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

The Sturtian glaciation, which occurred approximately 717 to 660 million years ago, represents a key interval in Earth’s Proterozoic eon, during which the planet experienced near-global ice coverage. For years, scientists have sought to unravel what processes triggered the eventual thawing of this frozen expanse. The work by Hu, Li, Wang, and their colleagues puts anaerobic oxidation of methane—specifically mediated by metal ions—in the spotlight as a decisive factor.

Methane, a potent greenhouse gas, can be produced through biological processes in anoxic environments. Traditionally, its anaerobic oxidation linked to sulfate reduction has been well documented, but in oceanic or sedimentary contexts depleted of sulfate, alternative pathways must prevail. This new study elegantly demonstrates that metal cofactors such as iron and manganese can drive AOM, providing a missing explanation for methane consumption during Earth’s deep past, especially under conditions characterized by limited sulfate availability.

Employing sophisticated geochemical analyses and isotopic tracing, the research team pieced together evidence from Proterozoic sediment records, revealing signatures indicative of metal-coupled methane oxidation. These signals correlate temporally with the decline in global ice coverage, implying that biological consumption of methane via metal-driven AOM contributed to the rise in atmospheric CO2 and greenhouse warming necessary for deglaciation.

What distinguishes this research is its interdisciplinary approach, integrating geochemistry, microbiology, and paleoclimatology. By simulating ancient environmental conditions in the laboratory and comparing these with natural sediment samples, the authors rigorously validate the hypothesis that iron and manganese oxides facilitated the anaerobic oxidation of methane. This mechanism would have served as a sink for methane, regulating its release into the atmosphere and modulating the climate.

The implications of this finding extend well beyond reconstructing a distant glacial event. Methane has long been recognized as one of the most influential yet elusive components in Earth’s climate puzzle. Understanding how ancient microbial communities harnessed metals for methane oxidation reveals a previously underestimated control on greenhouse gas fluxes, with parallels to modern anoxic marine environments and potential applications in climate models predicting future methane dynamics.

Furthermore, this study challenges existing paradigms that heavily focused on sulfate-driven methane oxidation. In the metal-rich, sulfate-poor oceans of the Neoproterozoic, alternative electron acceptors had to play significant roles. The research presented here fills this gap and suggests that biogeochemical cycling of metals was intricately intertwined with methane regulation, serving as an ancient thermostat and shaping Earth’s evolutionary trajectory.

Deglaciations like the Sturtian set the stage for significant biological innovation, including the rise of complex multicellularity. By deciphering the drivers behind these thawing episodes, Hu and colleagues contribute to a deeper understanding of how Earth’s biosphere and climate co-evolved, prompting reconsideration of feedback loops that enhanced habitability in a previously inhospitable frozen planet.

The metal-driven AOM mechanism also raises provocative questions about the extent to which microbial metabolisms have influenced Earth’s atmospheric composition and climate on geological timescales. It underlines the adaptive versatility of early life forms in exploiting available chemical energy sources, performing crucial ecosystem functions under extreme environmental constraints.

Technically, the research leverages novel isotopic markers and microanalytical techniques. For instance, iron isotope fractionation patterns, combined with methane-derived carbon isotope signals, provide compelling geochemical fingerprints of metal-mediated AOM. The detection of these signals within Sturtian sedimentary deposits substantiates the role of this metabolism during glaciation.

Additionally, the authors use thermodynamic modeling to demonstrate that metal oxides could energetically support AOM under the cold, anoxic conditions prevalent during the Snowball Earth events. These energetics underpin a feedback mechanism by which methane emissions were curtailed, preventing runaway greenhouse warming yet allowing sufficient warming to trigger deglaciation.

From an evolutionary standpoint, the work hints at an ancient origin of metal-based methane oxidation pathways, pointing to the potential existence of specialized microbial consortia that thrived in Precambrian oceans. This expands current knowledge about the metabolic diversity of early life and the complexity of primordial biogeochemical cycles.

By addressing previously unexplained isotopic anomalies and geochemical distributions in sedimentary rock formations, this study also contributes to refining the proxy records used to reconstruct Earth’s climatic and environmental conditions. It asserts that metals could have left lasting imprints in the geochemical archive, which modern analytical methods can now decode with unprecedented precision.

Finally, this research opens promising avenues for exploring the role of metals in methane cycling on other planetary bodies. For example, understanding metal-driven AOM on Earth could analogically inform the search for life and biomarkers in extraterrestrial environments, such as the icy moons of the outer solar system, where methane and oxides are also likely abundant.

Altogether, the discovery of metal-driven anaerobic methane oxidation as a key moderator in the terminal phases of the Sturtian glaciation offers a profound leap forward in Earth sciences. It showcases how minute microbial processes, operating on invisible chemical reactions, can substantially influence planetary climate stability and biosphere evolution. The integration of geochemical, microbiological, and climatic data underscores the interdisciplinary effort required to decode Earth’s ancient history and forecast its future.

As research continues to unfold, this pivotal mechanism promises to be a cornerstone in resolving long-standing questions surrounding early Earth climate dynamics, infection points of greenhouse gas flux control, and the co-evolution of life and the environment. The study by Hu, Li, Wang, and colleagues thus stands as a testament to the remarkable complexity of Earth’s past and its relevance to contemporary planetary science.

Subject of Research: Metal-driven anaerobic oxidation of methane in relation to the Sturtian deglaciation.

Article Title: Metal-driven anaerobic oxidation of methane and the Sturtian deglaciation.

Article References:

Hu, J., Li, S., Wang, SJ. et al. Metal-driven anaerobic oxidation of methane and the Sturtian deglaciation.
Nat Commun 16, 7249 (2025). https://doi.org/10.1038/s41467-025-62622-z

Image Credits: AI Generated