Interaction of earthworm burrows and cracks in a clayey, subsurface-drained, soil

Installation of subsurface tile lines in poorly drained soils is a beneficial management practice that enhances crop productivity. In some instances, however, they can contribute to offsite losses of agricultural chemicals and sediment in drain flow. Movement of these materials through soil macropores (earthworm burrows and cracks) has been shown to contribute to this phenomenon. In order to determine if there was any interaction between these two types of macropores and subsurface drains we investiuated water movement in a sandy clay field in southwest Finland that had I m-deep tile drains installed in the 1950s. Previous studies at this site suggested that cracks were important in terms of water movement and that Lumbricus terrestris L. populations were greater, and their burrows deeper, above the drains than in the area between drains. Mean infiltration rate for soil above the drains was twice that of the mid-drain position and the infiltration rates were positively correlated to L. terrestris numbers and biomass. Infiltration rates in individual L. terrestris burrows, measured with the plow layer removed to reduce the influence of cracks, ranged from 6 to 1043 ml min(-1) (average 358 nil min(-1)) and did not appear to be related to the position of the burrows relative to the buried tile. Consistently higher infiltration rates (average 1080 ml min(-1)) were noted when measurements were made with the plow layer intact. Dye poured into the cracks adjacent to these burrows indicated water movement to the base of the plow layer, which acted as a hydraulic barrier, followed by lateral movement until open earthworm burrows were encountered. Water movement to the depth of the tile was exclusively in L. terrestris burrows with 106 dyed burrows m(-2) observed 10 cm above the tile. These observations indicate that entry of water into this soil is probably dominated by cracks when it is dry enough for their formation, yet rapid movement of materials entrained by this flow to the subsurface drains depends on connection of the cracks to earthworm burrows. Published by Elsevier B.V.