The research team, led by Mengyuan Chang and Zongxing Wang of Zhejiang A&F University, took advantage of an extraordinary natural experiment: a chronosequence of Torreya grandis stands ranging from 100 to 1,600 years old. Because the trees in each age group grow under broadly similar climate and management traditions, differences among the groups can be attributed largely to age itself. The researchers sampled the full leaf-litter-soil continuum, measuring concentrations of carbon, nitrogen, phosphorus, and potassium in green leaves, fallen litter, and soil at two depths, and then calculated how efficiently the trees resorbed each nutrient before leaf drop.

The soil results told a story of slow but relentless change. Organic carbon and total nitrogen in the soil actually increased as stands aged, a pattern consistent with centuries of litter accumulation and organic matter buildup. Total phosphorus remained relatively stable across all age groups in both the 0-10 and 10-20 centimeter layers. But potassium broke the pattern dramatically: soil total potassium declined consistently with tree age, signaling what the authors describe as progressive potassium depletion. In other words, the very nutrient that plants need in large quantities for enzyme activation, stomatal regulation, and stress tolerance was steadily draining away from the system over the centuries.

The trees themselves reflected this depletion. Leaf concentrations of carbon, nitrogen, and potassium all decreased with tree age, while leaf phosphorus held comparatively steady. This divergence matters because leaf chemistry is a sensitive indicator of what a tree can actually extract from its environment. A falling leaf potassium concentration in the oldest stands suggests that the trees were no longer able to take up enough of the element to maintain the internal concentrations seen in their younger counterparts, even as they continued to photosynthesize and grow on the accumulated organic capital of their soils.

Perhaps the most striking result concerned nutrient resorption efficiency, the process by which trees withdraw valuable nutrients from senescing leaves before they fall, effectively recycling their own biochemical investments. Resorption efficiencies of nitrogen, phosphorus, and potassium all declined substantially as tree age increased, but potassium resorption showed by far the largest drop, plummeting from about 75 percent in the youngest stands to just 32 percent in the oldest. For a tree, losing the ability to salvage three-quarters of the potassium in its leaves before abscission represents a fundamental shift in nutrient economy, forcing greater dependence on an increasingly depleted soil pool.

The drivers behind these shifts were traced through statistical modeling of soil chemistry. Nitrogen resorption efficiency responded primarily to soil nitrogen availability, specifically the concentrations of nitrate and ammonium, the two inorganic forms plants can absorb directly. Phosphorus and potassium resorption, by contrast, were more sensitive to soil potassium levels and pH. This dissociation suggests that different nutrients are governed by different environmental levers, and that a one-size-fits-all fertilization strategy would fail to address the specific bottlenecks emerging in ancient stands.

To integrate these relationships, the team employed structural equation modeling, a technique that allows researchers to test networks of hypothesized cause-and-effect pathways simultaneously. The analysis revealed that tree age influences nutrient resorption efficiency primarily indirectly, through its effects on soil nutrient stoichiometry and on the stoichiometry of leaves and litter. Crucially, potassium-related imbalances exerted the most negative influence on resorption efficiency of any factor examined. The implication is that potassium is not merely one nutrient among several running low; it acts as a keystone element whose scarcity destabilizes the balance of nitrogen and phosphorus cycling throughout the ecosystem.

This finding resonates with a growing body of global evidence. A 2023 meta-analysis cited in the study highlighted that potassium limitation is far more widespread in terrestrial ecosystems than classical nutrient-paradigm thinking, which has long focused on nitrogen and phosphorus, would suggest. Potassium is unusual among macronutrients because it does not form part of any organic structural compound; it exists in plant tissue as a free ion, which means it is easily leached from litter and soils and cannot be locked into stable organic pools the way nitrogen and phosphorus can. Over sixteen centuries of continuous cultivation and harvest, that mobility appears to have worked against the ancient Torreya stands.

The study also touches on the concept of stoichiometric homeostasis, the ability of organisms to maintain stable internal elemental ratios despite variation in what their environment supplies. As the ancient trees aged, their capacity to buffer against shifting soil chemistry appears to have weakened, leaving them increasingly exposed to the elemental imbalances developing around their roots. Combined with the region’s history of high atmospheric nitrogen deposition, which earlier work on Torreya plantations suggested can blunt the benefits of conventional fertilization, the picture that emerges is one of multiple nutrient stresses compounding one another in the oldest stands.

