Warmer climates won't necessarily speed the return of nitrogen to soils as scientists once thought, according to a Purdue University study.

Increased temperatures from climate change have been expected to speed decomposition of plant materials and the return of nitrogen to soils, making the soil more fertile for plants. But Jeff Dukes, an associate professor of forestry and natural resources at Purdue, found that the microbes responsible for returning nitrogen to soils react differently to a range of climate scenarios.

"More nitrogen being available is not something we can count on in all ecosystems," said Dukes, whose findings were published in the journal Global Change Biology.

The findings suggest that while warming has been expected to accelerate nitrogen cycling, it may actually have little to no effect on the process in some ecosystems. This means that climate models that assume increased soil fertility in warmer conditions may overestimate the amount of plant productivity in those ecosystems.

Dukes runs the Boston-Area Climate Experiment, which measures ecosystem responses to climate change. His research group uses heaters, plastic roofs and sprinklers to change the climate over small plots of land and then tests plant and soil responses to increases in temperature and increases or decreases in precipitation.

In this experiment, Dukes tested 36 plots under 12 different climate scenarios - four levels of warming and three different levels of precipitation - over two years.

Although Dukes and his team expected to find that warming would speed the return of nitrogen to the soil, they instead found that warming and changes in precipitation rarely influenced this rate. Instead, warming and drought caused the processes involved in nitrogen cycling to become less sensitive to temperature.

"It seems that some ecosystems are less responsive than we expected," Dukes said. "It may be that as you warm up, the soil microbes in those ecosystems adjust and the rate of nitrogen cycling winds up being the same."

Novem Auyeung, a doctoral student who was involved in the study, added, "Soil microbes operate on very short time scales, and many adjustments could have happened in the microbial community over the course of our two-year experiment."

Dukes said it would take further study to understand just how soil microbes are affected by climate changes.

"These responses would have to be based on the abundance or composition of microbes or the activity of these microbes," Dukes said. "We need to learn which it is."

Dukes said he would like to test whether the results are similar in other soil types so that climate models can accurately simulate these processes. 

"Soil fertility can affect carbon storage, and ultimately the rate of climate change," Dukes said. "I want to see how general this result is so that we can better predict how ecosystems will function in the future. If lots of ecosystems work this way, then nature may not be as good at slowing climate change as we had thought."

The National Science Foundation and U.S. Department of Energy funded the research.