Species re-ordering due to long-term nitrogen and phosphorous inputs has consequences for plant community diversity and productivity

Background/Question/Methods

Increased nutrient inputs is one of many global change factors predicted to affect the composition and ecosystem function of plant communities. In general, nitrogen deposition decreases diversity and increases productivity. The effects of phosphorus addition have received less attention, however, and the interactive effect of both nutrients is likely to exacerbate diversity loss over time. Here we addressed whether chronic nutrient additions changed community structure and ecosystem productivity of a native tallgrass prairie. This study took place in a perennial C4 grassland at Konza Prairie in Manhattan, KS, within a watershed that is burned every two years. Two N treatments, 0 and 10 g m^-2, and four P treatments, 0, 2.5, 5 and 10 g m^-2 were crossed in a fully factorial experimental design. The experiment was initiated in 2002 and starting in 2003 nutrients were added at the beginning of each growing season. Plant species composition was surveyed both in the spring and late summer each year, and aboveground biomass was harvested at the end of each summer to estimate aboveground net primary productivity (ANPP). Mycorrhizal fungal biomass and/or colonization were assessed three times throughout the experiment.

Results/Conclusions

Overall, we found N addition initially increased ANPP, but its effects were dampened over time, while P addition, had a much weaker impact on ANPP.  Plant community composition was unchanged for the first five years of nutrient additions, but thereafter, N generally decreased diversity, whereas P generally increased diversity. Their combined effects at the high nutrient levels reduced species richness by 14% when compared to the controls in 2011. By 2008, the high N and P additions had drastically changed the plant community, with consequences for ecosystem productivity. The addition of N and P resulted in 267% increase in the cover of annual species, and a reduction in the dominant C₄ grasses. This reordering of species resulted in greater inter-annual variation in productivity. Mycorrhizal root colonization was increased by N and decreased by P additions, and the lowest levels found were in the combined high N and P treatment. The reduction in mycorrhizal root colonization may result in reduced abundance of the dominant C4 grasses, as they are mycorrhizal dependent, allowing subordinate forbs to increase in abundance. Our study highlights the complexity of the interactive effects of N and P additions, both directly and indirectly through effects on species interactions and plant community dynamics.