For conservationists, the practical implications are concrete. The authors argue that soil test-based nutrient management, including targeted potassium fertilization and measures to prevent soil acidification, could help sustain the ancient Torreya grandis forests, which are both culturally treasured and economically important for their edible nuts. More broadly, the study underscores that ancient trees are not simply younger trees scaled up in time; they occupy a distinct biogeochemical state shaped by centuries of nutrient cycling, and protecting them may require understanding and correcting elemental deficits that only become visible across millennial timescales. As the world’s oldest living trees face mounting pressures from climate change and land-use intensification, this research suggests that what lies beneath them, in the slow chemistry of their soils, may matter as much as what threatens them above ground.

Subject of Research: Long-term soil potassium depletion and nutrient resorption dynamics in millennial-aged Torreya grandis forests

Article Title: Potassium depletion and reduced nitrogen resorption intensifies the nutrient constraints in millennial-aged Torreya grandis forests

Article References: Chang, M., Wang, Z., Fan, Y., Jin, S., & Xie, H. (2026). Potassium depletion and reduced nitrogen resorption intensifies the nutrient constraints in millennial-aged Torreya grandis forests. Plant and Soil. https://doi.org/10.1007/s11104-026-09116-z

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09116-z

Keywords: Torreya grandis, potassium depletion, nutrient resorption, ecological stoichiometry, ancient trees, chronosequence, soil science, forest ecology, stoichiometric homeostasis, leaf-litter-soil continuum, nitrogen cycling, soil acidification

The research team, led by Jiayi Zhang of Northwest Normal University with colleagues including Yuki Tsujii of the Forestry and Forest Products Research Institute in Japan, worked at three arid marsh sites along a soil moisture gradient. At each site they collected green leaves and naturally senesced leaves of P. australis, the dominant wetland plant of the region. Instead of stopping at total phosphorus and nitrogen, they chemically partitioned each element into four operational fractions: easily soluble compounds, nucleic acids, lipids, and a residual pool of structural and recalcitrant material. The easily soluble phosphorus fraction was further split into inorganic phosphate and metabolite phosphorus, giving the team an unusually fine-grained view of how the element is deployed in living leaves and what remains behind in dead ones.

The distinction matters because resorption has two faces that ecologists track separately. Resorption efficiency is the percentage of a nutrient withdrawn from a leaf before it dies, while resorption proficiency is the final nutrient concentration left in the senesced leaf, a measure of how completely the plant can strip its dying tissue. A plant can be highly proficient, leaving almost nothing behind, yet show modest efficiency if its green leaves were nutrient-rich to begin with. Conversely, high efficiency can coexist with poor proficiency when starting concentrations are low. The new study found that these two metrics can come apart entirely once you look at individual biochemical fractions rather than whole-leaf totals.

The headline result concerns lipid phosphorus, the phosphorus locked in membrane molecules such as phospholipids. Across the moisture gradient, total phosphorus resorption looked stubbornly invariant: whatever the soil moisture, the reeds appeared to withdraw similar overall proportions of the element. But at the fraction level, a clear moisture signal emerged. At the wetter sites, senesced leaves contained significantly less lipid phosphorus than at drier sites, meaning the plants achieved higher resorption proficiency for this fraction. In other words, reeds growing with more water available were better at emptying their membrane phosphorus reserves before leaf death.

Yet the efficiency of lipid phosphorus resorption barely changed across sites, and the reason is a subtle ecological trade-off. Wetter soils did not only improve withdrawal; they also raised the amount of lipid phosphorus the green leaves contained in the first place. The plants invested more phosphorus into membrane biochemistry when water was plentiful, and then clawed back a larger absolute amount at senescence. Increased proficiency was offset by increased starting capital, so the percentage recovered stayed flat. This dissociation between proficiency and efficiency, invisible in bulk measurements, is exactly the kind of mechanism the fraction-level approach was designed to expose.

The biochemical logic behind the pattern is plausible in light of earlier work on phosphorus allocation. Lipid phosphorus sits largely in cell membranes, and plants under phosphorus stress are known to replace phospholipids with non-phosphorus galactolipids and sulfolipids to economize on the element. Where moisture relieves water stress and permits greater metabolic activity, membranes and their phospholipid content expand, creating a larger labile pool. During senescence, membrane turnover releases this phosphorus, and an active retrieval machinery can recover it. Under drier conditions, green leaves carry less lipid phosphorus and senesced leaves retain proportionally more, leaving a measurable biochemical fingerprint of water limitation on nutrient conservation.

