An extensive 24-year field study on grasslands has revealed that increased carbon dioxide levels are nearly tripling the loss of plant species due to nitrogen pollution.
Numerous studies have highlighted that nitrogen pollution, primarily resulting from fossil fuel combustion and agricultural activities, is leading to a decline in plant diversity around the globe.
However, it is still uncertain whether the rising concentrations of carbon dioxide, which contributes to climate change, are exacerbating or alleviating these biodiversity losses due to nitrogen. This has been a largely unexplored area of research.
The recent findings from a comprehensive 24-year field study, which involved 108 experimental grassland plots in Minnesota, provide concerning insights for biodiversity conservation efforts, particularly for grasslands.
In the study’s last eight years, elevated carbon dioxide levels were associated with a nearly threefold increase in species loss linked to long-term simulated nitrogen pollution.
Specifically, plots that received extra nitrogen experienced a 7% decrease in species richness—meaning the number of different plant species per plot—at normal carbon dioxide levels, which increased to 19% at higher carbon dioxide levels.
“If increased carbon dioxide makes the negative impacts of nitrogen pollution on plant diversity even worse, as our study shows, this poses a serious threat to grassland biodiversity worldwide,” stated Peter Reich, an ecologist at the University of Michigan and lead author of the study.
“Preserving biodiversity is essential because diverse plant communities provide important services, such as purifying water, aiding in crop pollination, maintaining healthy soils, mitigating climate change by sequestering carbon, and supporting various species of butterflies, birds, and mammals.”
The study’s findings are slated for publication on October 16 in the journal Nature.
Both nitrogen and carbon dioxide can stimulate plant growth. In the grassland experiment known as BioCON, the application of these two elements boosted growth, allowing a few dominant species to overshadow others, ultimately eliminating many less competitive plants.
This phenomenon is referred to by ecologists as competitive exclusion.
Such increased competition for light is likely occurring in many grasslands globally—leading to both winners and losers due to the higher concentrations of carbon dioxide and nitrogen pollution resulting from fossil fuel emissions, according to Reich.
“Concerns about changes in biodiversity, including those from habitat loss, altered fire regimes, and climate change, must also consider the impacts of rising carbon dioxide and varying nitrogen levels, which likely affect many ecosystems significantly,” added Reich, who directs the Institute for Global Change Biology at U-M’s School for Environment and Sustainability and holds a professorship in the Department of Forest Resources at the University of Minnesota.
“The urgency for biodiversity preservation and restoration is already high,” he noted. “Our results only add to this critical conversation.”
The BioCON experiment took place at the Cedar Creek Ecosystem Science Reserve in east-central Minnesota. From 1998 to 2021, up to 16 species of grasses and forbs (non-grass herbaceous plants, including wildflowers) were cultivated in each of the 108 7-foot by 7-foot plots.
Half of the plots received additional carbon dioxide gas pumped in from perforated vertical pipes, and half of those were treated with annual nitrogen fertilizer. The total number of plant species in each plot was assessed at the end of each growing season.
Surprisingly, in the first decade of the study, the higher carbon dioxide levels actually reduced the loss of species linked to nitrogen fertilization. At normal carbon dioxide levels, the introduction of nitrogen decreased species richness by an average of 16%, whereas at elevated CO2 levels, richness declined by 8%.
However, this interaction reversed over time, with elevated carbon dioxide significantly increasing the species loss attributed to nitrogen fertilization, nearly tripling those reductions in the final eight years of the study.
Big bluestem (Andropogon gerardii), a tall grass commonly found in the Great Plains and central and eastern North America’s grasslands, became the dominant species as its prevalence rose, leading to more shade and further loss of other plant species.
Plants that struggled in this experiment included the lead plant (Amorpha canescens), a purple-flowered species from the pea family that thrives in full sunlight, and the yellow-flowered Solidago rigida, a type of goldenrod often present in grassy areas across the U.S.
Nitrogen deposition remains elevated in numerous regions worldwide; however, the patterns and impacts differ, with some areas seeing decreases and others experiencing increases.
Previous studies indicate that nitrogen pollution can reduce the richness of plant communities by up to 20 to 30% across herbaceous ecosystems on multiple continents. Herbaceous plants are those that do not have a woody stem, including grasses, forbs, and ferns.
The Cedar Creek Ecosystem Science Reserve, managed by the University of Minnesota, hosts the BioCON experiment, the longest-running study—by over a decade—on how interactions between CO2 and nitrogen influence species diversity in grasslands.
