Objective To reveal the effect of root diameter classes on the occurrence of soil preferential flow in different depth of soil layer.

Method The study area located in the hillslope with hazelnut shrub forest in Zhangguangcailing of Changbaishan Mountains.The blue dye tracing method was applied to confirm and evaluate the changes in root biomass and root length density along preferential pathways in the soil matrix.

Result The macropore flow, funnel flow and surround flow were the main soil water flow movement patterns which occurred rapidly with surround features. The root length density decreased with the increase of soil depth. In addition, the contributions of root length densities with root diameter (d) < 1 mm, 1 < d < 3 mm, 3 < d < 5 mm, 5 < d < 10 mm to preferential flow were 67.9%, 64.9%, 55.2%, and 59.3%. The average contribution of plant roots d < 1 mm to preferential flow was almost 58.5% at all the experimental plots. The fluctuation ranges of root length density at different soil depth and different diameter were 2.59%~31.2% and 1.16%~11.07%. The proportion of root biomass in the preferential pathways was 1.1% higher than that in the soil matrix.

Conclusion The root length density of different root diameter classes in different soil layers has a significant effect on the soil preferential flow under hazel shrub forest. Root diameter d < 1 mm had the greatest effect. The root biomass does not play a decisive role in the occurrence of preferential flow paths.