Cation Exchange Capacity and Soil Pore System Play the Key Role in Water Vapour Sorption

Understanding the controlling factors of soil water vapour sorption at different water activities ( a w ) is essential for accurate estimation of soil properties, such as cation exchange capacity (CEC) and specific surface area (SSA). The objective of this study was to identify the role of CEC and soil pore system in water vapour sorption and sorption hysteresis over a range of a w values. We measured the CEC, SSA, pore volume, and water and nitrogen adsorption/desorption isotherms on eight soils with different clay contents and mineralogy. Regardless of a w and sorption direction, there was a significant correlation between water content and CEC. The correlation increased with increasing a w at a w < ~0.2, was almost unchanged at ~0.2 < a w < ~0.75, and decreased with increasing a w at a w > ~0.75. At a w < ~0.6, water content change related mainly to CEC and SSA H2O , and at a w > ~0.6, water content change related mainly to SSA N2 and pore volume. Similar to the forced closure phenomenon of soil nitrogen desorption branch at P/P0 of ~0.45, the soil water vapour desorption branch also showed a sudden drop at 0.2 < a w < 0.4. Soil water vapour hysteresis at a w < ~0.75 was mainly due to the difference in cation hydration during the adsorption and desorption process, but was mainly attributed to the different sizes of narrow pore necks and their connected pores at a w > ~0.75