Why Are the World’s Forests Changing the Way They Breathe?

Across the world’s forests, an invisible force is quietly altering one of Earth’s largest carbon cycles.

Picture a northern forest in spring: cool air, damp leaf litter, and a forest floor that feels springy under your boots. What looks quiet on the surface is powered by nonstop chemistry and biology belowground.

In the top layers of soil, microbes digest dead wood and fallen leaves, while threadlike roots release carbon dioxide as they burn energy to stay alive and grow. Together, that CO₂ output is called soil respiration. It is one of the biggest carbon flows on Earth, and in a healthy forest it tends to tick along with the consistency of a pulse.

In many places, that pulse is starting to change.

For decades, extra nitrogen has been arriving from human activity. Fertilizer used on farms can evaporate or wash into the air as reactive nitrogen compounds. Cars, trucks, and power plants also produce nitrogen emissions that can travel far before returning to the ground in rain, snow, or dust. Once it lands in a forest, nitrogen can act like a growth supplement at first.

The problem is what happens after years of buildup. In forests already loaded with nitrogen, additional inputs can push soil communities past their comfort zone. The biological network that supports decomposition and root activity can falter, and soil respiration can drop sharply.

A new global study published in Nature Communications argues that this two stage behavior is widespread and easy to miss, even though it reshapes how forests process carbon.

A global puzzle

Human activity has tripled nitrogen deposition since the Industrial Revolution. Fertilizers, exhaust fumes, and industrial emissions release vast amounts of reactive nitrogen, much of which comes back down to earth with rain, snow, or dust.

Ecologists have known for years that nitrogen deposition influences forest soils, but one stubborn question kept surfacing: Why does nitrogen pollution increase soil respiration in some forests, but decrease it in others?

Past studies seemed to contradict one another. Some reported large jumps in CO₂ coming out of the soil. Others found the opposite, with strong declines. The new work treats those outcomes not as disagreements, but as different points along a broader progression.

A global synthesis that reshapes the field

To find the answer, an international research team combined data on an unprecedented scale:

• 168 nitrogen-addition experiments across global forests
• 3,689 observations of soil respiration under natural conditions
• A worldwide map of nitrogen-limited versus nitrogen-saturated forests
• High-resolution nitrogen deposition data
• Measurements of both root and microbial respiration

Using machine learning techniques, the team modeled how soil respiration responds to varying nitrogen levels across every forested region of the planet.

Their conclusion was clear. Forests do not respond in a uniform way. Instead, two distinct pathways emerge.

Pathway 1: When nitrogen feeds the soil

In nitrogen‑limited forests, common in boreal regions and remote mountain areas, a little nitrogen acts like long‑awaited nourishment. Microbes multiply, roots grow more vigorously, decomposition speeds up, and soil respiration rises.

But only to a point.

As nitrogen increases, the boost weakens. Toxicity builds. Easily available carbon is used up. Eventually, the curve bends downward in an inverted U‑shape: a rise, a plateau, and then a decline.

It’s the ecological version of too much fertilizer burning a plant’s roots.

Pathway 2: When nitrogen breaks the system

In forests already saturated with nitrogen, the response is far more abrupt.

Additional nitrogen pushes ecosystems beyond their tolerance level. Microbial communities shift. Sensitive species disappear. Fine roots shrink and die. Soil becomes more acidic. And soil respiration doesn’t simply decline, it collapses.

These sudden transitions are common in regions with decades of heavy nitrogen pollution, including:

• parts of Europe
• eastern China
• the eastern United States

In these places, two forests receiving the same nitrogen input can respond in completely different ways: one breathing faster, another breathing significantly slower.

A hidden climate feedback

Soil respiration is immense: seven to eight times larger than global human fossil fuel emissions. Even small shifts matter.

Globally, the study finds that nitrogen deposition increases soil respiration by about 5%. Most forests are still nitrogen‑limited enough that added nitrogen speeds up the soil’s metabolism.

But where forests are saturated, collapsing respiration is not a positive sign. It often reflects declines in microbial biomass and root activity; the very processes that build and stabilize soil carbon.

Less CO₂ may be released in those regions, but the soil may also become less resilient.

A unified framework and new clarity

By combining thousands of datasets and decades of ecological theory, the researchers propose a new framework that explains both the gradual and the abrupt responses observed worldwide.

It incorporates:

• biochemical limits
• species‑level nitrogen tolerance
• shifts in community composition
• tipping points
• global nitrogen deposition patterns

For the first time, scientists can reliably predict how nitrogen pollution will alter soil respiration at the scale of the entire planet.

Why it matters

Reducing nitrogen pollution, whether from fertilizers, industry, or transport, is already high on the agenda for biodiversity and air quality.

This study adds another reason: Lower nitrogen levels could help stabilize the forest soil carbon pool.

By preventing ecosystems from crossing nitrogen‑saturation thresholds, we may help forests maintain their natural respiration rhythms, and their ability to store carbon in a changing climate.

Reference: “A general framework for nitrogen deposition effects on soil respiration in global forests” by Xiaoyu Cen, Peter Vitousek, Nianpeng He, Ben Bond-Lamberty, Shuli Niu, Enzai Du, Kailiang Yu, Mianhai Zheng, Kevin Van Sundert, Elizabeth L. Paulus, Liyin He, Li Xu, Mingxu Li and Klaus Butterbach-Bahl, 16 December 2025, Nature Communications.
DOI: 10.1038/s41467-025-67203-8

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.

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