The researchers used repacked columns to represent the upper 30 centimeters of soil, preserving material from the original 0–10, 10–20 and 20–30-centimeter depth intervals rather than mixing them together. Each column was compacted to a dry bulk density of 1.28 grams per cubic centimeter. The experimental design crossed two soil types, three nominal initial-moisture regimes and five freeze-thaw totals of 1, 3, 5, 10 and 15 cycles, with three replicate columns for every combination. Each 24-hour cycle consisted of 12 hours at minus 20 degrees Celsius followed by 12 hours of thawing at room temperature. The outer surface of each column was sealed during cycling to reduce evaporative moisture loss.

The two soils came from the same seasonally frozen region but had markedly different chemical properties. The saline soil had a pH of 9.37, total salinity of 0.27% and an exchangeable sodium percentage of 20.30%. The non-saline counterpart had a pH of 7.67, salinity of 0.04% and an exchangeable sodium percentage of 0.10%. Their carbonate and bicarbonate concentrations and cation exchange capacities also differed. As a result, the study treated comparisons between the soils as differences in the overall saline-soil chemical background, which included salinity, alkalinity and sodicity. The analysis did not isolate total salinity as the only cause of those differences.

After each prescribed freeze-thaw treatment, every column was vacuum-saturated with deionized water before low-field nuclear magnetic resonance measurement. This procedure standardized the amount of liquid pore water across treatments and allowed the researchers to compare the structures that remained after freezing and thawing. It also limits the interpretation: vacuum saturation can dissolve and redistribute soluble salts, so the resulting spectra do not directly represent the in situ water-salt distribution during a freeze or thaw. The measurements are indirect indicators of pore structure under a common saturated condition, not direct images of cracks, pore geometry or connectivity.

In low-field NMR, hydrogen nuclei in pore water produce a signal whose transverse relaxation time, T2, is controlled mainly by interactions with pore surfaces. Under surface-relaxation conditions, shorter T2 values are associated with smaller pores, whereas longer values are associated with larger pores. The researchers divided the T2 distributions into micropore, mesopore and macropore domains corresponding to 0.1–10, 10–100 and 100–1,000 micrometers. Because surface relaxivity was not independently calibrated using mercury intrusion porosimetry, X-ray computed tomography, gas adsorption or image analysis, these categories were interpreted as relative NMR-derived pore-size domains rather than exact measurements of pore diameter.

The averaged T2 spectra from both soil types were dominated by a main peak between approximately 0.1 and 100 milliseconds, showing that the detected water signal was concentrated mainly in micro- and mesopore domains. Some treatments also displayed a weaker shoulder or secondary peak at longer relaxation times, indicating a smaller contribution from larger-pore domains. In saline soil at low moisture, a weak long-T2 component became more evident after the first cycle and then weakened with further cycling. Under medium and high moisture, the saline soil’s long-relaxation-time signal generally declined as cycling progressed. The non-saline soil retained its long-T2 component more consistently, particularly at high moisture, although the main peak positions in the two soils remained broadly similar.

Standardized path analysis identified moisture content as the variable with the largest relative contribution to variation in the NMR-derived pore-size fractions, accounting for 56.0% of the variation within the variables examined. Soil depth contributed 21.3%, the number of freeze-thaw cycles 11.4% and the salinity indicator 11.3%. Moisture was negatively associated with microporosity, with a standardized coefficient of minus 0.655, and positively associated with mesoporosity at 0.694. Its positive association with macroporosity was smaller, at 0.296. The separate models explained 50.0% of microporosity variation, 51.5% of mesoporosity variation and 32.5% of macroporosity variation, indicating moderate explanatory performance for the first two fractions and more limited performance for the largest-pore category.

Cycle number and the salinity indicator had weaker and more selective associations than moisture. The number of cycles was negatively associated with macroporosity, with a standardized coefficient of minus 0.210, while its small positive association with microporosity was not statistically significant. Salinity was not significantly associated with either microporosity or mesoporosity, but it was negatively associated with macroporosity at minus 0.246. At high moisture, macroporosity increased in the non-saline soil but remained relatively low in the saline soil across the cycle sequence. Soil depth was positively associated with microporosity at 0.226 and negatively associated with mesoporosity at minus 0.198 and macroporosity at minus 0.215. The relatively low explanatory value for macroporosity leaves room for unreported influences such as local packing variability, slight compaction differences, aggregate fragmentation and the formation or collapse of isolated large pores.

