4. Discussion
The current study found that the SDC values of sand soil under all four
treatments were higher than those of loessal soil. Previous studies have
similarly found that SDC may be influenced by soil type (Su et al.,
2014). Line and Meyer (1989) showed that resistance of soil to erosion
increased with increasing soil clay particle content. In the current
study, the clay contents of sand soil and loessal soil were 6.82% and
17.59%, respectively. Thus, it may be concluded that the lower SDC for
loess soil was due to its higher clay content. However, the response of
SDC of sand soil to hydraulic parameters was weaker compared to that of
loessal soil. In particular, there was a weak relationship between the
hydraulic parameters and the SDC of sand soil with roots. Thus, the SDC
of soil may be strongly affected by the root system distribution and
soil properties. Soil particles bounded by the root system are likely to
be strongly consolidated and therefore would not easily be detached by
water erosion, whereas soil not consolidated by the root system would be
highly susceptible to erosion. In addition, the root system increases
the roughness of the soil surface, thereby reducing the energy of
runoff, decreasing flow velocity and increasing soil water infiltration
(Zheng et al., 2009). Soil that is not consolidated by the presence of a
root system experiences stronger scouring erosion, resulting in the
rapid detachment of erodible soil particles. Since the root system ofB. ischaemum is mainly distributed in the surface soil, the roots
of this plant protect the surface soil as well as the soil in deeper
layers. Under this scenario, there would be little variation in SDC with
increasing flow intensity until such a time that the flow is
sufficiently strong to dislodge the surface soil layer bounded by the
concentrated root system. Therefore, predicting the SDC of sand soil
containing a root system requires both hydraulic parameters as well as a
parameter reflecting root weight. However, due to the higher clay
content and lower erodibility of loessal soil, the soil detachment
characteristics of loessal soil both with and without a root system are
similar. Hence, models that considered only hydraulic parameters were
better able to predict the SDC of loessal soil with
roots.
Although the SDC of soils both with and without roots increased under
the freeze-thaw effect, the effect was not significant. This result is
consistent with that of previous research (Sun et al., 2018). The
stability of soil aggregate is an effective index to measure the SDC of
soil subjected to freeze-thaw (Bryan, 2000; DiAz-Zorita et al., 2002) as
the soil aggregate is broken down by successive freeze-thaw cycles
(Lehrsch et al., 1991; Oztas and Fayetorbay, 2003), and it has been
shown that soil SDC is increased with the destruction of soil
macro-aggregate (Edwards, 1991). However, once the number of freeze-thaw
cycles exceed 5, factors such as flow and slope become increasingly
important for explaining the contribution of freeze-thaw to soil SDC.
This is why the contribution of freeze-thaw to the SDC was not
significant (Sun et al., 2018). The clear consolidation effect of the
root system on soil results in a significant negative correlation
between root density and SDC (Wang et al., 2014), indicating that the
root system reduces the SDC of soil significantly. The present study
found that SDC of both unfrozen soil and freeze-thaw soil was reduced by
97.82–99.66% due to the presence of a root system. This result is
supported by the research results of Gyssels et al. (2006), who found
that the roots of cereal plants reduced SDC by > 90%. The
main reason for root systems reducing SDC is due to consolidation of
soil particles and the increase in soil adhesion and stability by the
presence of roots (Baets et al., 2006, 2007). A second reason is the
influence of the root system on increasing soil infiltration capacity by
increasing the soil surface roughness and thereby reducing the velocity
of surface runoff (Gyssels et al., 2006; Viles, 1990). Thus, the SDC
would be decreased by root system.
Although soil detachment was increased and decreased by freeze-thaw and
the root system, respectively, the inhibition effect of the root system
dominated when the effects of freeze-thaw were combined with that of the
root system. The SDC under the effect of freeze-thaw in combination with
the root system was 5 times higher than that of soil under only the
influence of the root system, but 295 times lower than that of the soil
affected only by freeze-thaw. On the one hand, the root system
significantly increased resistance to overland flow velocity during the
process of erosion, thereby weakening the SDC driven by stream scour. On
the other hand, the effect of soil disturbance through the freeze-thaw
cycle was weakened by the root system to some extent (Gao et al., 2015).
The freeze-thaw process has been shown to weaken bare soil by 20.6%,
whereas it only weakened soil containing a root system by 7.3% (Li et
al., 2012). Meanwhile, the present study found that combined factors of
freeze-thaw and the root system explained 36.9% of SDC variation,
whereas freeze-thaw as a single factor only explained 10.64% of SDC
variation. This result suggests hints at the complexity of the composite
effect of freeze-thaw and the root system on soil detachment and the
need for further research.
Shear stress and stream power had positive linear relationships with
SDC, whereas unit energy of the water carrying section and unit stream
power showed power function relationships with SDC. This result is
consistent with the previous research (Wang et al., 2016; Zhang et al.,
2002, 2003, 2008). The present study found that stream power was the
hydraulic parameter best able to explain SDC under the effect of
freeze-thaw and the root system. This result is consistent with the
findings of Cao et al. (2009) and Su et al. (2014). Since the root
system is an important parameter that cannot be ignored when
representing the process of soil detachment within model development
(Wang et al., 2016), the present study developed an SDC prediction model
for different soil types and treatment measures based on stream power
and root weight. While the model developed in the present study is
similar to those developed in previous studies (Li et al., 2015; Wang et
al., 2014), some important differences exist. First, since soil
parameters were not considered in the present study, only root weight
and stream power were included in the prediction model. Secondly, the
present study improved the regression relationship between the root
system and SDC as an improved correlation between the SDC and the square
of root weight was obtained. This result may be related to change to
soil permeability resulting from the root system (Gyssels et al., 2006).
Although the presence of a small root system in the soil limits the
consolidation effect of the root system on soil particles, mechanical
interludes of the root system can destroy soil structure, thereby
increasing water infiltration. This can result in the collapse of the
soil structure, particularly in low-viscosity soils such as sand,
resulting in an increase in stream scour-driven SDC after water
infiltration volume exceeds a certain critical value. However, an
increase in the root system above a certain threshold density will
result in great improvement in the soil consolidation capacity of the
root system, thereby enhancing soil cohesion and reducing the detachment
and transport capacity of stream scour on soil particles and reducing
SDC (Xu et al., 2019). Therefore, the present study identified a
prediction model based on the square of root weight and stream power
that had a higher prediction accuracy compared to models developed in
previous research (NSE = 0.8488) (Li et al.,2015). The model
developed in the present study could provide accurate prediction for SDC
under different conditions in the seasonal freeze-thaw region. Of
course, further field tests are required to verify the practicability of
this model in areas subject to seasonal freeze-thaw.