The Beinan River drains a catchment of roughly 1,600 square kilometers composed primarily of schist and slate, rocks that are being uplifted at rates of up to several centimeters per year. This rapid uplift, combined with torrential monsoon rains and frequent typhoons, generates extraordinary sediment loads that rank among the highest in the world. Rivers like the Beinan carry pulverized rock fragments downstream, keeping the water chronically turbid and physically unstable. For decades, scientists assumed that such conditions would suppress planktonic life, because suspended sediment blocks sunlight and because thin soils provide few organic substrates for microbial respiration. The new research demonstrates that this assumption is only partially correct, and that the reality is far more nuanced and biologically interesting than previously imagined.

To probe the metabolic activity of river microorganisms directly, the research team led by Jhen-Nien Chen and Pei-Ling Wang of National Taiwan University employed a technique that goes beyond the traditional reliance on dissolved oxygen sensors. They collected river water from five sites spanning the catchment, from forested headwater tributaries down to the river mouth near the estuary, and incubated it with carbon-13 labeled compounds. By adding isotopically labeled dissolved inorganic carbon, they could track how much carbon dioxide microorganisms fixed into biomass through photosynthesis and chemosynthesis. By adding carbon-13 labeled amino acids, specifically glycine and leucine, they could measure how rapidly heterotrophic microbes consumed organic molecules and respired them back into carbon dioxide. This dual approach allowed the team to separate the contributions of autotrophy, the process by which organisms build their own biomass from inorganic carbon, and heterotrophy, the process by which organisms consume organic matter and release carbon dioxide as a waste product.

The results revealed a striking seasonal pattern. Metabolic rates were consistently higher during the wet season, which runs from June through November, than during the dry season. Water temperatures were roughly six degrees Celsius warmer in the wet season, suspended sediment concentrations increased by an average factor of eleven, and ammonium concentrations rose significantly. All of these factors appeared to stimulate microbial activity. More remarkably, the team found that heterotrophic catabolic rates, meaning the speed at which microbes respired organic carbon into carbon dioxide, exceeded autotrophic carbon fixation rates by one to two orders of magnitude. This imbalance indicates that the planktonic community of the Beinan River is strongly net heterotrophic, functioning as a net producer of carbon dioxide rather than a net consumer.

When the researchers scaled their measured rates up to the entire catchment, they calculated that planktonic microbial metabolism produces approximately 7.89 times ten to the seventh moles of carbon dioxide per year. This figure represents about 3.8 percent of the total carbon dioxide flux generated by the catchment, which includes contributions from pyrite-induced carbonate weathering, the oxidation of ancient petrogenic carbon buried in the bedrock, and the physical exchange of carbon dioxide between the river surface and the atmosphere. While 3.8 percent may seem modest, the researchers emphasize that this is a substantial and previously unquantified component of the carbon budget in a system where geological processes overwhelmingly dominate. The total river-atmosphere carbon dioxide evasion for the catchment was estimated at roughly 2.08 times ten to the ninth moles per year, a figure consistent with the extraordinarily high weathering rates documented for active mountain belts worldwide.

One of the most intriguing discoveries concerns the identity of the microorganisms responsible for carbon fixation in the absence of light. At sites where suspended sediment concentrations were highest, dark incubations revealed significant uptake of labeled dissolved inorganic carbon even when photosynthesis was impossible. Genetic sequencing of the microbial communities showed that taxa capable of oxidizing sulfur compounds and ammonia were abundant during the wet season, particularly members of the genera Thiobacillus and Sulfuricurvum. These lithoautotrophic bacteria derive their energy not from sunlight but from chemical reactions involving reduced inorganic compounds, such as the sulfide and ferrous iron supplied by eroding pyrite-bearing rocks and by groundwater discharge. The team proposes that the constant pulverization and abrasion of slate and schist in the river channel exposes fresh mineral surfaces that fuel these chemoautotrophic communities, effectively linking geological erosion directly to biological carbon fixation in the water column.

The microbial community composition shifted dramatically between seasons and along the river’s length. In the dry season, when waters ran clearer and suspended sediment loads were lower, phototrophic organisms such as cyanobacteria dominated at upstream sites, taking advantage of the improved light penetration. In the wet season, these photosynthetic populations were displaced by sulfur-oxidizing and nitrogen-oxidizing lithoautotrophs that thrived on the chemical substrates released by sediment mobilization. At the most downstream site near the river mouth, heterotrophic bacteria capable of degrading complex dissolved organic matter predominated year-round, reflecting the accumulation of organic compounds transported from the entire upstream catchment. This longitudinal transition from rock-driven to organic-matter-driven metabolism echoes classical ecological frameworks such as the River Continuum Concept, but with a geological twist unique to tectonically active settings.

