When too much of a good thing becomes dangerous

On a cool spring morning in a northern forest, the ground feels soft underfoot. Mist hangs between the trunks, and the air smells of wet leaves and old humus; the slow alchemy that keeps a forest alive.

Beneath the surface, billions of microbes break down organic matter and hair-thin roots exhale, releasing steady pulses of carbon dioxide. This process, known as soil respiration, is one of the largest carbon fluxes on the planet, usually so stable it feels almost like a steady heartbeat.

But in many forests around the world, that heartbeat is changing.

For decades, invisible nitrogen drifting from agriculture, traffic and industry has settled over the ecosystems like a fine dusting of fertiliser. At first, nitrogen can boost growth. But increasingly, in forests already saturated with it, the risk is high that the biotic communities and soil respiration would collapse.

A new global study published in Nature Communications explains why and shows that this phased effect may be one of the most overlooked consequences of human pollution.

A global puzzle

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

Researchers have long known that this affects forests, but they have faced a persistent puzzle:

Why does nitrogen pollution increase soil respiration in some forests, but decrease it in others?

Previous studies pointed in opposite directions. Some saw dramatic CO₂ increases; others saw sharp declines. No single explanation could make sense of both.

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, they modelled how soil respiration responds to nitrogen in every forested region on Earth.

The result: forests don’t follow one pattern; they follow two.

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 fertiliser 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 stabilise 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 fertilisers, industry or transport, is already high on the agenda for biodiversity and air quality.

This study adds another reason:
Lower nitrogen levels could help stabilise 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.

More information

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.

Conflict of interest: None

Read more: The publication: “A general framework for nitrogen deposition effects on soil respiration in global forests” is published in Nature Communications. It is written 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. 

ContaCt: Postdoc Xiaoyu Cen, Land-Craft, Aarhus University. Mail: xcen@agro.au.dk 

Professor Klaus Butterbach-Bahl, Land-CRAFT, Aarhus University. Tel.: 93508238 or mail: klaus.butterbach-bahl@agro.au.dk 

Communications Advisor Camilla Brodam Galacho, Aarhus University. Tel.: 93522136 or mail: brodam@agro.au.dk