The central finding is stark: since the 1960s, the annual amount of reactive nitrogen entering the biosphere has increased roughly fivefold. This surge is driven overwhelmingly by human activity, above all the Haber-Bosch process that converts atmospheric N2 into ammonia for synthetic fertilizer, a technology that has underpinned agricultural expansion since the 1940s. Between 2015 and 2025, global bulk nitrogen inputs to terrestrial and aquatic ecosystems are estimated at 330 teragrams of nitrogen per year and 183 teragrams per year respectively, with wide uncertainty ranges. By comparison, equivalent inputs in the late 1950s stood at just 121 and 55 teragrams per year, a more than doubling of the flows that nourish the world’s ecosystems in barely two generations.

What makes the new analysis remarkable is what happens on the output side of the ledger. Emissions of nitrogen from the biosphere to the atmosphere increased by only about 12 percent between the oldest and the most recent estimates. In other words, humanity has poured vastly more reactive nitrogen into the Earth system than natural processes have been able to convert back into inert gas. The result is a terrestrial nitrogen accumulation on the order of 50 teragrams of nitrogen per year between 1959 and 2025, an amount sufficient to fundamentally alter soil chemistry, water quality and atmospheric composition across the globe.

The sinks themselves are expanding, but not nearly fast enough to keep pace. Terrestrial denitrification, the anaerobic microbial process that converts soil nitrate back into gaseous forms of nitrogen, is estimated at roughly 100 teragrams of nitrogen per year for the 2010 to 2020 period, up from 69 teragrams per year in 1955. Ocean denitrification estimates rose from 87 teragrams per year to approximately 200 teragrams per year over the same interval. Even these substantially larger losses, however, are dwarfed by the incoming flux, which is why reactive nitrogen continues to build up in soils, groundwater, vegetation and coastal sediments around the world.

This accumulation is not merely a bookkeeping curiosity; it carries real ecological and economic costs. Excess nitrogen availability pollutes soils, waters and air, driving eutrophication of lakes and coastal seas, hypoxic dead zones, acidification, loss of biodiversity and the release of nitrous oxide, a greenhouse gas nearly 300 times more potent than carbon dioxide over a century and now considered the dominant ozone-depleting substance emitted in the twenty-first century. The nitrogen cascade, the term scientists use to describe how a single atom of reactive nitrogen can cause harm sequentially in the atmosphere, on land and in water, means that each unit of nitrogen applied to a field can trigger a chain of environmental consequences far beyond the farm gate.

Part of the difficulty in managing the nitrogen cycle is that several of its largest components remain the most poorly constrained. The review identifies terrestrial biological nitrogen fixation and denitrification as the largest uncertainties in the global budget, and notes that improved estimates of anthropogenic nitrogen inputs to aquatic ecosystems are urgently needed to reduce the high uncertainties surrounding food systems in a changing world. Further quantification of the ocean biomass pool and of nitrogen release through rock weathering is also required. Processes only recently discovered, such as comammox, the complete oxidation of ammonia to nitrate by single microorganisms, and feammox, the anaerobic oxidation of ammonium coupled to iron reduction, are not yet represented in spatially explicit global models, representing a clear priority for future research.

The authors compiled their estimates by synthesizing published budgets across the atmospheric, terrestrial and aquatic reservoirs, drawing on datasets now available openly through the Zenodo repository. This assembly of historical fluxes allows, for the first time, a coherent view of how the modern nitrogen budget has evolved since the earliest global syntheses of the late 1950s. By comparing the earliest comprehensive estimates with the latest data, the review demonstrates that the gap between nitrogen inputs and nitrogen removal has widened dramatically, transforming the global cycle from a roughly balanced system into one with a persistent and growing surplus of reactive nitrogen stored in the terrestrial biosphere.

Closing the budget is not merely an academic exercise. Quantifying nitrogen pools and fluxes across reservoirs is critical for monitoring the imbalance and for evaluating mitigation strategies, from national fertilizer policies to international climate agreements. The review argues that better observations, measurement techniques and models are essential for identifying knowledge gaps and guiding the transition toward a more balanced and sustainable nitrogen cycle. Without such constraints, policymakers are effectively flying blind when attempting to design interventions, since it remains unclear how much of the applied nitrogen is retained, how much is lost to water and how much returns to the atmosphere in reactive or inert forms.

The good news embedded in the analysis is that a pathway toward rebalancing exists. The authors estimate that reducing fertilizer demand and use through dietary changes, deploying technological advances that enhance fertilizer use efficiency, and improving waste management and nutrient recycling could together support a net flux of 51 teragrams of nitrogen per year back to the atmosphere, a magnitude comparable to the current terrestrial accumulation. In practical terms, this means shifting diets toward less nitrogen-intensive protein sources, adopting precision agriculture and controlled-release fertilizers, and recapturing nitrogen from human and animal waste streams that would otherwise flow into rivers and coastal waters.

Nitrogen sits alongside carbon as a headline element of the Anthropocene, and its trajectory will help determine whether humanity stays within planetary boundaries. The review’s numbers make clear that the era of cheap, abundant reactive nitrogen has transformed the planet as profoundly as the rise of atmospheric carbon dioxide, and that correcting the imbalance will require coordinated action across agriculture, energy, sanitation and diet. What the new synthesis offers is a clearer map of where nitrogen is coming from, where it is going and how much remains unaccounted for, giving scientists and policymakers alike the baseline they need to begin steering the global nitrogen cycle back toward equilibrium.

Subject of Research: Fluxes and imbalances in the modern global nitrogen cycle

Article Title: Fluxes and imbalances in the modern global nitrogen cycle

Article References: Almaraz, M., Sun, X., Davidson, E. A., Zhang, X., Galloway, J. N., & Raymond, P. A. (2026). Fluxes and imbalances in the modern global nitrogen cycle. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-026-00821-y

Image Credits: AI Generated

DOI: 10.1038/s43017-026-00821-y

Keywords: nitrogen cycle, reactive nitrogen, Haber-Bosch process, denitrification, nitrogen fixation, fertilizer, nitrous oxide, eutrophication, biogeochemistry, global nitrogen budget, comammox, nitrogen pollution