Clay content and mineralogy, organic carbon and cation exchange capacity affect water vapour sorption hysteresis of soil

The hysteretic behaviour of the dry region (<−1.5 MPa) of the soil water characteristic, which is of the essence for accurate characterization and modelling of bio-physicochemical soil processes under dry conditions, is well documented. However, knowledge about how to best quantify water vapour sorption hysteresis and about the effects of soil properties on dry-region hysteretic behaviour is limited. To overcome this knowledge gap, we proposed a new method for quantifying sorption hysteresis and evaluated its applicability based on measured sorption isotherms of four source clay minerals and 147 soil samples. Furthermore, the effects of clay mineralogy, clay content, soil organic carbon (SOC) and cation exchange capacity (CEC) on the magnitude of sorption hysteresis were investigated. For the clay minerals, kaolinite did not exhibit hysteretic behaviour, illite showed some hysteresis, whereas Na- and Ca-smectite exhibited strong hysteretic behaviour. The average hysteresis, corrected for clay and SOC contents, was strongly reflective of the dominant clay mineralogy of the soil samples. For the soil samples with low SOC content, the average hysteresis significantly increased with increasing clay content (R2 = 0.92), except for the kaolinite-rich samples (R2 = 0.35). The SOC-rich samples that exhibited illitic clay mineralogy and similar soil texture showed a significant increase in average hysteresis with increasing organic carbon content (R2 = 0.93). For all soil samples combined, the CEC was the strongest indicator for the magnitude of water vapour sorption hysteresis. Highlights: A new index for quantification of soil vapour sorption hysteresis was proposed Large SOC and clay content increased sorption hysteresis For soil samples, dominant clay mineralogy controlled the magnitude of hysteresis Cation exchange capacity was the best predictor of hysteresis for all soil types.