Nitrogen told a parallel but distinct story. The team tracked the same four fractions for nitrogen, from easily soluble compounds such as amino acids and proteins through nucleic acids, lipids, and the residual pool. Fraction-specific responses to soil moisture were again evident, confirming that the pattern is not unique to phosphorus. Different nitrogen-bearing molecules follow different fates during senescence: chloroplast proteins are dismantled early, nucleic acids are degraded by nucleases, and structural residuals resist breakdown. Because soil moisture alters both the composition of green leaves and the pace of senescence, each fraction responds on its own schedule, producing a mosaic of resorption dynamics that a single total-nitrogen number would average into invisibility.

Why should this matter beyond the reed beds of an arid Chinese marsh? Nutrient resorption is one of the dominant pathways by which plants conserve limiting elements, and it shapes everything from litter decomposition rates to the nutrients available for the next growing season. In arid and semi-arid wetlands, where hydrology is increasingly erratic under climate change, shifts in soil moisture could quietly rewire nutrient cycling without any detectable change in bulk leaf chemistry. Models and ecosystem studies that rely on total leaf nutrient concentrations may therefore misjudge how wetland vegetation responds to drying or rewetting, because the underlying biochemical reallocation is where the action actually happens.

The study also connects to a broader movement in plant ecology toward fraction-level thinking. Work on tropical trees on Mount Kinabalu, on phosphorus-impoverished Proteaceae in Australia, and on desert shrubs in hyperarid China has all shown that how a plant allocates phosphorus among inorganic pools, metabolites, nucleic acids, and lipids is as informative as how much phosphorus it holds. The new results extend this framework to the resorption side of the cycle and to a moisture gradient, demonstrating that allocation and retrieval are two ends of a single biochemical strategy. A reed that banks more phosphorus in membranes when water is abundant is not being wasteful; it is running a larger, faster metabolism and then recovering the investment with higher precision at season’s end.

For the arid marshes of northwestern China, where P. australis anchors the ecosystem and groundwater fluctuations govern nearly everything, the findings offer a mechanistic handle on resilience. The research, funded by the National Natural Science Foundation of China, suggests that as moisture regimes shift, the plant’s nutrient conservation strategy will adjust at the level of individual molecular pools, maintaining overall nutrient balance while quietly changing which biochemical reserves are drawn down. Detecting and predicting those adjustments requires the kind of fraction-resolved measurements this study provides. What looked like a flat, unresponsive resorption profile in bulk data turns out to be a dynamic, fraction-by-fraction negotiation between a plant and its water supply, one that conventional methods have been averaging out of existence for fifty years.

Subject of Research: Soil moisture effects on biochemical fraction-level nutrient resorption in Phragmites australis in an arid marsh

Article Title: Leaf phosphorus and nitrogen fractions reveal the effects of soil moisture on nutrient resorption in Phragmites australis in an arid marsh

Article References: Zhang, J., Zhang, J., Tsujii, Y., Tang, K., Zhao, W., & Cheng, B. (2026). Leaf phosphorus and nitrogen fractions reveal the effects of soil moisture on nutrient resorption in Phragmites australis in an arid marsh. Plant and Soil. https://doi.org/10.1007/s11104-026-09159-2

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09159-2

Keywords: Phragmites australis, nutrient resorption, phosphorus fractions, nitrogen fractions, soil moisture, arid marsh, leaf senescence, plant ecology, wetland biogeochemistry, nutrient conservation, northwestern China, Plant and Soil

Nitrogen is the nutrient that most often limits plant growth, and human activities—from fossil fuel combustion to intensive agriculture—have more than doubled the amount of reactive nitrogen cycling through the global environment. When this nitrogen rains down on forests, it can fertilize trees, acidify soils, and shift the delicate balance of elements such as carbon, nitrogen, and phosphorus within living tissue. Ecologists have long predicted that forests receiving chronically high nitrogen inputs should eventually reach

