Background Riparian vegetation buffer zone has important ecological functions in soil and water conservation. However, serious vegetation degradation and soil erosion occur in this buffer zone due to various human disturbances. Thus, we characterized the root structure of five plant species and explored their relationships with soil infiltration.

Methods The soil and root systems of 5 species (Imperata cylindrica, Phragmites australis, Cynodon dactylon, Artemisia argyi and Juncellus serotinus) in three soil layers (0-10 cm, 10-20 cm, and 20-30 cm) in the riparian zone of lower Yellow River were sampled and analyzed. The soil infiltration was measured using the Double Ring method. The root morphology and structure analysis system (WinRHIZO) was used to analyze root length density, root surface density, and root volume density, while the root system was dried to determine the root biomass. The infiltration process was simulated by the Horton model, Philip model and Kostiakov model.

Results 1) The soil infiltration in 0-10 cm soil layer tended to be C. dactylon > I. cylindrical > P. australis > A. argyi > J. serotinus. The soil infiltration in the 10-20 cm and 20-30 cm soil layers was in this order: I. cylindrical > P. australis > A. argyi > C. dactylon > J. serotinus. 2) As the soil depth increased, the root structure parameters and soil infiltration of C. dactylon and J. serotinus decreased, that of A. argyi increased, while those of I. cylindrica and P. australis increased first and then decreased. 3) There were linear relationships among the root length density, root surface area density, root volume density, initial soil infiltration rate, stable infiltration rate, and average infiltration rate. The root biomass correlated with both soil stable infiltration rate and average infiltration rate. There was linear correlation between the characteristics of root system with diameter of 0.5-2.0 mm and soil infiltration characteristic index (P < 0.05). 4) Among the models, the Kostiakov model was more suitable to simulate the soil water infiltration processes. The model's parameter "a" decreased while parameter "b" increased with the increased root length density and root surface area density.

Conculsions Our results indicate that the root system characteristics of the plant species significantly affect soil infiltration. The overall soil infiltration of I. cylindrica and P. australis is the highest among five species. C. dactylon has a relatively better soil infiltration in the 0-10 cm soil layer, and the roots within 0.5-2.0 mm diameter play the key role in soil infiltration processes.