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).