New global research reveals that excess nitrogen pollution is significantly altering how forests regulate carbon through soil respiration, the process by which roots and soil microbes release carbon dioxide as they break down organic matter.Soil respiration is one of the largest carbon fluxes on Earth, exceeding annual human fossil fuel emissions several times over.

The study, conducted by an international team and published in Nature Communications, analyzed data from 168 nitrogen addition experiments, thousands of field observations, and global nitrogen deposition maps.

Using machine learning, researchers identified two distinct ecosystem responses to nitrogen inputs depending on whether forests are nitrogen-limited or nitrogen-saturated.

In nitrogen-limited forests, commonly found in boreal and remote mountainous regions, small increases in nitrogen initially stimulate microbial activity, root growth, and decomposition, leading to higher soil respiration.

However, this positive effect follows an inverted U-shaped curve, eventually peaking and declining as excess nitrogen accumulates, toxicity develops, and available carbon substrates diminish.

In contrast, nitrogen-saturated forests—often located in industrialized regions such as parts of Europe, eastern China, and the eastern United States—can experience abrupt declines in soil respiration when additional nitrogen is introduced.This is linked to soil acidification, loss of microbial diversity, and reduced root activity.

Overall, the study estimates that global nitrogen deposition has increased soil respiration by about 5%, though this average masks strong regional differences.Importantly, reduced respiration in heavily polluted areas may not indicate improved conditions but rather ecosystem stress and reduced resilience.

The findings highlight hidden tipping points in forest ecosystems and suggest that continued nitrogen pollution from agriculture, industry, and transportation could disrupt global carbon cycling.Reducing nitrogen emissions may help preserve soil health, maintain biodiversity, and stabilize forest carbon storage under climate change.