The pore-water fractal dimension, Dw, provided a complementary description of multi-scale heterogeneity in the T2 distributions. Values ranged from approximately 1.67 to 2.67 across all treatments. Under low moisture, Dw remained between 1.67 and 1.79 in both soils and fluctuated only slightly as the number of cycles increased. Under medium and high moisture, it was generally higher, spanning roughly 2.17–2.67, consistent with the pore-water signal being distributed across a wider range of size domains. At high moisture, Dw in the saline soil stayed within 2.18–2.29 without a clear monotonic trend, whereas the non-saline soil ranged from 2.53 to 2.67 and reached about 2.58 after 15 cycles. After cycling, the positive correlation between moisture and Dw became stronger, salinity showed a negative correlation and cycle number showed only a weak negative relationship. These correlations were descriptive and were not treated as causal paths.

Moisture and salinity followed distinct trajectories through the treatments. In saline soil, mean moisture under the low-moisture condition declined from 0.045 to 0.032, while the medium-moisture treatment rose from 0.250 to 0.295. The high-moisture treatment remained between 0.310 and 0.320. In non-saline soil, low moisture stayed near 0.065, while the medium and high treatments declined to 0.205 and 0.260, respectively. Salinity in the saline soil rose to approximately 0.33–0.36% after one to three low-moisture cycles and then declined slightly, remaining above its initial value. Under wetter conditions, salinity decreased during the early cycles, reached a relatively low level around five cycles and rose again with additional cycles. The non-saline soil remained within 0.015–0.045% throughout. Overall, the measurements associate the initial freeze-thaw event and moisture condition with the clearest pore-structure responses, while the indirect, saturated-state NMR method limits conclusions about field-scale salt movement and actual pore connectivity.

Subject of Research: Associations among moisture, freeze-thaw cycling and NMR-derived soil pore structure

Article Title: Moisture–dependent evolution of NMR–derived pore structure in saline and non–saline soils during freeze–thaw cycles

Article References: Qiu, Y., Gu, Z., Nie, S., & Zou, Y. (2026). Moisture–dependent evolution of NMR–derived pore structure in saline and non–saline soils during freeze–thaw cycles. Environmental Earth Sciences, 85(17), Article 436. https://doi.org/10.1007/s12665-026-13095-w

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13095-w

Keywords: saline soils, freeze-thaw cycles, soil moisture, low-field NMR, pore-size distribution, pore-water fractal dimension, seasonally frozen ground, path analysis, Jilin Province, Moisture, dependent, evolution

The team worked at the Sanjiangyuan Land Surface Process Observation Station in Maqu County, at 3436 meters elevation, where mean annual precipitation is roughly 604 millimeters and the mean air temperature is just 1.8 degrees Celsius. The region, often called the Asian Water Tower, hosts extensive alpine wetlands that regulate regional climate through evapotranspiration and influence downstream river hydrology at the basin scale. The wetland soils there form under persistently cold, near-saturated conditions, and partially decomposed Sphagnum and woody residues build a loose, porous, three-dimensional network with enormous surface area. That architecture is nothing like the tightly packed mineral soils for which most standard soil parameterizations were developed, and the discrepancy turns out to matter far more than modelers assumed.

To capture the hidden structure, the researchers collected eleven undisturbed soil cores spanning the profile from the surface down to 160 centimeters, one core per depth interval. They measured saturated hydraulic conductivity with an automated falling-head KSAT instrument, determined saturated water content gravimetrically, and quantified bulk density, soil organic carbon by potassium dichromate oxidation, fiber content by the von Post method, and particle size distribution by hydrometer. X-ray computed tomography of a surface sample visualized the pore architecture in three dimensions: a reconstruction built from roughly 3000 slices revealed a dense micropore-dominated cap in the upper 4 centimeters giving way below to increasingly frequent and partly connected macropores, with root channels and biopores threading the matrix at multiple scales.

The centerpiece of the laboratory work was a transient evaporation experiment using the HYPROP-2 system, in which two tensiometers tracked matric potential at two depths inside each intact core while an electronic balance logged mass loss during free evaporation under controlled temperature and wind speed. Applying the simplified evaporation method, which assumes linear profiles of potential, water content, and flux over short time steps, the team inverted unsaturated hydraulic conductivity via the Darcy–Buckingham law. Because HYPROP-2 covers only the near-saturated suction range, dry-end data from minus 0.1 to minus 300 megapascals came from a WP4C dew point potentiometer, and the merged datasets were fitted to the Clapp–Hornberger formulation using the shuffled complex evolution global optimizer, with parameter uncertainty assessed by Markov chain Monte Carlo.

