Effects of different tree species on infiltration and preferential flow in soils developing at a clayey spoil heap

• Infiltration and preferential flow were markedly affected by planted tree species. • Litter quality influenced number of earthworms, infiltration, and preferential flow. • Architecture of root system was important for distribution of infiltrated water. • Connections and size of pores were crucial factors for water flow in emerging soil. Soil recovery is crucial for successful ecosystem restoration at post-mining sites. Water infiltration and preferential flow in soil are important but little studied soil characteristics. In this study, we explore effects of tree species on infiltration, geometry of preferential flows, size distribution and origin of pores, and related properties of soil. An experiment was conducted at post-mining sites developing under monocultures of three tree species (alder, oak and spruce) after lignite mining near Sokolov, Czech Republic. A key hydraulic parameter of soil, field saturated hydraulic conductivity ( K s ), measured in topsoil, was highest under alder (mean value of 6.9 × 10 −5 ms −1 ) and lowest under spruce (mean value of 1.9 × 10 −5 ms −1 ). In subsoil, there were no significant differences in K s between tree species. For the alder site, dye tracer tests showed that water penetrates almost uniformly to the top 15 cm of soil depth and with the greatest lateral flow. In a deeper part of the alder soil profile, preferential flow was concentrated around earthworm burrows and roots, where the two often occur together. Thin soil sections of alder soil show that earthworm casts and burrows substantially contribute to porosity around the roots (59 % of dyed worm-created structures in alder-root microsite). A similar situation was observed in oak, but preferential flows along roots started from the surface and cross section of preferential flow was smaller than in alder soil. In spruce, preferential flow was concentrated around roots and cracks in claystones and the cross section was even smaller. Comparison among the forest stands indicates that overall porosity does not correspond with infiltration of dye tracer and K s , which seem to be more driven by pore size distribution and pore connectivity. This is consistent with larger occurrence of macropores (>300 μm) in alder. The results show that trees affect infiltration and preferential flow in soil by their specific root systems and to a large extent indirectly by effect of litter quality on earthworm populations, which then enhance macroporosity and pore connectivity.