Nitrogen is the currency of plant growth, and in the Arctic it is chronically scarce. Because the growing season is short and cold, ecologists have long assumed that nearly all nitrogen uptake happens during the brief summer. But a growing body of evidence has suggested that the long non-growing season is not biologically inert. Microbes can mineralize organic nitrogen at sub-zero temperatures, and the size of the nitrogen pulse released at spring thaw depends on what happens under the snowpack during winter. What remained unclear was whether tundra plants themselves can compete with microbes for winter nitrogen, and whether plants from different climate zones differ in this ability. The new study set out to answer both questions with an unusually direct experimental approach.

The researchers, led by Laura Helene Rasmussen of the University of Tromsø and the University of Copenhagen, together with Louise Rütting and Anders Michelsen, collected 34 intact tundra mesocosms during the summer of 2023. Each mesocosm contained the complete vegetation community, the crust organisms on the soil surface, and the soil down to 20 centimeters, preserving the living root systems of shrubs such as dwarf birch, arctic willow, crowberry, and the cushion-forming Dryas. The blocks were transported to controlled climate chambers in Copenhagen, where the team simulated the autumn freeze-in and the entire winter using light and temperature regimes matched to each site’s climate normals.

The key trick was a stable isotope tracer. Just before the simulated winter began, when soil temperatures had dropped to around 2 degrees Celsius, the team injected a tiny, highly enriched amount of nitrogen-15, in the form of ammonium sulfate, homogeneously throughout each mesocosm at 36 grid points. This labeled pulse mixed with the natural dissolved nitrogen in the soil, allowing the researchers to follow exactly where that nitrogen went. At four time points, from early winter in mid-October to late winter in mid-April, they sampled leaves, stems, fine and coarse roots, microbial biomass, and soil solution, measuring the isotope signature with isotope ratio mass spectrometry. Half of the mesocosms also received a simulated extreme winter warming event: six full days at 4 degrees Celsius in early April, mimicking the thaw episodes that are becoming more frequent across the Nordic Arctic.

The results reveal a stark hierarchy of nitrogen capture. Across all sites and all winter stages, soil microbes were the dominant players, retaining around 50 percent of the tracer nitrogen on average, with peak microbial recovery reaching 58 percent at the eastern Low Arctic site in early winter. Plant roots and aboveground biomass together took up and retained only about 3 to 8 percent of the labeled nitrogen. In total, roughly 40 to 68 percent of the early winter tracer was recovered across the measured ecosystem pools, with the remainder presumably locked in non-microbial soil fractions or lost through leaching and gaseous pathways. Microbes, in other words, are the gatekeepers of winter nitrogen, and plants must compete against them for every gram of this limiting nutrient.

Yet the plant side of the story is far from trivial, and it varies dramatically with climate. The High Arctic ecosystem, adapted to the coldest winters, captured the least nitrogen overall, leaving more of the tracer in soil solution during early winter. The Low Arctic sites kept the most nitrogen in their roots, and at the eastern site, root nitrogen concentrations actually increased steadily as winter progressed, suggesting that plants there are physiologically equipped to keep absorbing nutrients throughout the cold season. Most striking was the Subarctic: plants from the southern site allocated more tracer nitrogen to aboveground biomass than any other site, particularly into the stems of deciduous shrubs, while efficiently depleting nitrogen from the soil solution. This pattern supports the idea that vegetation adapted to milder winters runs a tighter nitrogen cycle and is better positioned to exploit the nitrogen that warmer winters release.

Plant functional type mattered as much as climate. Evergreen shrubs, which hold their leaves through the winter, retained more winter-released nitrogen overall than deciduous shrubs and bryophytes, storing much of it in their leaves. Deciduous shrubs, by contrast, concentrated their winter nitrogen in stems, a strategic placement that could fuel rapid leafing out in spring. When the researchers normalized for biomass, a different picture emerged: arctic willow, Salix arctica, was the most efficient nitrogen scavenger per gram of tissue, followed by dwarf birch and the dwarf willow Salix herbacea. Crowberry, Empetrum nigrum, dominated nitrogen uptake per unit ground area simply because of its sheer biomass, but per unit mass it was actually inferior to most other shrubs. Grasses, though poor competitors per unit area, outperformed several shrubs per gram of biomass, hinting that they too could benefit from nitrogen released in warmer early winters.