The derived Clapp–Hornberger parameters reveal coherent hydraulic contrasts along the profile. Surface layers, from 0 to 8 centimeters, showed saturated water contents of 0.64 to 0.70 cubic meters per cubic meter, far above the typical mineral soil value of less than 0.50, together with saturated conductivities of 36.6 to 86 centimeters per day, roughly an order of magnitude higher than most mineral soils. Bulk density was a mere 0.63 to 0.81 grams per cubic centimeter and soil organic carbon reached 45 to 99 grams per kilogram, confirming a loose, macropore-rich fabric that promotes rapid drainage and evaporation. The power law exponent b, which controls the slope of the water retention curve, measured about 3.2 near the surface, peaked at 5.08 in the 10 to 20 centimeter subsurface, and declined toward 1.05 at 130 to 150 centimeters, while air entry suction followed a partly opposite pattern from minus 0.378 meters near the surface to minus 0.518 meters at depth.

Translated into physical behavior, these numbers describe a vertical regime of rapid near-surface drainage, delayed mid-layer release, and long-lasting deep moisture storage. Pore size distributions derived from the water retention curves via the Young–Laplace relation sharpened the picture: surface peaks concentrated at 10 to 100 micrometers, indicating macropore connectivity that sustains fast infiltration and preferential flow; middle-layer pores narrowed to 1 to 10 micrometers, where mesopores enhance retention in finer pores; and deep-layer peaks shifted below 1 micrometer, marking a micropore-dominated fabric with strong capillary suction but poor permeability. Two diagnostic indices reinforced the state dependence of the system. The effective exponent b_eff, computed as the local logarithmic slope of the retention curve, jumped from 2 to 5 at intermediate saturation to 15 to 20 near saturation in surface and middle layers, while the near-saturated connectivity index, the logarithm of the ratio of conductivity at 90 percent to that at 50 percent saturation, reached 2.4 to 3.9 in shallow and middle layers, confirming that macropore flow dominates transport under wet conditions.

The decisive test came from the Noah-MP land surface model, version 5.0, run in single-point mode at the station for June through August 2023 with half-hourly meteorological forcing from the eddy covariance and automatic weather station systems. The team designed a control experiment with the model’s default loamy sand parameters and six one-at-a-time sensitivity runs in which each measured Clapp–Hornberger parameter replaced its default counterpart in turn. Frozen-soil processes used the Niu and Yang supercooled liquid water parameterizations, the Jarvis stomatal conductance scheme was retained to minimize vegetation-related disturbances, and outputs were evaluated against observed soil moisture at 5 and 20 centimeters, latent heat flux, and sensible heat flux using root mean square error, bias, and coefficient of determination.

The sensitivity ranking was unambiguous: b, then air entry suction, then saturated water content, then saturated conductivity. Replacing the default exponent of 4.26 with the measured surface value of 3.215 cut the summer latent heat flux root mean square error from 9.45 to 3.87 watts per square meter during the early-morning-to-noon window, while the subsurface value of 5.076 worsened it to 11.06 watts per square meter. Sensible heat flux responded in a compensating pattern, suppressed during the day but overestimated overall by up to 24 watts per square meter, underscoring how strongly the exponent governs surface energy partitioning. Saturated conductivity, by contrast, proved nearly irrelevant under the wet summer background, shifting latent heat flux errors by less than a watt per square meter, because evapotranspiration was controlled by capillary water rather than saturated pathways. Saturated water content adjustments improved 20-centimeter soil moisture, reducing the root mean square error from 0.063 to 0.047 cubic meters per cubic meter, reflecting the role of the storage upper bound in retention.

Event-scale analysis for a wet episode on 15 to 16 July and a dry episode on 12 to 13 August 2023 exposed the structural limits of the single power law. During the wet period, when macropores were fully filled and gravity-driven drainage ruled, the low exponent scheme matched the observed latent heat flux peak far better, cutting the root mean square error from about 42 to about 28 watts per square meter, though it raised sensible heat bias to 28 watts per square meter and drained shallow soil too fast. During the dry period, when capillary and micropore retention governed, the high exponent scheme improved storage and flux accuracy but blunted the short-term evapotranspiration response after rainfall. No single exponent could simultaneously capture rapid macropore drainage and slow micropore release, producing opposite biases under wet and dry conditions. A companion test showed that swapping the Ball–Berry for the Jarvis stomatal scheme changed flux errors far less than the soil parameters did, confirming that in these sparsely vegetated wetlands, where leaf area index is below one, soil water availability rather than stomatal control dominates land–atmosphere exchange.

