For centuries, forests have followed a remarkably consistent rhythm. Beneath the trees, roots and microscopic organisms steadily release carbon dioxide into the atmosphere as they break down organic material and fuel plant growth.
Scientists call this process soil respiration, and it represents one of the largest carbon flows on Earth.
New research suggests that this natural rhythm is being disrupted by a growing and often overlooked form of pollution: excess nitrogen.
Nitrogen Pollution Is Reaching Forests Worldwide
On a cool spring morning, a forest floor may seem calm and still. Yet beneath the surface, billions of microbes are hard at work decomposing leaves, wood, and other organic matter. At the same time, tiny roots release carbon dioxide as they grow and function.
Together, these processes create a steady exchange of carbon between the land and the atmosphere.
For decades, however, forests have been exposed to increasing amounts of nitrogen pollution. Fertilizers, vehicle emissions, and industrial activities release reactive nitrogen into the air, much of which eventually returns to the ground through rain, snow, or airborne particles.
Since the Industrial Revolution, human activities have roughly tripled global nitrogen deposition.
Scientists have long known that excess nitrogen affects forest ecosystems. What remained unclear was why some studies found that nitrogen increased soil respiration while others found the opposite effect.
Solving a Longstanding Forest Mystery
To investigate, an international team of researchers assembled one of the largest datasets ever used to study soil respiration.
The analysis combined:
- 168 nitrogen addition experiments conducted in forests around the world
- 3,689 observations of natural soil respiration
- Global maps showing nitrogen limited and nitrogen saturated forests
- High resolution nitrogen deposition data
- Measurements of both root respiration and microbial respiration
The team then used machine learning to model how forests worldwide respond to increasing nitrogen inputs.
Their conclusion was surprisingly simple: forests do not all react the same way. Instead, they generally follow one of two distinct pathways.
When Nitrogen Acts Like a Fertilizer
In forests where nitrogen is scarce, additional nitrogen can initially stimulate biological activity.
These nitrogen limited forests are often found in boreal regions and remote mountain landscapes.
When nitrogen becomes available, microbes become more active, roots grow faster, and organic matter breaks down more quickly. As a result, soil respiration increases.
But the benefits do not continue indefinitely.
As nitrogen levels keep rising, the positive effects begin to fade. Toxicity can develop, readily available carbon sources become depleted, and the increase in soil respiration eventually levels off before declining.
Researchers describe this pattern as an inverted U shaped response. Soil respiration rises, reaches a peak, and then begins to fall.
When Nitrogen Pushes Forests Past Their Limits
The picture looks very different in forests that already contain high levels of nitrogen.
In these nitrogen saturated ecosystems, additional nitrogen can push the system beyond its tolerance threshold.
Microbial communities change. Sensitive species disappear. Fine roots shrink or die back. Soil acidity increases.
Rather than showing a gradual response, soil respiration can drop sharply.
According to the study, this type of abrupt decline is common in regions that have experienced heavy nitrogen pollution for decades, including parts of Europe, eastern China, and the eastern United States.
As a result, two forests receiving similar amounts of nitrogen may respond in completely different ways. One may experience increased soil activity, while another may suffer a major decline.
A Hidden Climate Connection
The findings matter because soil respiration is enormous on a global scale.
Researchers estimate that carbon released through soil respiration is seven to eight times greater than annual fossil fuel emissions produced by humans.
Even relatively small changes can have significant implications.
Overall, the study found that nitrogen deposition increases global soil respiration by roughly 5%. Most forests remain nitrogen limited enough that additional nitrogen still stimulates biological activity.
However, the decline in respiration observed in nitrogen saturated forests is not necessarily good news.
Lower carbon dioxide emissions from soil in these areas often reflect declining root activity and shrinking microbial populations. These are key components of healthy ecosystems and play important roles in building and maintaining soil carbon stores.
In other words, less carbon dioxide release may sometimes signal a loss of ecosystem resilience rather than an environmental benefit.
A New Framework for Predicting Forest Responses
By combining thousands of observations with decades of ecological research, the scientists developed a new framework that helps explain both the gradual and abrupt responses observed around the world.
The framework incorporates:
- Biochemical limits
- Species specific nitrogen tolerance
- Changes in community composition
- Ecological tipping points
- Global nitrogen deposition patterns
For the first time, researchers say they can more reliably predict how nitrogen pollution will influence soil respiration across the planet.
Why Reducing Nitrogen Pollution Matters
Efforts to reduce nitrogen pollution are already underway because of concerns about biodiversity loss and air quality.
The new findings suggest another important benefit.
Reducing nitrogen inputs from agriculture, transportation, and industry could help protect the carbon stored in forest soils.
By preventing ecosystems from crossing nitrogen saturation thresholds, forests may be better able to maintain their natural carbon cycling processes and remain resilient as the climate continues to change.
Collaborators: Land-CRAFT at Aarhus University, Stanford University, National Forestry and Grassland Administration Harbin China, Pacific Northwest National Laboratory, Chinese Academy of Sciences, Beijing Normal University, Maastricht University, SLAC National Accelerator Laboratory, Duke University, and Karlsruhe Institute of Technology.
Funding: This work was financially supported by the National Natural Science Foundation of China (32430067, 32588202, 42141004) and the National Key R&D Program of China (2023YFF1305900, 2022YFF080210102) received by N.H., and the Pioneer Center for Landscape Research in Sustainable Agricultural Futures (Land-CRAFT), DNRF grant number P2 received by K.B.B.