A 24-year field study conducted in Minnesota has shed light on the compounded threat that nitrogen pollution and rising atmospheric carbon dioxide pose to grassland biodiversity.
The study, recently published in Nature, found that elevated carbon dioxide levels nearly tripled plant species losses caused by nitrogen pollution, marking a concerning trend for global biodiversity conservation efforts.
The study, led by University of Michigan ecologist Peter Reich, was conducted at the Cedar Creek Ecosystem Science Reserve. Researchers monitored 108 grassland plots, each spanning 7-by-7 feet, from 1998 to 2021.
The findings reveal that while both nitrogen and carbon dioxide can independently contribute to increased plant growth, their combined effects trigger significant biodiversity losses.
“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,” Reich stated.
Nitrogen pollution, primarily from fossil fuel combustion and agricultural activities, has been recognized for decades as a driver of biodiversity loss in ecosystems worldwide. This study highlights the potential for elevated carbon dioxide levels to intensify these nitrogen-induced biodiversity declines.
The researchers observed that during the last eight years of the experiment, plots exposed to both added nitrogen and elevated carbon dioxide showed a 19% decrease in species richness, compared to a 7% reduction under ambient carbon dioxide conditions.
The biodiversity loss can be attributed to competitive exclusion – a process by which certain dominant species outcompete others for light and resources. In the grassland experiment, named BioCON, these dynamics led to some species gaining a foothold at the expense of others. Reich explained that “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.”
Among the species affected by these altered environmental conditions were the tall native grass big bluestem (Andropogon gerardii), which emerged as the dominant species. As big bluestem proliferated, it cast shadows that limited light for other plants, driving further declines in biodiversity. Other species, such as the lead plant (Amorpha canescens) and rigid goldenrod (Solidago rigida), both of which prefer ample sunlight, saw marked reductions in abundance.
The implications of such changes extend beyond plant communities alone. Reich emphasized the importance of maintaining these ecosystems, noting that “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 BioCON experiment, which began in 1998, is one of the longest-running studies examining the combined impacts of carbon dioxide and nitrogen on plant diversity. Initially, the researchers observed that elevated carbon dioxide mitigated the effects of nitrogen pollution, reducing species losses in the early years. At the time, nitrogen enrichment at ambient carbon dioxide levels reduced species richness by 16%, compared to just 8% under high carbon dioxide. However, as the experiment progressed, the interaction between the two factors reversed, with elevated carbon dioxide eventually amplifying nitrogen-driven biodiversity losses.
According to the researchers, these findings underscore the complexity of biodiversity conservation in a world with rapidly shifting environmental conditions.
“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, who is Director the University of Michigan’s Institute for Global Change Biology (IGCB).
Nitrogen pollution remains elevated globally, though its impacts vary by region. Observational and experimental studies indicate that nitrogen deposition can decrease plant species richness by 20% to 30% in herbaceous ecosystems across multiple continents. Grasslands, which are composed mainly of herbaceous plants, are particularly vulnerable to these pressures. Despite some regional reductions in nitrogen deposition, the overall trend remains troubling.
With their critical role in the planet’s ecological stability, grasslands are increasingly being recognized as conservation priorities. Reich added, “Calls for biodiversity preservation and restoration are already at a fever pitch. Our results only further add to that chorus.”
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The study received funding from the U.S. National Science Foundation and the U.S. Department of Energy.
Journal Reference:
Reich, P.B., Mohanbabu, N., Isbell, F., Hobbie, S.E., Butler, E.E., ‘High CO2 dampens then amplifies N-induced diversity loss over 24 years’, Nature (2024). DOI: 10.1038/s41586-024-08066-9
Article Source:
Press Release/Material by University of Michigan
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