Nitrogen pollution and rising carbon dioxide: A joint threat to grassland biodiversity?

But whether rising levels of climate-warming carbon dioxide gas are amplifying those nitrogen-induced biodiversity losses or dampening them remains unclear and is an understudied topic.

The newly published findings of an ecologically realistic 24-year field study involving 108 experimental grassland plots in Minnesota provide an answer that doesn't bode well for biodiversity conservation efforts -- at least for grasslands.

During the most recent eight years of the study, experimentally elevated levels of carbon dioxide nearly tripled species losses attributed to the long-term application of simulated nitrogen pollution.

Specifically, plots that received added nitrogen saw species richness -- the number of plant species per plot -- reduced by an average of 7% at ambient carbon dioxide levels and by 19% at elevated carbon dioxide levels.

"If rising carbon dioxide generally exacerbates the widespread negative impacts of nitrogen deposition on plant diversity, as observed in our study, this bodes poorly for conservation of grassland biodiversity worldwide," said University of Michigan ecologist and study lead author Peter Reich.

"Maintaining biodiversity is critical because diverse plant communities provide services to people, such as water purification, crop pollination benefits, maintaining healthy soils, slowing down climate change by storing carbon, and being home to diverse butterfly, bird and mammal communities."

The findings are scheduled for publication Oct. 16 in the journal Nature.

Both nitrogen and carbon dioxide can promote plant growth. In the grassland experiment, called BioCON, application of the two resources spurred growth that allowed a few dominant species to hog the sunlight while throwing shade on plants beneath them, eventually eliminating many of them.

It's a phenomenon that ecologists call competitive exclusion.

This type of heightened light competition is likely to occur in many grasslands around the world -- resulting in both winners and losers -- due to the increased availability of carbon dioxide and nitrogen from fossil fuel emissions and nitrogen pollution, respectively, Reich said.

"Broad concerns about biodiversity changes, including those due to habitat loss, change in fire regime and climate change, need to be viewed within the context of rising carbon dioxide and varying nitrogen deposition, which likely also have significant effects in many ecosystems," said Reich, director of the Institute for Global Change Biology at U-M's School for Environment and Sustainability and a professor in the Department of Forest Resources at the University of Minnesota.

"Calls for biodiversity preservation and restoration are already at a fever pitch," he said. "Our results only further add to that chorus."

The BioCON experiment was conducted at the Cedar Creek Ecosystem Science Reserve in east-central Minnesota. Up to 16 species of grasses and forbs (herbaceous plants other than grasses, including wildflowers) were grown in each of the 108 7-foot by 7-foot plots from 1998 to 2021.

Half of the plots were treated throughout the growing season with additional carbon dioxide gas released from perforated vertical pipes. Half of those had nitrogen fertilized sprinkled on them annually. The number of plant species in each plot was counted late each summer.

During the first 10 years of the experiment, elevated levels of carbon dioxide actually reduced the species losses attributed to nitrogen enrichment. At ambient carbon dioxide levels, the added nitrogen reduced species richness an average of 16%; at elevated CO₂,species richness dropped by 8%.

But over time this interaction reversed, and elevated carbon dioxide amplified losses of diversity from nitrogen enrichment, nearly tripling those reductions over the last eight years of the study.

Big bluestem, or Andropogon gerardii, a tall grass native to much of the Great Plains and grassland regions of central and eastern North America, gradually emerged as the most dominant species. As its relative abundance increased, so did shading and the loss of other plant species.

Losers in the experiment included the purple-flowered Amorpha canescens, commonly called lead plant, a shrubby member of the pea family that prefers full sunlight and yellow-flowered Solidago rigida, one of the goldenrod species commonly found in grassy fields across the country.

Levels of nitrogen deposition remain elevated over much of the globe, though the trends and impacts are decreasing in some regions while increasing in others.

Previous observational and experimental studies suggest that nitrogen pollution decreases plant community richness by as much as 20 to 30% across herbaceous plant ecosystems on multiple continents. Herbaceous plants do not produce a woody stem and include grasses, forbs and ferns.

The Cedar Creek Ecosystem Science Reserve is owned and operated by the University of Minnesota. The 24-year BioCON experiment is the longest-running study -- by more than a decade -- to look at how interactions between CO₂ and nitrogen affect species diversity in grasslands.