The concept of nitrogen saturation, formalized in influential syntheses of temperate forest research in the late 1990s, describes a sequence of stages through which a forest ecosystem passes as chronic nitrogen inputs accumulate. In the earliest stages, added nitrogen is captured efficiently by plants and soil microbes, stimulating growth and enhancing nutrient uptake. As deposition continues, however, the system’s capacity to retain nitrogen becomes exhausted: excess nitrate leaches into streams, soils acidify, base cations are depleted, and the availability of other nutrients, particularly phosphorus, becomes the principal constraint on plant productivity. Under this framework, forests in regions with decades of elevated deposition were expected to exhibit symptoms of saturation, including diminished growth responses to further nitrogen inputs and declining foliar nitrogen relative to phosphorus. The new study from western China complicates this tidy progression, finding that a dominant tree in a high-deposition subtropical forest continues to respond nutritionally to added nitrogen rather than showing the plateau or decline that saturation would predict.

The setting matters enormously for interpreting this result. Subtropical China receives some of the highest rates of atmospheric nitrogen deposition anywhere on Earth, driven by dense industrial activity, intensive fertilizer use, and rapid urbanization in the region. Yet the soils and vegetation of these humid, warm forests differ in fundamental ways from the temperate and boreal systems where the saturation model was originally developed. Subtropical forests tend to be phosphorus-limited rather than nitrogen-limited, with highly weathered, acidic soils that hold relatively little labile phosphorus. In such systems, nitrogen deposition can act as a partial fertilizer even at high background rates, because the trees have evolved under conditions where nitrogen availability fluctuates and phosphorus scarcity, not nitrogen scarcity, sets the ceiling on productivity. The evergreen secondary forest where Michelia wilsonii grows represents exactly this kind of environment, where the interplay between abundant nitrogen and constrained phosphorus shapes every aspect of plant nutrient strategy.

Stoichiometry, the study of the ratios of elements such as carbon, nitrogen, and phosphorus in living tissue, provides a powerful lens for reading these nutrient dynamics. Leaf carbon concentrations are typically quite stable across environmental gradients, reflecting the structural and metabolic constancy of the photosynthetic apparatus. Nitrogen and phosphorus, by contrast, vary considerably with supply, because both are essential to proteins, nucleic acids, and the energy-transfer machinery of cells. The ratio of nitrogen to phosphorus in leaves is widely used as an indicator of which nutrient limits plant growth at a given moment, while carbon-to-nutrient ratios reflect how efficiently plants convert assimilated carbon into nutrient-rich tissue. When nitrogen deposition alters these ratios, it signals a shift in the internal economy of the plant, with cascading consequences for herbivores, decomposers, and the recycling of nutrients through the ecosystem.

The experimental design employed in the study followed a now-standard approach in deposition research: plots received supplemental nitrogen at rates of zero, twenty, and forty kilograms of nitrogen per hectare per year, spanning the range of additional inputs that forests in the region might plausibly experience. By measuring leaf carbon, nitrogen, and phosphorus concentrations alongside soil and microbial biomass pools, the researchers could trace how nitrogen moved through the ecosystem and where its effects originated. This multi-tiered sampling is critical because leaf chemistry does not respond to deposition in isolation; it reflects the integrated outcome of soil nutrient availability, microbial competition for nutrients, and the tree’s own physiological regulation of uptake and internal recycling.

One of the most intriguing patterns in the results is the non-linear response of leaf nitrogen and phosphorus, which rose at moderate nitrogen addition and then declined at the highest rate. This initial-increase-then-decline trajectory suggests that moderate nitrogen inputs relieve a nutrient constraint and allow the tree to enrich its foliage, but that heavier inputs trigger compensatory mechanisms or stress responses that pull nutrient concentrations back down. Possible explanations include increased leaching of nutrients from soils under heavier loading, soil acidification that reduces phosphorus availability, or physiological downregulation of uptake when the tree has accumulated sufficient nitrogen. The corresponding seasonal shifts in carbon-to-nitrogen and carbon-to-phosphorus ratios in summer samples reinforce the picture of a tree actively recalibrating its tissue chemistry as inputs change, rather than passively accumulating nitrogen.

Nutrient resorption, the process by which trees withdraw nitrogen and phosphorus from leaves before they are shed, is another key thread in the study. Resorption efficiency is a central component of nutrient conservation in evergreen species, which must sustain their foliage for multiple growing seasons in nutrient-poor environments. When soil nitrogen is abundant, trees typically reduce their reliance on resorption and instead draw more nitrogen directly from the soil, a shift that can loosen the tight internal cycling characteristic of infertile sites. The finding that resorption efficiencies followed the same rise-and-fall pattern as leaf nutrient concentrations indicates that deposition is reshaping not just what the leaves contain but how the tree manages its nutrient capital over time. Changes in resorption feed back into litter quality, which in turn alters decomposition rates and the release of nutrients back into the soil, closing a loop that connects deposition to the entire biogeochemical cycle of the forest floor.

