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