Hotter, drier soils are changing how forests breathe nitrogen

Forest soils are constantly talking – not in words but through quiet chemical exchanges between microbes, roots, and the air above them.

For years, scientists had assumed that warming temperatures would speed up this underground chatter and release more nitrogen gases into the atmosphere.

But long-term fieldwork is now revealing something different. Heat alone doesn’t control these reactions. Water scarcity, seasonal freeze-thaw cycles, and shifting microbial groups play far bigger roles than expected.

A new multi-year experiment shows just how surprising forest soil behavior can be when warming is studied in real landscapes instead of in lab jars.

Forest nitrogen in a warming world

Earlier forecasts suggested rising temperatures would accelerate forest nitrogen gas loss, robbing soils of nutrients vital for tree growth while contributing to atmospheric pollution.

This expectation was contradicted by a recent study from UC Riverside (UCR). The research was focused on a temperate forest in northeastern China and supported by more than 200,000 measurements.

The results showed that emissions of nitric oxide and nitrous oxide declined once warming began.

“These results flip our assumptions, capturing a central lesson drawn from long-term field observation,” said Pete Homyak, UCR associate professor of environmental sciences.

Insights from a related study reinforce that point. Laboratory incubations predicted strong temperature sensitivity for nitrogen gas production, yet real-world responses did not behave in that manner. 

Warming reduces soil moisture

Infrared heaters lifted soil temperature by roughly two degrees Celsius (3.6 degrees Fahrenheit), replicating projected mid-century conditions. As soils warmed, water content dropped in surface horizons.

Lower moisture was enough to suppress nitrogen transformations usually driven by microbial metabolism.

According to the study, moisture loss constrained net mineralization and nitrification. This reduced ammonium supply for nitrifying microbes and limited the production of nitric oxide and nitrous oxide.

Drying also altered seasonal dynamics. Earlier thawing reduced exposure to freeze-thaw events that normally stimulate denitrification pulses. This decrease lowered nitrous oxide release during late winter and early spring.

Shifts in moisture availability therefore played a stronger controlling role than thermal stimulation across much of the year.

Regional patterns stand out

Results from additional experimental forests reveal comparable patterns. Across sites receiving less than roughly one meter (3.3 feet) of annual precipitation, warming consistently produced drier soils accompanied by lower nitrous oxide emissions.

In contrast, wetter regions exhibited increased emissions under warming, aligning with older laboratory-based expectations. This broad comparison highlights the sensitivity of soil nitrogen behavior to regional moisture regimes. 

Findings from gene analysis add another dimension. Abundances of ammonia-oxidizing archaea increased under warming, while ammonia-oxidizing bacteria declined. This signals a shift in microbial guilds that mediate nitrification.

Yet even with such shifts, overall nitrogen gas output still fell whenever moisture supply tightened. 

Big impact on forest health

Moisture reductions of a magnitude similar to those observed in the experiment are already underway across many temperate regions.

According to the study, even small losses can strongly alter enzyme activity, substrate diffusion, and oxygen availability, collectively reshaping gaseous nitrogen production.

These changes ripple outward; forests rely on steady nutrient turnover to sustain carbon uptake.

“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 study lead author Kai Huang.

Despite reduced atmospheric nitrogen loss in the warmed plots, ongoing observations indicate tree growth did not accelerate. Drier forest soils limit nutrient uptake even when nitrogen remains in place.

“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,” said Huang.

Refining climate outlooks

Evidence from the research strengthens an emerging consensus: accurate climate forecasting requires explicit representation of soil moisture responses under warming.

Soil drying influences nitrogen cycling, carbon storage, energy balance, and local to regional climate feedbacks.

Many models assume enhanced microbial metabolism under hotter conditions, yet findings here demonstrate that insufficient moisture can override expected thermal reactions. 

“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.

Complex interactions among heat, moisture, soil chemistry, and microbial life mean real ecosystems rarely behave in simple or linear ways.

Carefully maintained warming experiments such as the one described here offer valuable clarity, revealing how future forests may respond under increasingly variable conditions.

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

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