Background Plant roots affect the soil strength by their mechanical role and capability of influencing soil infiltration. At present, most studies mainly focus on the effects of root quantity, while the effect of root architecture remains unclear.

Methods In order to explore the influence of different root architecture on soil infiltration, water-head infiltration experiments were carried out with six plant species of Sapium sebiferum (V-type), Gordonia acuminate (VH-type), Nerium indicum (M-type), Symplocos lucida (R-type), Cunninghamia lanceolate (W-type) and Neolitsea aurata (H-type) in Jinyun Mountain of Chongqing. Each kind of tree was planted in a PVC bucket and the change of water seepage was measured by a rain gauge cylinder. The length density, bulk density, growth direction, and fractal dimension were measured for each root-soil composite. The relationship between steady infiltration rate and parameters representing root architecture was analyzed. Three methods including Kostiakov formula, Philip formula and Horton formula were used to fit the relationship between infiltration rates and time.

Results Root length density and root bulk density of W-type were the largest, and the root length density of M-type and root bulk density of H-type were the minimum. The steady infiltration and initial infiltration rate of six root-soil composites were higher than sole soil. The H-type was the tree species with horizontally and widely extending roots. The VH-type was the tree species with a strong tap root and widely extending lateral roots with low orientation to horizontal plane. Most of the main R-type roots extended obliquely. Majority of the M-type roots extended in various directions and the V-type root was nearly vertical. The W-type roots' lateral roots extended widely and the tap root was shallow. The volume fractal dimension of six root architecture ranged from 1 to 2. The maximal and minimal volume fractal dimensions were 1.87 and 1.03 for W-type and H-type respectively. Assuming that the root volume was given, the steady infiltration rate for V-type root-soil composite was the largest, while for H-type roots was minimum. The best fitted function of the steady infiltration rate versus time of all six root-soil composite was Horton's formula. However, the pair-sample t test results revealed that Horton's formula fitted for H-type root-soil composite and M-type root-soil composite, while Kostiakov formula fitted for M-type root-soil composite, R-type root-soil composite, and W-type Root-soil composite.

Conclusions 1) The steady infiltration rate of root-soil composite was 2-5.23 times higher than sole soil. Root length, bulk densities, volume and fractal dimension were positively related to steady infiltration rate. 2) Under the same external conditions, the steady infiltration rate with same root volume was in an order of M-type > R-type > W-type > VH-type > H-type. 3) The infiltration rate-time curves of H-type and the M-type root-soil composite can be fitted with the Horton's formula, while the M-type, R-type and W-type root-soil composite can be fitted with the Kostiakov formula.