The study also revealed fascinating details about how these microbes process organic substrates. Glycine was consistently preferred over leucine as an energy source, a selectivity the authors attribute to the metabolic efficiency of glycine-centered biochemical pathways. The reductive glycine pathway and the glycine cleavage system require less energy to operate than the multi-step enzymatic breakdown required for leucine catabolism. In the dry season, when nutrient supplies were scarce, microbial communities diverted nearly all available amino acids toward respiration for basic cellular maintenance rather than biomass production. In the wet season, with abundant particulate organic matter available, a larger fraction of consumed substrates was channeled into growth and biosynthesis. These physiological shifts underscore how tightly microbial carbon processing is coupled to the hydrological and geological rhythms of the catchment.

The broader implications of this work extend well beyond Taiwan. Mountainous catchments cover 27 percent of Earth’s land surface and drain more than 30 percent of global runoff, yet they remain severely underrepresented in global carbon models. Inland waters collectively release between 0.65 and 2.1 petagrams of carbon per year as carbon dioxide, a flux comparable in magnitude to the ocean’s annual carbon dioxide uptake. If planktonic metabolism contributes a measurable fraction of this flux even in the most physically hostile river environments, then current global estimates that neglect or underestimate microbial contributions from mountain rivers may require revision. The researchers note that their planktonic measurements exclude the benthic and hyporheic compartments, the riverbed and subsurface zones where additional microbial processing almost certainly occurs, particularly during the dry season when clear water allows light to reach coarse streambed sediments.

Looking forward, the team advocates for high-frequency monitoring during storm events, kinetic experiments that measure substrate uptake at environmentally realistic concentrations, and RNA-based molecular approaches that can distinguish actively metabolizing microorganisms from merely abundant ones. They also emphasize the need for parallel measurements of benthic and hyporheic metabolism to construct a complete picture of river carbon cycling in these dynamic systems. What is already clear, however, is that even in oligotrophic, sediment-choked, tectonically restless rivers, planktonic microorganisms constitute a vital and quantifiable link in the chain that connects mountain erosion to atmospheric carbon dioxide. As climate change intensifies both monsoon precipitation and the frequency of extreme hydrological events in mountainous regions worldwide, understanding how these microbial communities respond to disturbance will become increasingly important for predicting future carbon fluxes and refining the global carbon inventory.

Subject of Research: Planktonic microbial metabolism and carbon dioxide fluxes in a tectonically active mountainous river catchment in Taiwan

Article Title: Roles of planktonic metabolism in CO2 fluxes along the river-estuary continuum in a rapidly uplifting catchment of eastern Taiwan

Article References: Chen, J.-N., Chen, P.-E., Yen, Y.-S., Tu, T.-H., Wang, L.-Y., Lien, W.-Y., Lin, Y.-T., & Wang, P.-L. (2026). Roles of planktonic metabolism in CO 2 fluxes along the river-estuary continuum in a rapidly uplifting catchment of eastern Taiwan. Biogeosciences, 23(19), 6781-6802. https://doi.org/10.5194/bg-23-6781-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-6781-2026

Keywords: river metabolism, carbon dioxide flux, planktonic microbes, mountain rivers, Taiwan, Biogeosciences, heterotrophy, autotrophy, carbon-13 labeling, microbial communities, chemical weathering, tectonic uplift

The study, published in the journal Microplastics and Nanoplastics, stands out for its methodological rigor. Rather than relying on bulk measurements that can be confounded by background carbon, the researchers used polyethylene enriched with carbon-13, a stable isotope that acts as a molecular fingerprint. By tracking the appearance of carbon-13 in carbon dioxide released from the incubated soil, they could measure microbial mineralization with extraordinary precision. Any carbon-13 dioxide detected had to come from the plastic, because natural soil carbon carries a far lower abundance of this heavy isotope. This allowed the team to separate the slow metabolism of plastic-eating microbes from the vast background noise of ordinary soil respiration.

The plastic itself was not simply dropped into the soil as pristine beads. The researchers first aged it with ultraviolet light, mimicking the weathering that plastic undergoes in the field before it is tilled into the ground. UV exposure breaks polymer chains and introduces oxygen-containing chemical groups at the surface, which is widely considered a prerequisite for microbial attack. Even under these favorable conditions, the soil microbial community managed to oxidize only a tiny fraction of the polymer over the 22-month incubation. The mineralization rate was highest early in the experiment and declined over time, suggesting that the most accessible, oxidized surface material was consumed first, leaving behind a polymer core that microbes could barely touch.

