Vacuum drying water-repellent sandy soil: Anoxic conditions retain original soil water repellency under variable soil drying temperature and air pressure

Monitoring land degradation due to soil water repellency (SWR) through space and time is often complicated by different soil drying temperatures and drying times that generate artefacts causing inconsistent SWR results. This study aimed to develop a reproducible soil drying method at room temperature for the severity of potential SWR determination. We compared conventional 105 °C oven drying under oxic conditions (101.3 kPa) and vacuum drying under anoxic conditions (20 °C and low air pressure 1.3 kPa)—both drying methods removed the same amount of water from soil. We then investigated the effect of aeration condition (anoxic or oxic), temperature (20 °C or 105 °C) and air pressure (1.3 kPa or 101.3 kPa) on SWR and residual soil water content after drying using a complete factorial design. We hypothesised that 1) SWR will decrease with low air pressure under both oxic and anoxic conditions, 2) SWR will remain unchanged with temperature under anoxic conditions, and 3) conventional air drying treatments will have the highest residual soil water content. We used sieved, air-dried soil (<2 mm) that had been collected from a cropping field. Soil water repellency, determined by the molarity of ethanol droplet (MED) test, did not change following drying under anoxic conditions, relative to soil that was air-dried at 20 °C and 101.3 kPa under an oxic condition but decreased significantly under oxic conditions at high temperature (MED from ~1.58 to 1.34 mol L⁻¹) or low air pressure (from ~1.58 to 1.42 mol L⁻¹). We recommend vacuum drying water-repellent sandy soil under anoxic conditions at 20 °C and 1.3 kPa when assessing SWR.