A riverbank ecosystem flooded thirty meters every winter reveals a surprising split: flooding changes plant carbon, nitrogen, and phosphorus concentrations, yet the plants maintain similar nutrient ratios along the inundation gradient. Carbon, nitrogen, and phosphorus are indispensable elements in plant growth.

Carbon serves as the fundamental energy supplier in ecosystems, while nitrogen and phosphorus play crucial roles in electron transfer during respiration and act as key limiting factors for primary production. Plant carbon-to-nitrogen ratios can impact microbial mineralization rates and influence the global carbon cycle.

As plant nitrogen-to-phosphorus ratios increase, nutrient limitations may shift from nitrogen to phosphorus, changing vegetation composition and ecosystem function. In riparian ecosystems, frequent flooding results in plant organ mortality, exacerbates soil nutrient loss, and alters soil nutrient ratios.

Under flood stress and nutrient co-limitation, plant photosynthesis and nutrient mineralization are constrained. That is why investigating plant carbon, nitrogen, and phosphorus stoichiometry patterns and their drivers can improve understanding of plant adaptation strategies and ecosystem functioning in dynamic riparian habitats.

The Three Gorges Reservoir Riparian was formed by the full operation of the Three Gorges Dam in 2010, and it is a recently established ecosystem on the Yangtze River in China. This ecosystem experiences thirty meters of inundation annually during winter compared with summer or the growing season, making it a distinctive natural laboratory for investigating inundation impacts on plant elemental stoichiometry.

Because riparian vegetation is sensitive to hydrological changes, and because river damming and unpredictable precipitation events are increasing, nutrient stoichiometry could transform under varying inundation and differ from patterns in other terrestrial ecosystems. The central gap is that patterns and drivers of carbon, nitrogen, and phosphorus stoichiometry in different riparian plant organs under diverse inundation scenarios remain poorly documented.

Figure one has two maps: panel A locates the Three Gorges Reservoir study area within China and the Yangtze River basin, while panel B shows the reservoir’s riparian zone, the Yangtze River, the Three Gorges Dam, and the distributed sampling sites. The authors surveyed thirty-six representative sites across this region during the peak plant-growth period, making the map important for showing the geographic scope and spatial coverage of the study.

To address this research gap, plant samples were gathered from annuals and perennials, and from leaves, stems, and roots, in the Three Gorges Reservoir Riparian in China. The objective was to examine carbon, nitrogen, and phosphorus stoichiometry across organs and life forms in predominant TGRR plants while identifying the environmental factors associated with those patterns.

The first hypothesis was that plants would show distinct carbon, nitrogen, and phosphorus stoichiometric patterns among organs and life forms because of varying physiological functions and adaptive strategies. During peak plant growth, from June to August 2019, an extensive survey and sampling effort collected plants and soils across elevation zones within the TGRR.

A total of thirty-six representative sample sites were identified, with help from TGRR jurisdiction staff who knew the area and could select sites with minimal anthropogenic disturbance. Each sample site in every one-hundred-meter elevation zone was designated with a one-hundred-meter survey transect.

At the reservoir tail, sampling was confined to zones from one hundred sixty-five to one hundred seventy and from one hundred seventy to one hundred seventy-five because of elevated water levels. In the laboratory, dried leaves, stems, roots, and soil samples were finely ground into powder.

Carbon and nitrogen contents were determined using an elemental analyzer, while phosphorus contents were determined by ICP-OES. Soil pH was determined by an acidimeter with the soil-water mix maintained at a ratio of one to two point five. Soil water content was assessed by drying soil in an oven at one hundred five degrees Celsius for forty-eight hours.

Soil bulk density was calculated as the ratio of the volume of a soil core sample to the mass of dry soil. Differences in carbon, nitrogen, and phosphorus concentrations and in carbon-to-nitrogen, carbon-to-phosphorus, and nitrogen-to-phosphorus ratios among roots, stems, and leaves, and between annual and perennial species, were tested with a nonparametric Kruskal–Wallis test and pairwise Wilcox multiple comparison test.

Random forest analysis was used to assess the relative contributions of drivers to nutrient concentrations and ratios. The Random Forest algorithm can incorporate numerous predictors, and variable importance was quantified using the percentage increase in mean square error.

Segmented structural equation modeling examined the direct and indirect effects of drivers influencing carbon, nitrogen, and phosphorus stoichiometry within different organs. Segmented SEM extends traditional SEM by considering the contribution of random variables to response variables, and variables with non-significant or covariate effects were eliminated before SEM analysis.

A linear mixed model with sample sites as random factors accounted for random effects. Model fit was estimated with Fisher’s C-value, and a model was considered adequately fitted when the Fisher’s C-value had a P value greater than zero point zero five.

The results revealed variation in plant carbon, nitrogen, and phosphorus concentrations, as well as carbon-to-nitrogen, carbon-to-phosphorus, and nitrogen-to-phosphorus ratios, among organs and across life types within the TGRR ecosystem. Leaf carbon, nitrogen, and phosphorus concentrations were three hundred eighty-six point six five, nineteen point three one, and five point two seven milligrams per gram, respectively.

Figure two compares carbon, nitrogen, and phosphorus concentrations, plus their stoichiometric ratios, across leaves, stems, and roots in Three Gorges Reservoir riparian plants. Across all species, leaves contain more nitrogen and phosphorus than stems and roots, while stems show the highest carbon concentration and carbon-to-nitrogen ratio.