Extrapolated to real-world timescales, the numbers are sobering. If 0.12 percent mineralizes in less than two years, and the rate continues to fall as the remaining plastic becomes less accessible, complete degradation of polyethylene in soil would take centuries, if it happens at all under natural conditions. Agricultural soils are among the most plastic-contaminated environments on Earth, receiving fragments from mulch films, plastic-coated fertilizers, irrigation pipes, sewage sludge, and atmospheric deposition. The new findings imply that virtually every gram of polyethylene ever tilled into farmland is still there, either as visible fragments or as microscopic and submicroscopic particles dispersed through the soil matrix.

But persistence is only half of the story. The second major finding concerns where the plastic goes. Using nanoscale secondary ion mass spectrometry, or NanoSIMS, the team mapped the location of the labeled plastic inside individual soil aggregates — the small, crumb-like clusters of mineral particles and organic matter that give soil its structure. They found microplastics and even nanoplastics lodged inside pores within 1-to-2-millimeter aggregates, spaces that are typically sheltered from water flow and physical disturbance. This means plastic particles are not merely sitting on the soil surface; they are being transported into the interior architecture of aggregates, where they can reside for very long periods and become increasingly difficult to extract or study.

The physical integration of plastic into soil structure has consequences that go beyond simple contamination. Soil aggregates regulate water infiltration, aeration, root penetration, and the protection of organic carbon from decomposition. Introducing hydrophobic polymer surfaces into these delicate structures can alter how water and gases move through the soil, and may change how aggregates form and break apart. The study also found small but measurable amounts of polyethylene-derived carbon-13 incorporated into soil organic matter and into the microbial biomass itself. This indicates that some carbon from the plastic does enter the soil’s biological and chemical cycles — not through rapid mineralization, but through slow assimilation into the organic pool that sustains soil fertility.

That incorporation, however, was minor. The overwhelming majority of the labeled carbon remained as intact or partially oxidized polymer. For the researchers, this combination of extreme persistence and gradual integration is the key takeaway. Polyethylene does not vanish in soil; it becomes part of the soil. Over years and decades, fragments fragment further, migrate into smaller pores, associate with mineral surfaces and organic matter, and effectively become a permanent, synthetic component of the terrestrial environment. Unlike organic amendments that decompose into nutrients, this material accumulates, and its long-term effects on soil health remain largely unknown.

The work was carried out under the MINAGRIS project — MIcro- and Nanoplastics in AGRIcultural Soils — funded by the European Union’s Horizon 2020 research and innovation program. The project brings together institutions across Europe to assess how plastic debris affects soil biodiversity, productivity, and function. The new results provide a quantitative foundation for those assessments, offering hard numbers on mineralization rates that can feed into models of plastic accumulation in farmland. They also validate the use of isotope labeling combined with high-resolution imaging as a powerful toolkit for studying the fate of plastics in complex environmental matrices, where traditional extraction methods miss particles embedded deep within aggregates.

For farmers and policymakers, the message is clear: prevention matters far more than remediation. No known technology can remove microplastics from soil once they are incorporated, and the new data suggest there will be ample time for them to spread. Reducing plastic inputs to agricultural land — through biodegradable mulch alternatives, better recovery of plastic films, restrictions on sewage-sludge application, and improved waste management — is currently the only effective strategy for limiting the buildup. As the researchers demonstrate, every year of continued plastic input adds material that will remain in the ground long after current farming practices have changed.

The study also raises questions for future research. The incubation captured a single soil type under controlled laboratory conditions; field soils experience freeze-thaw cycles, wetting-drying pulses, root growth, and tillage, all of which can physically fragment plastic and redistribute it. Whether these processes accelerate mineralization or simply enhance the physical dispersion of particles into aggregates is an open question. What is already certain, however, is that polyethylene’s reputation as an inert, harmless filler material in soil is untenable. It persists, it infiltrates, and it slowly becomes one with the ground beneath our feet — a legacy that future generations of soil scientists, and farmers, will have to live with.

Subject of Research: Fate, mineralization, and physical integration of polyethylene microplastics in agricultural soil

Article Title: Polyethylene microplastics mineralize slowly in soil but integrate into soil structures and organic matter

Article References: Forsyth, H., Schweizer, S., Stricker, K., Höschen, C., Velescu, A., Wilcke, W., & Bigalke, M. (2026). Polyethylene microplastics mineralize slowly in soil but integrate into soil structures and organic matter. Microplastics and Nanoplastics, 6(1), Article 54. https://doi.org/10.1186/s43591-026-00223-3

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00223-3

Keywords: polyethylene, microplastics, nanoplastics, soil pollution, mineralization, carbon-13 labeling, soil aggregates, soil organic matter, biodegradation, agricultural soils, NanoSIMS, plastic degradation