The authors conclude that pore-scale structure demonstrably regulates profile-scale hydrothermal responses in Qinghai–Tibetan Plateau wetlands, and that vertical hydraulic heterogeneity rather than any single-layer parameterization governs the timing and magnitude of evaporation and heat fluxes. They acknowledge limitations: the Clapp–Hornberger formulation cannot resolve macropore drainage near saturation, the inversion shifts inferred pore distributions toward medium and small pores, and the single-core-per-depth design limits quantification of local spatial variability. As a way forward, they recommend evaluating the more physical van Genuchten formulation and its Peters–Durner–Iden extension, which combines capillary and film flow pathways and achieved coefficients of determination above 0.95 in prior validations, to represent the bimodal pore networks of organic-rich wetland soils. For a plateau that supplies water to more than a billion people downstream, the message is clear: the hidden layering of its wetland soils is not a modeling detail but a first-order control on how the Asian Water Tower breathes.

Subject of Research: soil hydraulic heterogeneity

Article Title: Evidence of vertical soil hydraulic heterogeneity regulating hydrothermal simulations in Qinghai–Tibetan Plateau wetlands

Article References: Liu, R., Wang, X., Wang, Z., Pan, Y., & Song, Y. (2026). Evidence of vertical soil hydraulic heterogeneity regulating hydrothermal simulations in Qinghai–Tibetan Plateau wetlands. Hydrology and Earth System Sciences, 30(20), 6289-6313. https://doi.org/10.5194/hess-30-6289-2026

Image Credits: AI Generated

DOI: 10.5194/hess-30-6289-2026

Keywords: Qinghai–Tibetan Plateau, alpine wetlands, soil hydraulics, Clapp–Hornberger, Noah-MP, land surface modeling, evaporation experiments, water retention curve, pore size distribution, energy fluxes, Sanjiangyuan, Hydrology and Earth System Sciences

The research team, led by Tim-Patrick Fellinger with contributions from Simon W. J. Dietzmann, Asad Mehmood, Jian Liang Low and colleagues, set out to understand a puzzling observation: commercial activated carbons and advanced nitrogen-doped carbons frequently show extremely sharp peaks in their calculated ultramicropore distributions, with full widths at half maximum far below what physical considerations should allow. For pores differing in size by even a single carbon layer, theory suggests a peak width of at least 0.125 nanometres, yet some measurements report peaks as narrow as 0.04 nanometres. Such precision would imply pores defined to a fraction of an atom’s diameter, which is physically implausible in the disordered, amorphous world of non-graphitic carbon.

To resolve the mystery, the researchers prepared a pair of elegantly matched model materials. Starting from a zinc-containing zeolitic imidazolate framework, they synthesised a zinc-nitrogen-carbon material containing tetrapyrrolic ZnN4 sites, atomically dispersed metal centres coordinated by four nitrogen atoms embedded in the carbon plane. Acid extraction of the zinc then produced a second material in which the metal ions were removed but the carbon framework remained essentially unchanged, leaving behind tetrapyrrolic H2N4 cavities with nitrogen-nitrogen distances of roughly 0.4 nanometres. Spectroscopic analysis using extended X-ray absorption fine structure confirmed the tetrapyrrolic coordination in both materials, while X-ray photoelectron spectroscopy tracked the transformation from ZnN4 to H2N4 sites. Powder X-ray diffraction confirmed that no change in graphitisation occurred, meaning only the zinc was removed and the carbon skeleton was preserved.

With these precisely defined materials in hand, the team measured gas sorption using three different probe gases: nitrogen at 77.4 kelvin, carbon dioxide at 273 kelvin, and argon at 87.3 kelvin. Both materials displayed type I isotherms characteristic of microporous solids, and zinc extraction increased the apparent specific surface area by about 23 percent. But then came a surprise. At the lowest accessible relative pressures, around ten to the minus seven, the zinc-containing material actually adsorbed more nitrogen and more carbon dioxide than the supposedly more porous metal-free version. This trend reversal was counterintuitive: if the removal of zinc created new ultramicropores, the leached material should have taken up more gas, not less.

The key clue came from argon. Unlike nitrogen and carbon dioxide, argon is essentially non-polar and possesses no quadrupole moment, the asymmetry in its charge distribution that allows it to interact with electrically polarised surfaces. Argon adsorption followed the expected trend, with the metal-free material consistently taking up more gas across the entire pressure range, and no reversal appeared at low pressures. Because the anomalous behaviour depended on the quadrupole moment of the gas, with carbon dioxide showing the strongest effect, followed by nitrogen and effectively none for argon, the researchers concluded that the origin was microscopic and chemical rather than macroscopic and structural. The specific interactions between polar gas molecules and the embedded nitrogen-metal sites were distorting the measurements.

