Grassland study shows high CO₂ levels have nearly tripled species losses attributed to nitrogen pollution

Dozens of studies have demonstrated that nitrogen pollution, mainly due to the burning of fossil fuels and agricultural practices, is leading to a loss of plant biodiversity worldwide.

But it remains unclear whether increasing levels of carbon dioxide, responsible for global warming, amplify or mitigate nitrogen-induced biodiversity losses, and remains an understudied subject.

The results of a 24-year ecologically realistic field study of 108 experimental grassland plots in Minnesota provide an answer that does not bode well for biodiversity conservation efforts, at least for grasslands. The article is published in the journal Nature.

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

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

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

“Maintaining biodiversity is essential because diverse plant communities provide services to humans, such as water purification, crop pollination benefits, maintaining healthy soils, slowing climate change by storing carbon and habitat for diverse communities of butterflies, birds and mammals.”

Nitrogen and carbon dioxide can promote plant growth. In the grassland experiment, called BioCON, the application of the two resources stimulated growth that allowed a few dominant species to hog sunlight while casting shade on plants below. them, ultimately eliminating many of them.

This is a phenomenon that environmentalists call competitive exclusion.

This type of increased competition for light 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 respectively from fossil fuel emissions and nitrogen pollution, Reich said.

“Major concerns about changes in biodiversity, including those due to habitat loss, changing fire regimes and climate change, must be seen in the context of increasing carbon dioxide and variation in nitrogen deposition, which likely also has significant effects on many ecosystems,” said Reich, director of the Institute for Global Change Biology at UM’s School for Environment and Sustainability and professor in the Department of Forest Resources at the University of Minnesota.

“Calls for the preservation and restoration of biodiversity are already at a fever pitch,” he said. “Our results only add to this chorus.”

The BioCON experiment was conducted at the Cedar Creek Ecosystem Scientific Preserve in east-central Minnesota. Up to 16 species of grasses and non-grass forbs (including wildflowers) were grown in each of 108 7-by-7-foot plots from 1998 to 2021.

Half of the plots were treated throughout the growing season with supplemental carbon dioxide released through perforated vertical pipes. Half of them received nitrogen fertilizer each year. The number of plant species present in each plot was counted at the end of each summer.

During the first 10 years of the experiment, high levels of carbon dioxide actually reduced species losses attributed to nitrogen enrichment. At ambient carbon dioxide levels, added nitrogen reduced species richness by 16% on average; high CO₂specific richness fell by 8%.

But over time, this interaction reversed and increased carbon dioxide amplified diversity losses from nitrogen enrichment, nearly tripling these 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 prairie regions of central and eastern North America, has gradually become the most dominant species. As its relative abundance increased, so did shading and loss of other plant species.

Losers in the experiment included the purple-flowered Amorpha canescens, commonly known as lead plant, a shrubby member of the pea family that prefers full sun, and the yellow-flowered Solidago rigida, one of the species of verge ‘gold commonly found in grassy fields across the country.

Levels of nitrogen deposition remain high across much of the globe, although 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 up to 20–30% in herbaceous plant ecosystems on several continents. Herbaceous plants do not produce a woody stem and include grasses, forbs, and ferns.

The Cedar Creek Ecosystem Scientific Reserve is owned and operated by the University of Minnesota. The BioCON experiment, spanning 24 years, is the longest-running study (over a decade) examining how interactions between CO₂ and nitrogen affect species diversity in grasslands.

The other authors of Nature were Neha Mohanbabu, Forest Isbell, Sarah Hobbie and Ethan Butler of the University of Minnesota.