Boreal Forests Rewrite the Rules of Nitrogen Uptake Under Long-Term Deposition

Deep in the Dahurian larch forests of northeastern China, one of the planet’s most nitrogen-starved ecosystems is quietly reorganizing the way it handles a growing flood of human-made nitrogen. A new field experiment, published in Plant and Soil, has tracked exactly where ammonium and nitrate—the two dominant forms of nitrogen arriving from the atmosphere—end up inside a mature boreal forest. The findings reveal that years of chronic nitrogen addition do not simply make trees absorb more nitrogen. Instead, they fundamentally reshape which chemical form trees prefer, how quickly that nitrogen moves through roots and stems, and where it ultimately settles within the plant–soil system.

The research team, led by Enyue Zou and Miao Wang of Northeast Forestry University together with colleagues from Qufu Normal University and the Institute of Applied Ecology of the Chinese Academy of Sciences, worked in a boreal forest dominated by Larix gmelinii, the Dahurian larch. The site sits at the heart of one of Earth’s largest continuous boreal belts, a biome where cold temperatures slow decomposition and keep nitrogen locked away in soil organic matter. Because plant growth in these forests is typically constrained by nitrogen availability, scientists have long assumed that any extra nitrogen arriving from the atmosphere would be snapped up eagerly by trees and soil microbes alike. The new study shows the reality is far more nuanced.

To follow the nitrogen, the researchers used one of the most powerful tools in ecosystem science: isotopic labeling. They applied paired tracers of nitrogen-15, a rare heavy isotope of nitrogen, in the form of both 15NH4+ and 15NO3−. Because the isotope behaves chemically like ordinary nitrogen but can be detected with a mass spectrometer, every atom of labeled nitrogen that enters a leaf, a fine root, or a soil aggregate leaves a measurable fingerprint. The team established four long-term treatments—a control plus low, medium, and high nitrogen addition—and then traced the labeled nitrogen through five distinct pools: leaves, branches, fine roots, coarse roots, and soil.

The first surprise came at the very start of the tracer’s journey. Ammonium-derived nitrogen entered fine roots more rapidly in the early stages, producing a higher short-term uptake rate than its nitrate counterpart. This makes physiological sense: ammonium is already in a reduced form and can be assimilated directly into amino acids inside the root, whereas nitrate must first be reduced by the enzymes nitrate reductase and nitrite reductase before it can be used—a process that costs energy and takes time. Conifers, in particular, have historically been described as ammonium specialists, and the early uptake pattern in these larch trees fits that classical picture.

But the story changed dramatically as time passed. Nitrate-derived nitrogen proved to be the long-distance traveler of the pair. During the middle and later stages of the experiment, especially under medium and high nitrogen addition, nitrate-derived nitrogen was transported more readily to the leaves and branches above ground. This suggests that once nitrate is taken up, it moves through the xylem stream with relative ease, delivering nitrogen to the canopy where it fuels photosynthesis. In other words, the two forms of deposited nitrogen follow fundamentally different routes through the tree: ammonium is captured quickly and processed locally in the roots, while nitrate behaves more like a mobile courier service running from soil to crown.

Perhaps the most consequential finding concerns what long-term nitrogen addition did not do. Contrary to the expectation that chronic fertilization would continuously boost the fine roots’ appetite for nitrogen, the study found no sustained enhancement of fine-root uptake of the labeled tracer. What changed instead was the plumbing. Nitrogen addition promoted the transfer of labeled nitrogen from fine roots—the short-lived, high-turnover organs that do most of the absorbing—into coarse roots and then into aboveground organs. Under elevated nitrogen deposition, the forest essentially rerouted its internal nitrogen traffic, pushing a larger share of newly acquired nitrogen upward through the woody transport network rather than letting it linger below ground.

The soil, meanwhile, played the role of a temporary vault. Immediately after both nitrogen forms were applied, the soil was the major short-term sink, holding the bulk of the labeled nitrogen through microbial immobilization, exchange with mineral surfaces, and physical fixation. This initial retention is critical for the global carbon cycle, because nitrogen retained in soils can influence how much carbon forests store; previous work, including a 2022 analysis in Nature Communications, has linked the retention of deposited ammonium and nitrate to the strength of the forest carbon sink. But the vault does not hold forever. Over the course of the study, accumulation and recovery of the labeled nitrogen in plant pools steadily increased while soil recovery declined—a clear signature of substantial redistribution of deposited nitrogen from the soil into the living biomass.

This shifting balance carries implications that ripple far beyond a single larch stand. Global nitrogen deposition has risen sharply over the past century, particularly across Asia, and models of future forest carbon sequestration depend heavily on assumptions about where deposited nitrogen goes. If soils initially lock up most incoming nitrogen but gradually release it to trees, then the timescale of observation matters enormously: a snapshot taken weeks after deposition tells a very different story than one taken months or years later. The study’s multi-pool, multi-stage design captures exactly this temporal evolution, which is why its authors argue that long-term deposition reshapes not just how much nitrogen trees use, but the relative use of different nitrogen forms and the long-term fate of exogenous nitrogen across the whole plant–soil system.

The work also speaks to a long-running debate in forest ecology. Classic experiments in the 1990s suggested that conifer roots discriminate strongly against nitrate, implying that boreal and temperate conifers rely almost exclusively on ammonium. More recent studies, including work showing that mature conifers can assimilate nitrate as efficiently as ammonium in several plantation settings, have chipped away at that dogma. The new larch data land squarely in the revisionist camp: nitrate is not merely tolerated by these trees, it is preferentially exported to the canopy over time, particularly when nitrogen supply is elevated. The physiological machinery for nitrate uptake, transport, and signaling appears fully operational in this boreal species, and chronic deposition may even upregulate it.

For a warming world, the timing could not be more relevant. Boreal forests store an enormous fraction of terrestrial carbon, and their response to nitrogen deposition is woven into every major Earth system model. By demonstrating that deposited ammonium and nitrate follow distinct pathways—rapid root capture for ammonium, delayed canopy delivery for nitrate—and that years of enrichment reroute nitrogen from soil sinks into wood and leaves, the study provides the kind of mechanistic detail that modelers need to predict how much anthropogenic nitrogen will actually boost carbon sequestration, and how much will simply cycle through soils and back into the atmosphere. The larch forests of northeastern China, it turns out, are not passive sponges for pollution. They are active accountants, balancing a ledger of two very different currencies and deciding, season by season, where every atom should go.

Subject of Research: Fate of ammonium- and nitrate-derived nitrogen in a nitrogen-enriched boreal forest traced with 15N labeling

Article Title: Long-term nitrogen addition alters the uptake, transport, and fate of ammonium- and nitrate-derived nitrogen in a boreal forest

Article References: Zou, E., Wang, M., Liu, G., Huang, B., Yin, L., Liang, C., Xing, Y., Wang, X., & Wang, Q. (2026). Long-term nitrogen addition alters the uptake, transport, and fate of ammonium- and nitrate-derived nitrogen in a boreal forest. Plant and Soil. https://doi.org/10.1007/s11104-026-09077-3

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09077-3

Keywords: nitrogen deposition, boreal forest, Larix gmelinii, 15N tracer, ammonium, nitrate, fine roots, nitrogen uptake, plant-soil system, carbon sequestration, forest ecology, soil nitrogen