Forest soils may hold nitrogen better as temperatures rise

A six-year study in northern China has upended a long-held assumption about how warming forests respond to climate change. Scientists expected rising temperatures to unlock nitrogen from soil, releasing it into the atmosphere and starving trees of an essential nutrient. Instead, they found the opposite—at least in drier climates.

Researchers from UC Riverside and the Chinese Academy of Sciences installed infrared heaters above forest plots in Qingyuan County, warming the soil to simulate a 2°C temperature increase (roughly what climate models predict by mid-century). Over six years, they took more than 200,000 gas measurements from the soil. When warming arrived, emissions of nitric oxide dropped 19%, while nitrous oxide—a potent greenhouse gas—fell 16%.

"We've always thought warming would accelerate microbial processes and release more nitrogen," says Pete Homyak, the UC Riverside environmental scientist who led the work. "But in the field, especially under dry conditions, the microbes slow down because the soils dry out."

The finding matters because forests are critical carbon sinks. Trees need nitrogen to grow, and if warming strips it away too quickly, forests lose their ability to absorb carbon from the atmosphere. A forest that can't grow is a forest that can't help us.

The moisture threshold

But the story has a crucial boundary. The team discovered the effect only holds in forests receiving less than 1,000 millimeters (about 40 inches) of rain per year. In wetter climates, warming still increases nitrogen loss, exactly as earlier laboratory models predicted. This distinction matters enormously. It means climate models that treat all forests the same—or that ignore soil moisture—are missing half the picture.

Kai Huang, the postdoctoral researcher who first authored the study, frames it plainly: "Our concern is about what warming does to the nitrogen cycle, and whether forests will have enough nutrients to keep absorbing carbon as the planet heats up."

Yet the research carries an uncomfortable caveat. While nitrogen stayed in the soil of warmed plots, the trees didn't grow faster. Unpublished measurements suggest they may actually be growing more slowly, possibly because drought stress is limiting their ability to use the nitrogen that remains. "We may not be losing nitrogen to the atmosphere in drier soils," Huang notes, "but if trees can't use it because of drought, that's another problem entirely."

This is not a signal to relax about climate change. It's a refinement—the kind that makes climate models more accurate and predictions more trustworthy. The team is continuing to monitor these experimental plots and others worldwide, tracking microbial responses, soil chemistry, and forest health as the planet warms. The longer these studies run, the clearer the picture becomes.