Effect of soil organic matter on sorption of water vapor and associated hysteresis

In‐depth knowledge about the impact of organic matter (OM) on soil water vapor sorption (SWVS) and associated hysteresis related to various water activity ( a w ) levels is limited. The objective of this study was to elucidate the effect of OM on SWVS and sorption hysteresis for different a w levels. We measured water sorption isotherms and physicochemical properties of eight soils with varying clay (6%–47%) and OM (2.0%–7.8%) contents before and after OM removal. In general, OM increased SWVS, but the effect of OM on SWVS varied for different a w ranges. The presence of OM generally increased water vapor sorption during the adsorption process. For soils with OM content above 4.6%, the presence of OM decreased water vapor sorption during the condensation process. The local hysteresis ( H w ) of soils with OM was higher than that of soils without OM for soils with OM contents above 3.4%. For a w < ∼0.75, the water vapor sorption hysteresis of soils with and without OM showed a significant correlation with cation exchange capacity (CEC), and the H w values of soils with OM were lower than that of soils without OM for the same CEC. The OM can affect water vapor sorption and the associated hysteresis at low a w levels through adsorbed cations and organic functional groups, and at high a w levels through the structures of pores and OM.