# Influence of Vegetation Filter Strip on Slope Runoff, Sediment Yield and Nutrient Loss
## 4 Discussion
### The Offset Effect of Influencing Factors on Erosion Control
[55], the contribution to soil loss is ranked as runoff > rainfall intensity > slope > alfalfa coverage > soil protection measures in the following order. It can be speculated that the combined effect of VFS and slope gradient can be equivalent to the effect of rainfall intensity on sediment yield. Many studies have confirmed that the presence of plant roots increases soil porosity (through the formation of seepage channels) and retains surface soil particles, thereby reducing runoff and soil erosion. The rainfall intensity, slope gradient and vegetation combinations represented by No. 2, 22 and 23 also supported this view. However, according to the grouping of No. 17, 26 and 27, it was indicated that under the condition of rainfall intensity of 90 mm/h, no matter how the VFS was set, the results of sediment yield at 20\({}^{\circ}\) and 35\({}^{\circ}\) slope were similar. Therefore, using VFS does not prevent all runoff or soil loss in extreme conditions (such as steep sloping croplands and regions with frequent heavy rain) [55]. The influence of excessive rainfall intensity on slope sediment yield may be greater than that of slope gradient and VFS. However, the effect of No. 6 (60 mm/h, 35\({}^{\circ}\), bare slope) was close to that of No. 7 (90 mm/h, 5\({}^{\circ}\), bare slope), indicating that sediment yield had little change when rainfall intensity increased but slope had a corresponding amplitude of reduction simultaneously. In addition, according to the other three dissimilarity groups, it can be seen that when rainfall intensity was low at 30 mm/h, there was little difference in setting VFS1 or VFS2 on the same slope. It may be due to the small amount of sediment yield on the slope under light rain intensity, and further research is needed to obtain the interaction effect of the three factors. According to the dissimilarity group of runoff nutrient content, TN and TP content were different in different combinations of rainfall intensity, slope gradient and vegetation setting mode. For TN in runoff (Figure 8c), under the same rain intensity and vegetation coverage, the contents of TN in three slopes were similar. It can be concluded that the influence of slope on TN was not obvious from the group of No. 1, 2, 3; the group of No. 16, 17, 18; the groups of No. 22, 23, 24 and 25, 26, 27. On slope with the same VFS, the contents of runoff TN with 30 mm/h, 60 mm/h and 90 mm/h were in the same group, indicating that rainfall intensity has little influence on TN content in runoff, and its effect can be offset by slope gradient and VFS. For TP content in runoff (Figure 8d), when the rainfall intensity was 30 mm/h, TP contents in runoff of the same slope with and without vegetation (No. 1 and 19) were similar, and TP contents on different slopes with VFS2 (No. 20 and 21) were also similar. It is speculated that the runoff nutrient changes were not obvious due to the small rainfall intensity and little runoff. However, the TP content of No. 2 (30 mm/h, 20\({}^{\circ}\), bare slope) was similar to that of No. 7 (90 mm/h, 5\({}^{\circ}\), bare slope), indicating that the effect of rainfall intensity and slope gradient can be equivalent under certain conditions. These two were in the same group with No. 17, 26 (90 mm/h, 20\({}^{\circ}\), VFS) and No. 27 (90 mm/h, 35\({}^{\circ}\), VFS2), which indirectly indicates that VFS can effectively reduce runoff TP loss [59]. Since the concentration of runoff nutrients increased with the increase of raindrop kinetic energy and decreased with the increase of infiltration rate [60], when the rainfall intensity and slope increased, raindrop kinetic energy increased, and the rapid infiltration of surface runoff decreased, resulting in the increase of TP content in runoff. In terms of runoff nutrient loss, VFS can offset the influence of rainfall intensity and slope on nutrient loss. From the distribution of TN and TP contents in sediment, the loss of different nutrients in soil was also not completely synchronous.