First-of-its-kind global study shows grasslands can withstand climate extremes with a boost of nutrients

"Resources such as nutrients and water have been fundamentally altered by humans on a global scale, and this can disrupt how plants grow," said Amber Churchill, an assistant professor of ecosystem science at Binghamton University and co-author on the study. "Extreme changes in these resources are therefore predicted to have an even larger potential impact, with implications for a range of economic sectors. This is especially true for global grasslands, where resource availability for water and nutrients directly supports livestock and pastoralism on all inhabited continents."

To address this issue, the researchers assessed how grasslands respond to extreme drought and increased nutrient availability through field experiments at 26 sites across 9 countries.

"It took what are often very site-specific methodologies, where we're interested in the impacts of nutrients or the impacts of drought and water availability, and it scaled a single site experiment up to something at a much larger spatial scale," said Churchill. "The ability to test both variation in nutrients, as well as this global change driver of impacts of drought in combination at such a huge spatial extent -- that's the really novel aspect of this experiment."

The researchers added nitrogen, phosphorus, and potassium -- essential nutrients that all plants need -- as well as a one-time addition of a series of micronutrients. They found that while drought alone reduced plant growth by 19%, adding fertilizer increased plant growth by 24%. Importantly, the combination of the two resulted in no net change in growth, largely driven by grasses that were able to take advantage of the added nutrients even under drought.

"The really big takeaway is that adding nutrients can offset the impact of drought, and this is really true in areas that are already pretty dry," said Churchill.

Churchill worked at two of the 26 sites. At the Yarramundi site at the Hawkesbury Institute for the Environment of Western Sydney University, she managed data, recording the number of plants present in the area. At the Cedar Creek Long Term Ecological Research Site in Minnesota, she was in charge of cleaning all data and organizing it to be shared with the network of researchers.

"In terms of drought, we get less growth; we add fertilizer, we get more growth. As you're seeing some of those idiosyncrasies, the sort of follow-up lines are where it gets a little bit more interesting," said Churchill. "Traditionally, we might hypothesize that if plants are already limited by water at, say, an arid site, plants may not be able to respond to adding nitrogen. But we actually found the opposite of that, where plants are able to better respond to the nitrogen addition under these more arid conditions. And so that's a really sort of a striking difference than what we might have expected."

Churchill will be creating similar treatments at Binghamton University as part of the Pasture and Lawn Enhanced Diversity Global-change Experiment (PLEDGE), at Nuthatch Hollow, a 75-acre, "open-air lab" at Binghamton University.

While adding fertilizer might temporarily offset the effects of drought, said Churchill, it's not a feasible long-term solution.

"In a forage production system where you need to offset the effects of drought, adding fertilizer will remove that effective drought," she said. "That's a great benefit, but that costs a lot of money. So there's a tradeoff there. It can be a tool used, but it's not going to be the long-term solution."

Churchill said that in terms of management, the number of plant species growing might be a more important factor in surviving drought.

"We have a prediction that as you have more species, one of those species is more likely to withstand the drought, so you'll get at least some biomass, even if each species doesn't make it. And so the idea is you'll have more stable biomass over the long term if you have more species present. That's something we can't test with this data set, because we're only looking at one year. But longer-term data sets can look at that sort of a question."