Alpine ecosystems struggle to recover from nitrogen deposition

Alpine ecosystems struggle to recover from nitrogen deposition
Afternoon light shines on an alpine dry meadow community on Niwot Ridge, Colorado, surrounded by the Indian Peaks of the southern Rocky Mountains. Elevated inputs of atmospheric nitrogen pollution from nearby urban and agricultural developments are altering species composition and soil biogeochemistry in these alpine ecosystems. Credit: William Bowman

What happens to high mountain ecosystems when you take away air pollution? Not much, not very quickly. A new CU research study finds that degraded alpine ecosystems showed limited recovery years after long-term inputs of human-caused nitrogen air pollution, with soil acidification and effects on biodiversity lingering even after a decade of much lower nitrogen input levels.

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

"The legacy of the impacts of 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, 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 can recover or reverse the effects of 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 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.


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More information: William D. Bowman et al, Limited ecosystem recovery from simulated chronic nitrogen deposition, Ecological Applications (2018). DOI: 10.1002/eap.1783
Journal information: Ecological Applications

Citation: Alpine ecosystems struggle to recover from nitrogen deposition (2018, September 4) retrieved 16 June 2019 from https://phys.org/news/2018-09-alpine-ecosystems-struggle-recover-nitrogen.html
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