Effect of salinity on soil water vapor sorption and associated hysteresis

The mechanisms by which soil salinity impacts on water vapor sorption behaviors are not completely clear. The study aimed to investigate the effects of natural salts on soil water vapor sorption and associated hysteresis under different a_w levels, and to elucidate the role of salt crystals in the water vapor adsorption and desorption process. We measured the water vapor sorption isotherms (WSIs) and physicochemical properties of seven soils with salt content (Q) ranging from 0.5 to 22.7 g kg⁻¹ before and after salt removal. The WSIs of low-salinity soils (Q < 9.0 g kg⁻¹) exhibited classical Type II isotherm, but the WSIs of high-salinity soils (Q > 15.7 g kg⁻¹) displayed atypical linear adsorption (a_w = ∼0.7 to ∼ 0.85) and desorption (a_w = ∼0.5 to ∼ 0.75) trends, deviating from Type II behavior. The presence of salts had little effect on water content at low a_w levels (a_w < ∼0.4), but significantly increased water content at high a_w levels (adsorption: a_w > ∼0.6; desorption: a_w > ∼0.5). The presence of salts increased local hysteresis (H) across all a_w levels, with untreated soils exhibiting 1.3 to 6.2 times higher average H values than salt-removed counterparts. The maximum H differences for soils with Q < 15.7 g kg⁻¹ occurred at a_w > 0.8, while for the soil with Q = 22.7 g kg⁻¹, it occurred at a_w = 0.7. Water vapor sorption in saline and non-saline soils at low a_w levels was mainly through hydration interactions. The effects of salts on soil WSIs at high a_w levels were mainly attributed to the deliquescence of salts (sulfates and chlorides), increased electrostatic forces, and the osmotic potential generated by salt dissolution. The presence of salts amplified H primarily through cation hydration-dehydration asymmetry at low a_w levels, pore-throat blockage via capillary condensation at high a_w levels, and asymmetric deliquescence-crystallization thresholds in high-salinity soils.