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

Saline soils in seasonally frozen regions experience pore–size redistribution under repeated freeze–thaw cycling, which can influence water and solute transport. Here, repacked columns of a saline soil and a non–saline counterpart from Jilin Province, China, were subjected to 1, 3, 5, 10, and 15 freeze–thaw cycles under three initial moisture levels. Pore–size fractions were derived from LF–NMR T 2 spectra and evaluated together with the pore–water fractal dimension (D w ), while SEM–based path analysis was used to examine the relative associations between environmental factors and NMR–derived pore–size fractions. Across treatments, pore–size variation generally showed micropore reduction, mesopore expansion, and macropore fluctuation. Moisture content showed the largest relative contribution to pore–size–fraction variation, whereas freeze–thaw cycle number and the salinity indicator showed smaller contributions. D w further highlighted a moisture–dependent response: D w remained low under the low–moisture treatments (1.67–1.79) but was higher under the medium– and high–moisture treatments (2.17–2.67). These results suggest that moisture condition, freeze–thaw cycling, and salinity–related soil properties jointly influence NMR–derived pore–size variation in seasonally frozen saline soils.