The soil and microbial measurements add an important belowground dimension to the story. Soil organic carbon and microbial biomass carbon emerged as the primary determinants of variation in leaf stoichiometry, implicating the microbial community as a gatekeeper controlling nutrient flows to tree roots. Microbes and plant roots compete directly for nitrogen and phosphorus in the soil, and the elemental composition of microbial biomass determines whether immobilized nutrients are locked up in microbial tissue or released for plant uptake. Nitrogen deposition is known to shift microbial communities, favoring some groups over others and altering the balance of fungal and bacterial dominance, with consequences for carbon storage and nutrient turnover. The strong correlations observed between microbial biomass composition and leaf chemistry in this forest suggest that belowground responses are not merely a side effect of deposition but an active mediator of how trees experience added nitrogen.

The absence of nitrogen saturation in this system deserves particular attention. Several factors could explain why the forest has not crossed the saturation threshold despite high background deposition. The humid subtropical climate supports rapid plant growth and high nutrient demand, allowing trees and microbes to absorb substantial nitrogen inputs. Deep soils and abundant organic matter may provide large exchange and retention capacities. Moreover, if phosphorus availability, while low, is sufficient to support continued growth, then added nitrogen can still be converted into biomass rather than accumulating as excess. The authors’ conclusion that high deposition continues to enhance the nutrition and growth of dominant species implies that these forests remain in the fertilization stage of the saturation sequence, a finding that extends the applicability of the saturation framework by revealing how differently it can unfold in subtropical versus temperate settings.

The ecological implications of this continued fertilization are far-reaching. If dominant species such as Michelia wilsonii gain a nutritional advantage under sustained deposition, they may outcompete subordinate species that are less able to exploit the extra nitrogen, simplifying forest composition and altering canopy structure. Shifts in leaf chemistry also propagate upward and downward through the food web, affecting insect herbivores whose foliar diets become more nitrogen-rich, and decomposer communities whose litter inputs change in quality. Over longer timescales, the combination of enhanced growth, altered litter chemistry, and modified microbial activity could change how much carbon these forests store, a question of global relevance given the role of subtropical forests in the terrestrial carbon sink. At the same time, the non-linear responses observed here caution against assuming that fertilization benefits will persist indefinitely; the decline in leaf nutrients at the highest addition rate hints that thresholds may exist beyond which negative effects emerge.

More broadly, the study underscores the value of examining plant responses to pollution through the integrated framework of ecological stoichiometry, which links leaf chemistry, soil processes, and microbial ecology into a single analytical picture. Rather than treating nitrogen deposition as a simple dose of fertilizer or toxin, this approach reveals it as a force that reorganizes the flow of multiple elements through an ecosystem, with effects that depend on season, soil properties, and the identity of the organisms involved. For forests across subtropical Asia, where deposition rates remain high and may continue to rise, understanding these element-coupled responses will be essential for predicting which species thrive, which decline, and how the structure and function of some of the world’s most biodiverse ecosystems will be reshaped in the decades ahead.

Subject of Research: Effects of nitrogen addition on leaf carbon, nitrogen, and phosphorus stoichiometry of the dominant tree Michelia wilsonii in a high-nitrogen-deposition subtropical forest in western China.

Article Title: No N saturation, N addition still alters leaf stoichiometry of Michelia wilsonii in a high-N input forest

Article References: Liu, S., Zheng, X., Xiao, Y., Wang, L., Li, H., You, C., Xu, L., Xu, H., Xu, Z., Tan, B., Yuan, Y., & Zhang, L. (2026). No N saturation, N addition still alters leaf stoichiometry of Michelia wilsonii in a high-N input forest. Plant and Soil. https://doi.org/10.1007/s11104-026-09111-4

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09111-4

Keywords: nitrogen deposition, nitrogen saturation, leaf stoichiometry, Michelia wilsonii, ecological stoichiometry, soil organic carbon, microbial biomass, nutrient resorption, subtropical forest, phosphorus limitation, forest ecology, Plant and Soil