Forest soils release less nitrogen as moisture disappears

For years, scientists assumed one thing about warming forests: heat up the soil, and microbes would speed up, venting more nitrogen gases into the air.

That extra nitric oxide and nitrous oxide would mean worse air pollution, a stronger greenhouse push, and fewer nutrients left behind for trees.

A research team led by UC Riverside has now tested that idea where it counts – outdoors, for six continuous years -and found the opposite in a dry summer forest.

Dry soils and microbial activity

When researchers warmed a temperate woodland in northeastern China by about 3.6°F (2°C), nitric oxide emissions fell by 19 percent and nitrous oxide by 16 percent.

The reason wasn’t that microbes sped up, but that they slowed down. As soils dried under the heat, microbial activity stalled instead of accelerating.

“These results flip our assumptions,” said Pete Homyak, a UC Riverside associate professor of environmental sciences.

“Warming can boost nitrogen losses in the lab, but in the field – especially under dry conditions – microbes hit the brakes because the soils dry out.”

Testing the soil warming theory

The experiment unfolded in Qingyuan County, near Shenyang City, a site chosen because its soils and climate are sensitive to weather swings.

To mimic atmospheric warming, the team mounted infrared heaters above six 108-square-meter (about 1,160-square-foot) forest plots, warming the ground from above the way future summers are expected to.

Automated chambers on each plot periodically sealed, captured, and analyzed the gases rising from the soil.

Over six years, graduate students and postdocs based in China kept the system working, ultimately logging more than 200,000 measurements. These are enough to watch gas fluxes rise and fall with seasons, rain pulses, and heat waves.

Dry forest soils change everything

The results point to a key environmental lever that’s sometimes glossed over: water. In drier settings – roughly, places averaging less than 1,000 millimeters (about 40 inches) of annual rainfall – warming dried the surface soils enough to tamp down microbial activity.

That slowdown also weakened the nitrogen-cycling pathways that generate nitric oxide and nitrous oxide.

In wetter forests, the team notes, warmth can have the expected effect: moister soils stay active and nitrogen losses go up, matching many lab predictions.

“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,” said ecologist Kai Huang, the study’s first author.

“This study shows that moisture, not just heat, is key.” Put simply, temperature alone doesn’t tell you how nitrogen will move. You have to factor in how warming reshapes soil water.

Impact on climate models

Forests are one of Earth’s biggest carbon sponges, pulling more CO2 out of the air than they emit. But trees only keep absorbing carbon if they can grow, and growth depends on nitrogen.

If warming consistently stripped soils of nitrogen as gas, forests could falter as carbon sinks.

If, instead, nitrogen tends to stay put in drying soils, that specific risk eases. The new field data carve out a middle path: in dry summer regions, fewer nitrogen gases escape, but drought can still throttle tree growth even when nitrogen remains in the soil.

“This is a major refinement,” Homyak said. “Climate models that overlook soil moisture are missing a crucial part of the story.”

Integrating heat-moisture interactions into Earth system models should sharpen projections of both greenhouse gas emissions from soils and forest productivity under future climates.

Nitrogen drops, trees stall

One surprise lurks in the team’s ongoing measurements. If less nitrogen is leaving the soil as gas, you might expect trees to grow faster. They did not.

Early, unpublished observations suggest trees in the warmed plots may be growing more slowly than those in the controls, most likely because water stress is overriding any nutrient benefit.

“We may not be losing nitrogen to the atmosphere in drier soils, but if trees can’t use it because of drought, that’s another problem entirely,” Huang said.

This nuance underscores the tangle of trade-offs warming creates: soils leak less nitrogen, but plants may be too thirsty to capitalize on it.

A clear look at forest soil responses

Methodologically, the study’s strength is time and resolution. Automated chambers provided a high-frequency view of how subtle shifts, such as an extra-hot week, a thin rain front, or a prolonged dry spell, ripple through the soil’s microbial machinery.

The team’s approach also aligns the field with the lab: the same gases and pathways can be studied under natural weather and root conditions, not just in controlled pots.

The upshot is a clearer sense of thresholds: below roughly 1,000 millimeters (about 40 inches) of annual rain, warming tends to dry soils enough to mute nitrogen emissions. Above that, warmth often amplifies them.

Uncovering what drives change

The UC Riverside team and their collaborators are continuing to track microbial communities, soil chemistry, and tree health at the Qingyuan site and in parallel forest experiments worldwide.

The goal is to pin down the mechanisms. Researchers want to know who which microbes are speeding up or slowing down, which nitrogen pathways dominate under different moisture regimes, and how often short, intense rain events reset the system. The next step is to translate those findings into model parameters.

“As the planet warms, these long-term studies help us fine-tune climate models and better understand how forests will behave in a world that’s changing quickly,” said Homyak.

Takeaway for a hotter, dryer world

The headline isn’t that warming is “good” for nitrogen. It is that context rules. In dry forests, extra heat can dry soils enough to suppress nitrogen gas losses. In wet forests, it can do the reverse.

Either way, forests still face a dual constraint: water scarcity can choke growth even when nutrients remain in place, and moisture surpluses can speed nitrogen loss.

The path forward for climate forecasting is to model heat and moisture together, not in isolation, if we want realistic expectations for how forests feed, breathe, and buffer the atmosphere in the decades ahead.

The study is published in the journal  Proceedings of the National Academy of Sciences.

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