## 4 Discussion
Among the pollutants in this experiment, TP and TSS were cut off at a high rate, TN at a moderate rate, and NH\({}_{3}\)-N and COD at the lowest rate. Based on a comprehensive analysis, physical interception is the most simple and effective measure for reducing TSS solids. Significantly, the reduction rate for TSS was higher when the specific surface area of contact was large between surface runoff and specific density herbaceous vegetation [10; 39; 40]. Phosphorus compounds in sewage are divided into organic and inorganic phosphorus; inorganic phosphorus exists in almost all phosphate forms, most incompatible with water, and in colloid and granular forms. However, soluble phosphorus and Ca\({}^{2+}\), Fe\({}^{3+}\), and Al\({}^{3+}\) ions form precipitates, and the relative mobility of phosphorus is poor [41; 42]. Therefore, TP is more easily removed than TN and NH\({}_{3}\)-N. Compared with phosphorus, nitrogen is more mobile; it is not easily adsorbed or intercepted. NH\({}_{3}\)-N mainly comprises nitrate nitrogen (NO\({}_{3}\)\({}^{-}\)) in water, which is released into the air after nitrification-denitrification chemical reactions. In addition, when the surface runoff velocity is high, on the one hand, it reduces the reaction time between pollutants and riparian herbaceous vegetation buffer strips; on the other hand, it weakens the physical interception effect of riparian herbaceous \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Plant Species**} & **Rate of** & **Rate of** & **Rate of** & **Rate of** & **Rate of** & **Synthesis** \\  & **TSS (\%)** & **TN (\%)** & **NH\({}_{3}\)-N (\%)** & **TP (\%)** & **COD (\%)** & **Score** \\ \hline _Cynodon dactylon ([PERSON].) [PERSON]_ & 64.6 & 44.5 & 27.8 & 58.1 & 27.6 & 44.5 \\ _Dichondra repens Forst_ & 57.2 & 36.8 & 36.3 & 69.6 & 25.3 & 45.0 \\ _Zoysia matrella_ & 46.9 & 33.0 & 32.0 & 65.7 & 24.2 & 40.4 \\ _Festuca elata Keng ex [PERSON]. [PERSON]_ & 39.0 & 29.2 & 32.0 & 65.7 & 25.3 & 38.2 \\ _Lolium preenne_ & 29.8 & 23.4 & 25.6 & 36.6 & 18.4 & 26.8 \\ bare soil & 3.0 & 7.2 & 21.4 & 20.1 & 16.1 & 13.6 \\ \hline \end{tabular}
\end{table} 
Table 8: The comprehensive reduction capacity of five herbaceous vegetation buffer strips. vegetation buffer strips and further weakens their ability to reduce pollutants [43]. Under high runoff velocity and short time conditions, the chemical action is insufficient, and the reduction rate is low [44; 45; 46]. Herbaceous morphology and biomass are important factors affecting the rate of reduction. Physical adsorption is the most direct way to reduce pollutants, and herbaceous vegetation biomass and growth morphology are the main factors affecting physical adsorption efficiency. Vegetation biomass reflects the total amount of living organisms per unit area at a particular moment. Under the same herbaceous vegetation and external environment, the greater the biomass, the stronger its ability to reduce pollutants [47]. In shallow surface runoff situations, herbaceous vegetation with well-developed leaves and stems that grow close to or along the ground have a stronger ability to reduce pollutants [48]. _Dichondra repens Forst_ is a small creeping herb with adventitious roots on nodes fixed to the ground; _Cynodon dactylon ([PERSON]. [PERSON]_ is a low, stalking ground with a firm root-spreading force, spreading widely on the ground. The above two kinds of herbaceous vegetation grow fast, prostrate on the ground, and have suitable biomass during the experiment. This is the main reason for its strong ability to reduce pollutants. _Zoysia mattrella_ is a perennial plant with slender fibrous roots, a stem height of 20 cm, dense growth, and a good cutting ability. _Festuca elata Keng ex E. [PERSON]_ has sparse or solitary stems, which can be up to 120 cm in height, and ryegrass, which has thin rhizomes and 90 cm in length, has relatively poor vegetation coverage, resulting in a general reduction in surface runoff.