions and interactions between N and P are understudied. We added both N (10 g m −2 ) and three levels of P (2.5, 5 and 10 g m −2 ) to a native, ungrazed tallgrass prairie burned biennially in northeastern K ansas, USA , to determine the independent and interactive effects of N and P on plant community composition and above‐ground net primary productivity ( ANPP ). After a decade of nutrient additions, we found few effects of P alone on plant community composition, N alone had stronger effects, and N and P additions combined resulted in much larger effects than either alone. The changes in the plant community were driven by decreased abundance of C 4 grasses, perhaps in response to altered interactions with mycorrhizal fungi, concurrent with increased abundance of non‐ N ‐fixing perennial and annual forbs. Surprisingly, this large shift in plant community composition had little effect on plant community richness, evenness and diversity. The shift in plant composition with N and P combined had large but variable effects on ANPP over time. Initially, N and N and P combined increased above‐ground productivity of C 4 grasses, but after 4 years, productivity returned to ambient levels as grasses declined in abundance and the community shifted to dominance by non‐ N ‐fixing and annual forbs. Once these forbs increased in abundance and became dominant, ANPP was more variable, with pulses in forb production only in years when the site was burned. Synthesis . We found that a decade of N and P additions interacted to drive changes in plant community composition, which had large effects on ecosystem productivity but minimal effects on plant community diversity. The large shift in species composition increased variability in ANPP over time as a consequence of the effects of burning. Thus, increased inputs of N and P to terrestrial ecosystems have the potential to alter stability of ecosystem function over time, particularly within the context of natural disturbance regimes.