- Grasslands are critical global ecosystems, but pollution from agricultural fertilizers and nitrogen-laden smog threatens to dump too many nutrients onto the grasses.
- Ecologists applied various fertilizers to grassland plots at 45 sites on five continents annually and measured biodiversity and plant growth.
- Excess nutrients decreased biodiversity more and more over time, even when plant growth remained constant.
At first glance, grasslands may look like monotonous stretches of green or gold. But closer inspection reveals a bounty of diverse grasses, clovers and wildflowers coexisting within small areas. These rich ecosystems cover nearly one-third of Earth’s land surface and provide food and fuel to humans and animals alike. However, they face increasing pollution from agricultural fertilizer and industrial smog—sources of excess nutrients, the elements that plants use to grow.
Now, a large international team has examined how adding various nutrients affects how many species grow within global grasslands. It’s the first time that ecologists have studied multiple nutrients in combination, said lead author Dr. Stan Harpole, a professor at the German Centre for Integrative Biodiversity Research in Leipzig. Harpole estimated that 90 percent of previous nutrient studies have examined either nitrogen or phosphorous, but no other factors.
Humans are “shifting where nutrients, like nitrogen and phosphorous, are in the world,” said co-author Dr. Elizabeth Borer, associate professor of ecology at University of Minnesota, St. Paul, in the U.S. “Does nitrogen do something fundamentally different from phosphorous? Or is it just that we’re dumping a bunch in of one or the other?”
The team’s study, published recently in the journal Nature, is the culmination of a 10-year international effort by more than 30 ecologists on five continents. By applying various combinations of fertilizers to grassland plots, the scientists demonstrated that excess nutrients actually decreased the number of grass species, or biodiversity, within those plots.
In 2005, Harpole, Borer, and five other ecologists founded an international collaboration of grassland ecologists, called the Nutrient Network, with a standard experimental process that can be conducted anywhere in the world. Each year, scientists apply nitrogen, phosphorous or potassium fertilizers, either singly or in combination, to square-meter-sized plots. At the end of the seasonal growing cycle, they cut the grasses to measure biodiversity and biomass—the amount of vegetation within a plot—and reapplied fertilizer to begin the next year’s cycle.
The new study used 45 sites that had at least three years’ worth of data. Locations ranged from the savannas of South Africa and Tanzania to Swiss mountain meadows, and from North American prairies to the Australian Outback. In some cases, researchers had to hike to plots perched precariously on remote mountainsides, carrying bags of fertilizer and garden tools.
Biodiversity declined at all these varied sites when the researchers applied fertilizer, especially when they added more than one type of nutrient. Over time, the loss of plant species became even more pronounced. Increased nutrient availability means the grasses that best siphon up those nutrients can outgrow the less-efficient plants, eventually hogging the sunshine and crowding them out.
Plant growth, or biomass, increased with added nutrients at most sites, as the team expected. At 11 sites, however, the biomass remained unchanged. Still, even at those sites, the number of plant species went down.
“An increase in biomass production means an increase in competition, [but] apparently you can get the same effect without an increase in competition,” said Dr. Wim van der Putten, a terrestrial ecologist at the Netherlands Institute of Ecology, who was not involved with the study. “To me, this is the key novelty.”
He also pointed out that the study did not measure water limitation, another critical resource for grasslands. Harpole acknowledged this, and told Mongabay that moving forward would require “more expensive data,” including soil moisture sensors.
Looking ahead, Harpole hopes to expand this project. “We know that nutrients besides nitrogen and phosphorus are important” in grassland ecosystems, he said, but “we don’t know which ones.” He also wants to learn what happens to the grassland species that disappear, and how invasive species might invade and disrupt the fertilized plots.
“There are all these hypotheses we still can’t answer,” he said. “On the one hand [this is] a major step, but on the other hand it’s just the first step.”
Harpole, W. S. et al. (2016). Addition of multiple limiting resources reduces grassland diversity. Nature 537, 93-96. doi:10.1038/nature19324
Emma Hiolski is a graduate student in the Science Communication Program at the University of California, Santa Cruz. Other Mongabay stories produced by UCSC students can be found here.