The simulated winter warming event delivered a sobering message about climate extremes. At the High Arctic site, the six-day warm spell reduced plant nitrogen uptake while boosting microbial immobilization, leaving less nitrogen available in the soil solution. The same pattern, at larger scale, had been observed in previous field studies, and related work has shown that freeze-thaw cycling can cut root nitrogen uptake dramatically and retard root growth. Because the warming event was imposed without adding snowmelt water, the effect reflects temperature alone, and soil moisture remained essentially unchanged. The implication is that High Arctic vegetation, having evolved under stable, deeply frozen winters, is the most sensitive to the destabilizing thaw events that climate models project will become more common, while Subarctic plants, which already experience such events, appear better buffered.

These findings carry weighty implications for how the Arctic will change. Shrub expansion, the so-called shrubification of the tundra, is one of the most visible signatures of Arctic warming, visible even from satellites. The new results suggest that this expansion is not driven only by longer growing seasons and warmer summers, but also by below-ground competition during winter. Species such as dwarf birch and arctic willow, which can capture nitrogen released during freeze-in and early winter, may gain a competitive edge as winters warm and more nutrients become available in the cold season. Because nitrogen availability ultimately constrains how much carbon the tundra can sequester, the winter nitrogen economy also feeds directly into predictions of the future Arctic carbon sink, a critical uncertainty in global climate models.

The study also underscores that simple measures such as root biomass cannot explain winter nitrogen uptake on their own. The Subarctic sites achieved the highest aboveground nitrogen capture despite relatively low root biomass, pointing to the importance of root efficiency, species-specific traits, and the legacy of adaptation to different winter regimes. The authors conclude that species-level strategies must be built into models of Arctic tundra nitrogen and carbon dynamics, rather than lumping vegetation into broad functional categories. As winters across the Arctic grow milder and more erratic, the invisible contest between roots and microbes under the snow may prove to be one of the decisive forces shaping which plants will green the landscape of the twenty-first century Arctic, and how much carbon that landscape will hold.

Subject of Research: Winter nitrogen uptake by plants and microbes along an Arctic climate gradient in East Greenland

Article Title: Winter climate or plant functional type? Controls on tundra winter nitrogen uptake along an Arctic climate gradient in East Greenland

Article References: Rasmussen, L. H., Rütting, L., & Michelsen, A. (2026). Winter climate or plant functional type? Controls on tundra winter nitrogen uptake along an Arctic climate gradient in East Greenland. Plant and Soil. https://doi.org/10.1007/s11104-026-09022-4

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09022-4

Keywords: Arctic tundra, nitrogen cycling, winter ecology, nitrogen-15 tracer, soil microbes, shrubification, climate gradient, Greenland, plant functional types, winter warming events, dwarf birch, arctic willow

The research, led by corresponding author Lili Wang and colleagues including Qiannan Yang, Guilong Zhang, Jie Li, Hu Li, and Lukas Van Zwieten, examined an alkaline paddy field in Tianjin, China, where rice-straw biochar had been applied annually for more than five years. Rather than sampling only the familiar topsoil layer where most agronomic studies stop, the team collected soil from four depths extending from the surface down to 80 centimeters below ground. This vertical perspective proved essential, because the study’s central finding is that biochar’s influence on nitrogen cycling changes dramatically with depth.

Using a nitrogen-15 tracer technique, the researchers simultaneously quantified three major nitrate reduction processes that operate in waterlogged paddy soils. The first is denitrification, in which microbes convert nitrate step by step into nitrogen gases that escape to the atmosphere. The second is anaerobic ammonium oxidation, or anammox, a process in which ammonium and nitrate react to produce dinitrogen gas, again removing reactive nitrogen from the system. The third is dissimilatory nitrate reduction to ammonium, known as DNRA, a fermentation-like pathway that reduces nitrate all the way back to ammonium. From a farmer’s standpoint, these pathways could hardly matter more: DNRA keeps nitrogen in the soil where roots can potentially access it, while denitrification and anammox effectively bleed nitrogen out of the field.

The study revealed a striking depth-dependent pattern in how these three pathways partition the nitrate supply. Across the soil profile, the contribution of DNRA increased from 25.7 percent in the surface layer to as high as 91.1 percent at depth, while the contribution of denitrification fell from 67.9 percent to just 7.2 percent. In other words, the deeper the soil, the more nitrate reduction shifted from a nitrogen-losing process to a nitrogen-retaining one. Biochar amplified this trend. Under the biochar treatment, DNRA became increasingly favored below the surface, and in the deepest layer sampled, 60 to 80 centimeters below ground, biochar increased the DNRA contribution by 37.0 percent while reducing the contribution of denitrification by 27.8 percent.

