Influencing factors
Soil erosion is related to both runoff strength and soil erodibility (Tang, 2004; Wang et al., 2012; Wang, 1993). Runoff serves as a vector for sediment (Hudson, 2015), and the final sediment yield is based on both runoff strength and soil erodibility (Wang, 1993). Runoff strength can be illustrated by volume and velocity, representing its amount and energy, respectively (Prosdocimi et al., 2016a). Splash-erosion was adopted to evaluate soil erodibility (Hu et al., 2016). As shown in Table 2, both Hr and Cm could restrict runoff (García-Orenes et al., 2012), and mulching could effectively alleviate splash-erosion (Gholami et al., 2013). These are the fundamental principles for weakening soil erosion. Generally, in our study, the treatments with higher runoff strength also suffered worse soil erosion (Figure 4 and 5). However, grievous splash-erosion, i.e., worse erodibility, did not always correspond to high soil loss (Figure 8; Table 4). Therefore, runoff strength should be a direct influencing factor to predict soil erosion.
Except for the effects introduced by tillage, runoff strength was determined by rainfall intensity (Gholami et al., 2013). Our results indicated higher-strength runoff (Woolhiser & Goodrich, 1988) and more soil loss (Römkens et al., 2002) with heavier rainfall (Figure 4 and 5), which indicated that the hydrological response of the soil is based on Hortonian flow type (Bombino et al., 2021). For example, in the Vr treatment, the runoff volume, runoff velocity, and sediment yield at 100 -mm h-1 were 1.67-, 2.26-, and 2.66-fold greater than those at 50 mm h-1, respectively; other treatments also showed similar trends (Table 3; Figure 4 and 5).
At the seedling stage, maize plants could protect the surface soil from splash-erosion by preventing direct hitting of raindrops (Table 4) and reducing their kinetic energy (Table 1) and by changing the distribution of raindrops (Table 2) because of canopy gaps (Ghahramani et al., 2011; Miyata et al., 2009). Nevertheless, as discussed earlier, splash-erosion has a limited influence on total soil loss amount. Under 100 mm h-1, the amount of soil loss of Vr significantly dropped from 10.6 to 0.28 kg by Vr+Cm, while the runoff amount was non-significantly reduced from 703.25 to 620.9 0 L, which indicated that the primary function of mulching is not reducing runoff volume. Thus, the excellent effects of mulching on erosion control shown in this experiment should mainly result in two other functions, reducing runoff strength and filtering out runoff soil particles (Prosdocimi et al., 2016a, b). Both functions caused a reduction in sediment concentration because of the effects of mulching as buffer strips (Fang, 2017).