Alpine ecosystems struggle to recover from nitrogen deposition

The study, which was recently published in the journal Ecological Applications, indicates that even a dramatic reduction in nitrogen emissions may not be sufficient to reverse changes to various ecosystem processes after decades of high exposure.

"The legacy of the impacts of nitrogen pollution is strong, and our results emphasize that sensitive standards are needed to minimize enduring environmental impacts," said William Bowman, lead author of the study and a professor in CU Boulder's Department of Ecology and Evolutionary Biology (EBIO).

Nitrogen is a key nutrient for life, but agricultural and industrial activities have increased global levels significantly over the last two centuries, with previous research indicating harmful effects on water quality, soil acidity and biodiversity. Nitrogen emission rates have slowed in most of the U.S. and Europe in recent years, but continue to increase in developing regions.

The new study explores the extent to which alpine ecosystems can recover or reverse the effects of nitrogen deposition even after input levels have slowed. To test the difference, CU Boulder researchers used a long-running set of field plots first established in 1997 on Colorado's Niwot Ridge at an elevation of 11,400 feet. The plots had been artificially exposed to varying levels of additional nitrogen over the course of 12 years.

Beginning in 2009, the researchers divided the plots in half, continuing to fertilize one half at the same rate while cutting off nitrogen to the other. Then, they followed the changes in the plots' biotic composition and ecosystem processes for nine more years, tracking changes in plant diversity, microbial abundance and soil acidity.

Overall, the researchers found that vegetation recovery was more limited in the areas that had received the highest levels of nitrogen previously, even after gaining a reprieve in subsequent years. Bacteria and fungi abundances also remained lowered and soil remained acidic, indicating sustained impacts that cannot be easily reversed.

"The altered chemistry and biology of the ecosystem stimulated the rates of nitrogen cycling in the soil, extending the negative impacts of a high nitrogen condition," Bowman said. Additionally, some recovery processes operate at geologic scales, relying on the breakdown of the rocks and soil particles that can take decades or longer.

The findings indicate that many of the effects of human-caused nitrogen deposition may already be baked into ecosystems and hamper their recovery regardless of future decreases in emission rates, a crucial consideration for setting environmental regulations and pollution standards.

Additional co-authors of the research include Cliff Bueno de Mesquita, Noah Fierer, Stefanie Sternagel and Teal Potter of CU Boulder and Asma Ayyad of University of California Riverside. The National Science Foundation provided funding for the study via the Niwot Ridge Long-Term Ecological Research Project and a Research Experiences for Undergraduates Grant.