Corresponding author Lili Wang emphasized that this depth sensitivity is the study’s key message. As she put it, the effect of biochar on nitrogen cycling cannot be understood by looking only at the surface soil. As soil depth increased, biochar shifted nitrate reduction toward the pathway that retains nitrogen as ammonium, while suppressing the pathways associated with nitrogen loss. For years, biochar research has concentrated overwhelmingly on the top 20 or 30 centimeters of soil, where most sampling protocols end. The new results suggest that this focus may have missed some of the most consequential chemistry happening far below the plow layer.

Why would biochar favor one microbial pathway over another, and why would that preference change with depth? To answer this, the researchers examined the environmental conditions and microbial communities at each depth. Statistical modeling identified several important controls on the pathway balance: soil pH, the ratio of soil organic carbon to nitrate, dissolved organic carbon, and ferrous iron. These factors are known to shape the competitive hierarchy among nitrate-reducing microbes. Organisms capable of DNRA tend to thrive when carbon is abundant relative to nitrate, while denitrifiers often dominate under different carbon and oxygen regimes. Iron chemistry adds another layer of complexity, because ferrous iron can participate in chemically coupling nitrate reduction to iron oxidation in flooded soils.

Biochar altered these environmental conditions differently at different depths, and those alterations cascaded through the nitrogen-transforming microbial communities and the functional genes they carry. In the surface soil, biochar stimulated several nitrate reduction processes at once, apparently by improving carbon availability and modifying pH in the zone where the amendment was concentrated. Deeper in the profile, however, the picture changed. The soil became more alkaline and carbon availability declined with depth. These conditions increasingly suppressed denitrification and anammox while allowing DNRA to account for a larger and larger share of nitrate transformation. The result is a vertical gradient in nitrogen fate that no surface-only study could have detected.

The practical implications extend well beyond the paddy fields of Tianjin. Nitrogen transformations below the plow layer can determine whether nitrate remains in the soil, moves downward toward groundwater, or is converted into gaseous products that contribute to greenhouse gas emissions and the loss of fertilizer value. In alkaline rice-growing regions, where high pH already shapes microbial activity in distinctive ways, the finding that long-term biochar application may help conserve nitrogen in deeper horizons offers a potential strategy for improving nitrogen use efficiency. More ammonium retained in the profile means less fertilizer needed, fewer nitrate pulses into aquifers, and potentially reduced emissions of nitrous oxide, a potent greenhouse gas produced during denitrification.

The authors are careful to note that biochar’s effects are not universal. They caution that the outcomes depend strongly on soil depth, initial soil pH, the aging of the biochar over successive seasons, and changes in available carbon. Biochar is not chemically static; over years in a flooded field it weathers, its labile carbon fraction diminishes, and its surface chemistry evolves. The Tianjin experiment captured the consequences of more than five years of annual application, but the researchers stress that future studies should compare fresh and naturally aged biochar directly, and should measure nitrogen leaching and gaseous nitrogen losses explicitly rather than inferring them from pathway partitioning alone. Such measurements would close the loop between microbial process rates and actual field-scale nitrogen balances.

What the study delivers now is mechanistic evidence for a principle that soil scientists have increasingly advocated: biochar-based nitrogen management should consider the entire soil profile rather than focusing only on topsoil. The nitrogen-15 tracer approach allowed the team to watch three competing pathways respond in real time to five years of amendment, and the answer that emerged is unusually clear. In deep alkaline paddy soil, biochar nudged the microbial economy toward conservation—toward ammonium that stays put rather than gases that drift away. For a crop grown on roughly 160 million hectares worldwide, much of it in alkaline or calcareous soils, that shift could translate into meaningful gains in fertilizer efficiency and meaningful reductions in nitrogen pollution. The deeper soil, long treated as a black box beneath the sampling auger, turns out to be where some of biochar’s most valuable work is done.

Subject of Research: Long-term biochar effects on nitrate reduction pathways and nitrogen retention in deep alkaline paddy soils

Article Title: Long-term biochar use may help conserve nitrogen deep in alkaline paddy soils

Article References: Long-term biochar use may help conserve nitrogen deep in alkaline paddy soils. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: biochar, nitrogen cycling, paddy soil, DNRA, denitrification, anammox, nitrogen-15 tracer, alkaline soil, soil depth, nitrogen retention, rice straw, soil microbiology