The pore size distributions told the same story. Analysis of nitrogen and carbon dioxide isotherms produced razor-thin apparent ultramicropore peaks, with full widths at half maximum of roughly 0.09 and 0.04 nanometres respectively, at apparent pore diameters between 0.53 and 0.82 nanometres depending on the gas used. Argon, in contrast, revealed only broader supermicropores centred around 1.3 nanometres with a full width of about 0.6 nanometres, features the team assigns to genuine physical porosity. The apparent surface areas themselves also diverged depending on the gas: for the zinc-containing material, nitrogen analysis suggested 750 square metres per gram while argon gave 566, with nitrogen consistently overestimating the accessible surface.

To explain these observations at the molecular level, the team turned to electronic density functional theory calculations of adsorption energies for each gas on three sites: plain graphitic carbon, the tetrapyrrolic H2N4 cavity, and the ZnN4 centre. For all three gases, adsorption at ZnN4 was stronger than on graphitic carbon, and the deviation grew with increasing quadrupole moment of the adsorbate. The zinc atom carries a substantial partial charge of about plus 1.6 electrons due to its ionic bonding character, creating a strongly polarised site that attracts quadrupolar molecules like carbon dioxide and nitrogen through quadrupole-ion interactions. Binding energies at these sites fall squarely in the range normally associated with ultramicropore filling, so the porosity analysis software interprets them as pores smaller than 0.7 nanometres, even though no pore physically exists there. Carbon dioxide also binds preferentially to the H2N4 cavity through quadrupole-dipole interactions aligned with the nitrogen-hydrogen bonds, though more weakly than at ZnN4.

The most striking validation came from a new analytical approach. Because standard pore size distribution fitting cannot directly yield adsorption energies, the researchers applied a multi-Langmuir analysis to the carbon dioxide isotherms, deconvoluting the data into contributions from three classes of binding sites: the carbon support, real micropores, and the strongest specific sites. For the strongest site class, the fitted adsorption free energies differed by 0.036 electron volts between the zinc-containing and metal-free materials, a value that matches almost exactly the 0.035 electron volt difference predicted by the density functional theory calculations for carbon dioxide binding at ZnN4 versus H2N4 sites. The internal consistency between experiment and simulation is compelling evidence that the phantom pores are indeed the chemical fingerprints of atomically dispersed metal-nitrogen sites. Encouragingly, the extracted maximum capacities at these sites, between 0.08 and 0.13 millimoles per gram, are comparable to active site densities independently reported for iron-nitrogen-carbon catalysts, hinting that simple gas sorption could one day serve as a quantitative probe of catalytically active sites, a quantity that has remained notoriously difficult to measure.

The practical implications for the field are substantial. The authors recommend that narrow ultramicropore peaks arising from in-plane functionalities be flagged as artefacts, for example with an asterisk, and that corresponding pore volumes be reported as apparent rather than true micropore volumes. For atomically dispersed metal-nitrogen-carbon materials and related nitrogen-doped carbons, surface areas and total pore volumes are best determined from argon sorption, which avoids the overestimation caused by specific adsorption. Beyond correcting current practice, the findings open a genuine opportunity: gas sorption, one of the most widely accessible characterisation techniques in materials chemistry, could be transformed into a tool for detecting and quantifying specific surface sites and in-plane functionalities, provided new analysis kernels are developed. That would help researchers disentangle morphological properties such as porosity and surface area from chemical properties such as active site geometry and concentration, both crucial for optimising carbon-based materials in energy storage, conversion and catalysis. What looked like a flaw in the measurement may turn out to be one of its most useful features.

Subject of Research: Pore size distribution analysis artefacts in non-graphitic carbons and atomically dispersed M-N-C materials caused by specific gas adsorption

Article Title: A Pore or not a Pore? Understanding Pore Size Distributions of Non‐Graphitic Carbon and Atomically‐Dispersed M‐N‐C Materials

Article References: Dietzmann, S. W. J., Mehmood, A., Low, J. L., Wu, S.-H., Prinz, C., Buzanich, A. G., Radnik, J., Appel, P. A., Emmerling, F., & Fellinger, T.-P. (2026). A Pore or not a Pore? Understanding Pore Size Distributions of Non‐Graphitic Carbon and Atomically‐Dispersed M‐N‐C Materials. Advanced Science, 13(50), Article e76048. https://doi.org/10.1002/advs.76048

Image Credits: AI Generated

DOI: 10.1002/advs.76048

Keywords: pore size distribution, gas sorption, M-N-C materials, nitrogen-doped carbon, ultramicropores, ZnN4 sites, density functional theory, activated carbon, BET surface area, argon sorption, Langmuir analysis, electrocatalysis