Grassland Biodiversity Emerges as Key Factor in Climate Crisis

An aerial view of a research plot at the Cedar Creek Ecosystem Science Reserve. Photo: UMN/Cedar Creek.

A trio of University of Minnesota scientists has found that the degree of biodiversity in the world’s grasslands is vital to their ability to continue functioning as carbon “sinks” as global carbon dioxide levels rise.

Grasslands, said researcher Melissa Pastore, cover about a fifth of the world's land surface and play a critical role in the global carbon cycle.

“They contain more than 10 percent—and up to 30 percent, based on some estimates—of the total carbon stored by Earth's terrestrial ecosystems, most of it below ground,” she said.

Pastore, corresponding author of a report published in the Proceedings of the National Academy of Sciences, was a PhD student advised by her two co-authors: Professor Sarah Hobbie, College of Biological Sciences; and Regents Professor Peter Reich, College of Food, Agricultural, and Natural Resource Sciences. Pastore is now a postdoc at the University of Vermont.

The researchers tackled the question of whether rising levels of carbon dioxide—which provides carbon for plant tissues—will stimulate plant growth and lead to decades-long carbon storage in plant tissues and soils. They also asked whether other factors, such as today’s downward trend in plant biodiversity and ongoing human-generated inputs of nitrogen (a plant nutrient), would affect carbon storage.

“The world faces human-caused changes on multiple fronts, from changes in atmospheric composition and climate to shifts in the species that compose Earth's ecosystems,” Pastore said. “By the year 2100, atmospheric CO2 is projected to more than double compared to pre-industrial times, and grassland biodiversity is projected to decline substantially because of climate change, land-use change, and other human disturbances like high rates of nitrogen deposition.”

The Exemplary History of Grasslands

Decades of experiments at UMN’s Cedar Creek Ecosystem Science Reserve have shown that areas with lots of plant species produce more biomass than those with few or, especially, just one species. And in the short term, extra CO2 has been found to boost carbon storage, but it wasn’t clear whether that depended on the soil nitrogen supply or species richness.

In a 19-year grassland experiment (1998-2016) at Cedar Creek, the three scientists tested the relative abilities of elevated CO2, nitrogen inputs, and biodiversity to enhance carbon storage. It was part of the longstanding BioCON experiment, which concurrently manipulates plants’ carbon dioxide and soil nitrogen supplies, as well as species richness.

The Shape of Things to Come

The researchers studied plots planted with either one, four, nine, or 16 species of plants. Some plots were given enough extra (pumped) CO2 to simulate the level projected for the end of the century. Other plots received enough nitrogen (as ammonium nitrate, a fertilizer) to simulate the current high rates of nitrogen deposition in industrialized areas of the Northern Hemisphere.

Still other plots received both treatments or neither. All plots were subject to scheduled burns, which simulate prairie fires.

The researchers monitored levels of carbon for major ecosystem components—all plant material in a plot, plus soil down to 60 centimeters—as well as plant tissue chemistry.

On average over 19 years, increasing the species richness from one species to four, nine, or 16 boosted total carbon storage by 22 to 32 percent. But even though the soil was nutrient-poor, the added nitrogen and CO2 increased carbon stores by only about 5 percent. Soil carbon—which excluded any in aboveground plants and roots down to 20 centimeters—accounted for 90 percent of total ecosystem carbon. Therefore, as soil carbon went, so also did ecosystem carbon.

One Caveat

While all treatments increased soil carbon inputs, they also boosted carbon losses, mostly from enhanced soil respiration (which releases CO2) and the effects of burning. With nitrogen and CO2 treatments, the gains from enhanced plant growth barely outweighed the losses, leaving those plots with little net carbon storage. In comparison, increasing species richness led to both greater gains and greater losses of soil carbon. However, the gains outstripped the losses by large enough margins that carbon storage rose by a substantial amount.

“Our results from BioCON suggest that some model projections may overestimate the potential for grasslands to increase carbon sequestration in response to atmospheric CO2 rise,” Pastore warned. “To promote soil carbon storage in these systems, grassland biodiversity conservation—increasing plant diversity or maintaining existing diversity—is key.”