The lower panels separate annual and perennial plants, revealing life-type differences in nutrient concentrations and ratios, supporting the authors’ interpretation that nutrient allocation varies among organs and growth strategies. Carbon, nitrogen, and phosphorus concentrations showed distinct responses among annual and perennial plants along the inundation gradient.

As elevation increased, carbon concentrations rose in various organs of annuals and in perennial leaves, while perennial stems and roots showed no substantial variation. Nitrogen concentrations remained relatively stable in annual organs and in perennial stems and roots as elevation increased, but increased significantly in perennial leaves.

Carbon, nitrogen, and phosphorus stoichiometric ratios in annuals and perennials showed similar responses to the inundation gradient. Carbon-to-phosphorus and nitrogen-to-phosphorus ratios in all organs of annual and perennial plants first increased and then decreased with elevation, while the carbon-to-nitrogen ratio remained relatively unchanged.

Figure three compares carbon, nitrogen, and phosphorus concentrations, along with their carbon-to-nitrogen, carbon-to-phosphorus, and nitrogen-to-phosphorus ratios, across leaves, stems, and roots in annual and perennial plants along four elevation zones. The authors report that carbon-to-phosphorus and nitrogen-to-phosphorus ratios generally rise and then fall with elevation, while carbon-to-nitrogen ratios remain relatively stable.

The contrasting nutrient responses between annuals and perennials matter because they indicate different strategies for coping with inundation stress. The variation was predominantly attributed to soil properties rather than plant communities, and different factors influenced carbon, nitrogen, and phosphorus concentrations and ratios across organs and life types.

For annual leaves, the main factors were community height, bulk density, water content, and total carbon. For annual stems, they were pH, bulk density, and water content. For annual roots, the significant factors were bulk density, water content, total nitrogen, and total phosphorus.

In perennials, community cover, elevation, water content, and bulk density were major factors for leaves. For perennial stems, water content, elevation, and community height were crucial factors. For perennial roots, the primary factors were pH, community cover, elevation, and water content.

Figure four partitions the relative influence of environmental factors across carbon, nitrogen, phosphorus, and their stoichiometric ratios for leaves, stems, and roots. Panels A through C show annual species, while D through F show perennial species; the colored segments represent community properties and soil variables, including elevation, water content, bulk density, and soil nutrients.

The authors use this comparison to show that different organs and life forms are associated with distinct driving factors, supporting the conclusion that soil properties often play a central role. Structural equation modeling showed that elevation indirectly influences carbon, nitrogen, and phosphorus concentrations and stoichiometric ratios in different organs, mainly through changes in soil physicochemical properties and plant community properties.

Life forms directly impact carbon, nitrogen, and phosphorus concentrations and stoichiometric ratios. Plant community properties, including community height and cover, significantly affected leaf carbon and nitrogen concentrations. Soil physical and chemical properties had both direct and indirect effects on concentrations and stoichiometric ratios in different organs.

Leaf carbon, nitrogen, and phosphorus concentrations were directly influenced by soil total nitrogen, total carbon, and bulk density, respectively, and were directly or indirectly affected by water content. Leaf stoichiometric ratios were strongly impacted by soil total phosphorus.

Figure five maps the proposed causal pathways linking elevation, life form, plant communities, and soil factors to leaf carbon, nitrogen, phosphorus, and their stoichiometric ratios. Standardized coefficients show the direction and strength of each pathway, while arrow styles mark statistical significance; marginal and conditional R-squared values summarize explained variation.

The figure matters because it separates direct effects from mediated effects, showing how environmental conditions and community structure jointly shape leaf nutrient traits. Carbon concentrations in leaves, stems, and roots were significantly lower than those observed in grasslands and forests globally and in China, while nitrogen and phosphorus concentrations were relatively high.

A shift from phosphorus limitation to nitrogen limitation in riparian plants might be influenced by higher nutrient inputs from anthropogenic activities. However, using nitrogen-to-phosphorus ratios to evaluate nutrient limitation has uncertainties, so future investigations need to consider more reliable metrics, such as the ratio of leaf nitrogen-to-phosphorus uptake efficiency.

Several limitations should be considered. The study focused on total nutrients rather than available nitrogen and phosphorus, although total nutrients remain a valid indicator of soil nutrient levels. The study included many environmental factors but omitted climatic factors such as average annual precipitation and temperature because their range was relatively narrow in the study area.

Because of sampling difficulties and high labor costs, only one field survey and sample were conducted. Continuous monitoring of carbon, nitrogen, and phosphorus interactions in riparian plant-soil systems is therefore needed to improve understanding of biogeochemical processes and ecosystem functions in wetlands.

The study found that riparian plants had higher nitrogen and phosphorus concentrations but lower carbon concentrations and elemental stoichiometry than grasses and forests, indicating nitrogen-limited plant growth in the recently established TGRR riparian habitats.

That nitrogen limitation was associated with plant-specific ecological strategies, including diminished carbon sequestration, lower nutrient-use efficiency, and heightened nutrient uptake capacity. Nitrogen and phosphorus concentrations were higher in leaves than in stems and roots, and annuals exhibited higher concentrations than perennials.

Although carbon, nitrogen, and phosphorus concentrations showed divergent responses to inundation stress, annuals and perennials maintained similar stoichiometric ratios along the inundation gradient. Variations in stoichiometry were more closely tied to soil characteristics than to plant community structure, while inundation induced indirect changes by altering plant community properties and soil factors.

In this recently established riparian ecosystem, soil characteristics and plant life form shape nutrient stoichiometry more strongly than plant community structure, while flooding acts mainly through changes in soil and communities.