A new study published in Nature Communications resolves a decades-old debate about how nitrogen pollution affects forest soil respiration. Researchers analyzed 168 nitrogen experiments from forests worldwide and found that the direction of change depends on how much nitrogen a forest already holds. Forests low in nitrogen see a temporary boost in soil carbon dioxide release as microbes and roots become more active. Forests already saturated with nitrogen can suffer a collapse in soil respiration, signaling a failing ecosystem rather than a climate benefit. The practical takeaway is significant for carbon accounting and forest management. Globally, nitrogen pollution still raises forest soil respiration by about 5% because most forests remain nitrogen limited. But in heavily polluted regions like the eastern United States, western Europe, and eastern China, additional nitrogen can push soils past a tipping point. Cutting nitrogen pollution to protect air quality also helps safeguard the carbon stored in forest soils, giving policymakers a clear lever for climate mitigation.
