For centuries, forests have followed a surprisingly consistent rhythm. Beneath trees, roots and microorganisms break down organic matter and fuel plant growth, steadily releasing carbon dioxide into the atmosphere.
Scientists call this process soil respiration, and it represents one of the largest flows of carbon on Earth.
New research suggests that this natural rhythm is disrupted by a growing but often overlooked form of pollution: excess nitrogen.
Nitrogen pollution is affecting forests around the world
On a cool spring morning, the forest floor may seem calm and still. But beneath the surface, billions of microorganisms are hard at work breaking down leaves, wood, and other organic matter. At the same time, the tiny roots release carbon dioxide as they grow and function.
These processes work together to create a steady exchange of carbon between land and the atmosphere.
However, for decades, forests have been exposed to increasing amounts of nitrogen pollution. Fertilizers, vehicle exhaust, and industrial activities release reactive nitrogen into the air, much of which ultimately returns to the ground through rain, snow, or airborne particles.
Since the Industrial Revolution, human activities have approximately tripled the amount of nitrogen deposited on Earth.
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 long-standing forest mysteries
To investigate, an international team of researchers collected 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 map showing nitrogen-limited and nitrogen-saturated forests
- High-resolution nitrogen deposition data
- Measures both root respiration and microbial respiration
The team then used machine learning to model how forests around the world would respond to increased nitrogen input.
Their conclusion was surprisingly simple. Not all forests respond in the same way. Instead, they typically follow one of two different routes.
When nitrogen acts like a fertilizer
In nitrogen-deficient forests, additional nitrogen may initially stimulate biological activity.
These nitrogen-limited forests are commonly found in northern regions and remote mountainous areas.
When nitrogen is available, microbial activity increases, resulting in faster root growth and faster decomposition of organic matter. As a result, soil respiration increases.
However, the benefits do not last indefinitely.
As nitrogen levels continue to rise, the positive effects begin to wane. Toxicity can occur, readily available carbon sources are depleted, and increases in soil respiration level off before eventually decreasing.
Researchers describe this pattern as an inverted U-shaped response. Soil respiration increases, reaches a peak, and then begins to decline.
When nitrogen exceeds forest limits
The situation is very different in forests, which already contain high levels of nitrogen.
In such nitrogen-saturated ecosystems, additional nitrogen can push the system beyond tolerance thresholds.
Microbial communities change. Sensitive species will disappear. The thin roots will shrink or die. Soil acidity increases.
Instead of a gradual response, soil respiration can decline rapidly.
The study found that such rapid declines are common in regions that have experienced decades of heavy nitrogen pollution, such as parts of Europe, eastern China, and the eastern United States.
As a result, two forests that receive the same amount of nitrogen can respond very differently. Some people experience an increase in soil activity, while others experience a significant decrease.
Relationship with the hidden climate
This discovery is important because soil respiration is huge on a global scale.
Researchers estimate that soil respiration releases seven to eight times more carbon than the annual fossil fuel emissions produced by humans.
Even relatively small changes can have significant effects.
Overall, the study found that nitrogen deposition increases global soil respiration by about 5%. Most forests are sufficiently nitrogen limited that adding nitrogen will stimulate biological activity.
However, the reduced respiration observed in nitrogen-saturated forests is not necessarily good news.
Reduced carbon dioxide emissions from soils in these regions often reflect reduced root activity and reduced microbial populations. They are important components of healthy ecosystems and play a key role in building and maintaining soil carbon stores.
In other words, reduced carbon emissions may represent a loss of ecosystem resilience rather than an environmental benefit.
A new framework for predicting forest responses
By combining thousands of observations and decades of ecological research, scientists have developed a new framework that helps explain both gradual and sudden reactions observed around the world.
The framework includes:
- biochemical limits
- Species-specific nitrogen tolerance
- Changes in community composition
- ecological tipping point
- Global nitrogen deposition patterns
For the first time, researchers say they can now more reliably predict how nitrogen pollution will affect soil respiration across the planet.
Why reducing nitrogen pollution is important
Efforts have already begun to reduce nitrogen pollution due to concerns about biodiversity loss and air quality.
The new findings suggest another important benefit.
Reducing nitrogen inputs from agriculture, transportation, and industry can help conserve carbon stored in forest soils.
By preventing ecosystems from exceeding nitrogen saturation thresholds, forests may be able to better maintain natural carbon cycling processes and remain resilient as the climate continues to change.
Collaborators: Land-CRAFT from 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 study was financially supported by the National Natural Science Foundation of China (32430067, 32588202, 42141004) and the National Key Research and Development Program of China (2023YFF1305900, 2022YFF080210102) received by NH, and the Pioneer Center of Landscape Research in the Future of Sustainable Agriculture. (Land-CRAFT), KBB receives DNRF permit number P2
