Nitrogen Falling From the Sky Changes What Forests Breathe Out

A forest does not only take carbon in. On a warm afternoon it hands some straight back, as a thin chemical breath of isoprene, monoterpenes and heavier molecules, the family of compounds that gives a pine wood its smell and a eucalyptus stand its blue haze. Plants make them for their own reasons, among them heat tolerance and defense against insects. What a century of fertilizer, farm ammonia and combustion exhaust settling onto those leaves has done to that breath is a question the field has largely left open.

Yanli Zhang and Xinming Wang of the Guangzhou Institute of Geochemistry, together with Alex Guenther of the University of California, Irvine, and colleagues, answered it by pooling other people's experiments. Their synthesis compiles 885 observations spanning nine plant functional types, from crops and grasses to broadleaf trees. All of them come from studies in which nitrogen was added on purpose and the emissions measured afterward. The paper went up on Aug. 29, 2026, in Communications Earth & Environment.

Nitrogen raised all three classes. Isoprene emissions ran 52% higher under added nitrogen, monoterpenes 39% and sesquiterpenes 107%, roughly a doubling for the heaviest and least volatile of the three, the authors report.

None of those molecules stay as they are for long. In sunlight and in the company of the nitrogen oxides that traffic and industry supply, they help build ground-level ozone. They also oxidize and condense into secondary organic aerosol, the fine particulate matter that air-quality agencies measure. The gases a forest gives off are feedstock for two pollutants usually filed under human activity.

The response is not a straight line. A meta-regression across the pooled data traces an arc: emissions climb with nitrogen input, reach compound-specific peaks somewhere between 49 and 118 kilograms of nitrogen per hectare each year, then fall away above that. Those peaks sit well above what falls out of a real sky. Monitoring networks across China have measured deposition averaging around 20 kilograms per hectare a year, less than half the bottom of the paper's peak range. The descending arm is drawn by long-running fertilization experiments that dose a plot far harder than the atmosphere ever does, so it is not a prediction that more nitrogen will now quiet a forest down.

The third result is a map. The team ran the dose-response over the way nitrogen deposition actually shifted between 1980 and 2020. The potential for isoprene emission rises by up to about 29% across tropical regions in that calculation, and falls where deposition has come down, most visibly over China, where an independent analysis has documented the decline. Potential is the word carrying the sentence. No emissions were measured over those four decades; what changed is what the dose-response implies when it is applied to deposition fields.

There is a seam in that step. The observations come from nitrogen addition: fertilizer or solution applied to a plot in known amounts, usually across a few growing seasons. Deposition is slower, more dilute and chemically mixed, and it keeps arriving for decades. Treating the first as a stand-in for the second is a modeling assumption, not a measured equivalence, which is why the map represents a projected potential rather than a direct physical record.

The paper was published as an accepted manuscript; while peer review is complete and the DOI is permanent, the text may undergo minor formatting and copyediting before the final version of record is released. The three percentages represent one team's synthesis of a historically varied literature, where earlier studies have not always reached a consensus.

The study's final conclusion is presented as a carefully qualified proposal. Nitrogen management may provide air-quality co-benefits by modulating the biological supply of ozone and aerosol precursors. While current nitrogen regulations are written for rivers, soils and drinking water, this newly documented dose-response suggests policymakers may soon need to account for the air as well.