Chronic nutrient enrichment in grasslands increases vulnerability to drought: multiple studies in a single long-term experiment

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Increased nutrient inputs is one of many global change factors predicted to affect the ecosystem function of plant communities, including their ability to withstand drought. A likely mechanism for this reduced resistance is changes in plant community composition. 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 at Konza Prairie LTER. Starting in 2003, 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. These nutrient additions altered plant composition and structure, particularly in plots receiving both nitrogen and phosphorus additions. Plant communities in these plots switched from a perennial C₄-dominated grassland to a perennial and annual forb-dominated community and became more variable across space and time. Additionally, these altered plant communities impacted ecosystem function, with ANPP responding less predictably to annual precipitation than native tallgrass prairie.

A comprehensive understanding of the sensitivity of plant communities subjected to multiple environmental drivers is crucial for understanding the interactive effects of global environmental change. Therefore, within the context of this experiment, we also investigated the resistance and resilience of plant communities altered by increased nutrient availability to seasonal drought. Three consecutive years (years 8-10 of the nutrient experiment) of summer drought were imposed via complete rainout shelters over a portion of each replicate reducing growing season soil moisture by 50%. This has been followed by three years of monitoring recovery. We predicted that the plant communities that were dramatically altered by chronic nutrient additions would be less resistant to drought stress compared with control plots. Control plots showed little response to three years of soil moisture reduction. However, the weedy communities in the N10P10 plots exhibited severely reduced ANPP, increased bare ground, and further altered community composition. Even three years after cessation of the drought treatment, these plots have yet to recover and in parts still have visible bare ground. Throughout the 13 years of this experiment, we have not only shown that nutrient additions cause drastic alterations to plant communities and ecosystem function but also that chronic nutrient additions increase the vulnerability of an ecosystem to other global change factors such as drought. And perhaps these interactive effects with other global change drivers will be the most severe consequences of increased nutrient availability.