DISCUSSION
Reasons for single-peak curve
Figure 4(b) shows that CI appears to have an inflection point, and Fig. 5(b) shows that IR is a single-peak curve that has a higher value after the peak than before. We think this may be caused by the following factors: (1) Disappearance of WR: The water drop penetration time and ISWC have a one-peak distribution (Gao et al., 2018), suggesting that WR and ISWC follow a normal distribution. The wetting body ISWC above the wetting front increases gradually with the continuous infiltration of water. When ISWC becomes greater than the critical ISWC (corresponding to the maximum WR), the WR begins to decrease, and the repellency effect disappears after prolonged contact with water (Filipović et al., 2018). Thus, the IR increases over a longer duration after the WR has disappeared (Alagna et al., 2017).(2) Soil volume expansion: We conducted rainfall runoff tests and one-dimensional vertical water infiltration experiments using W-RS. The results show that water-repellent humus soil has a degree of expansibility (Fig. 14(a)), with volume expansion of ~17.5% under fully saturated conditions. Higher values of OMC (Table 1, row 6, column 2) enhance aggregate stability (Algayer et al., 2014) and increase soil porosity. Volume expansion leads to an increase in porosity, and especially an increase in macropores (Menon et al., 2011), which allow the penetration of water in the saturated surface soil. (3) Root growth of plants: Strong root activity in humus soil (Agnelli et al., 2016) means there may be several plant shoots (0–0.5 cm high) on the soil surface, and a large number of white root systems (0–10 cm) growing inside the soil (Fig. 14(b)). When water infiltrates the soil, the biological activity of plant seeds in the soil is activated (Maškováet al., 2019). Thus, the growth of root cells accelerates the absorption of water by soil particles. More importantly, some closed pores in the soil are connected by roots, and infiltration accelerates with preferential flow along those roots. (4) Increasing soil temperature: In our experiments, we found that the temperature of wetted soil was significantly higher than that of the dry parts of soil (Fig. 14(c)), and also higher than the water temperature in the Mariotte bottle. The maximum difference in temperature between the wetted and dry regions was 3.4°C (maximum temperature of 18.3°C, minimum temperature of 14.9°C). This indicates that the temperature increases come from biochemical reactions inside the soil. The viscous coefficient of water decreases as the soil temperature increases, thus enhancing the ability of water movement (Ben Amar et al., 2007) and accelerating the infiltration.
In summary, the mutation phenomenon of CI and IR results from the physical structure combined with chemical and biological factors. The results further prove that WR is a transient and spatially distributed physical soil property affecting soil hydrological, chemical, and biological processes (Liu et al., 2019).