Earthworm macropores and preferential transport in a long-term manure applied typic hapludalf

Deep burrowing earthworm species have been found to be present in soils with a history of manure application. This study was designed to quantify the effects of long-term application of liquid dairy manure and inorganic fertilizer on the distribution of earthworm macropores and in turn on the preferential transport of water and tracer through a typical soil of the karst area of the upper mid-western USA. Large (≈30 cm diam. by 90 cm long) undisturbed soil columns were taken from plots where liquid dairy manure or inorganic fertilizers had been applied continuously for 8 yr. The number and size distribution of macropores in soil columns were nearly the same for both inorganic and manure treatments, however, visible surface macropores were continuous to much deeper depth in soil columns taken from manure than from the inorganic fertilizer plot. Identification of the earthworms a year later showed the presence of Apporectodea tuberculata, A. trapezoides, and Lumbricus rubellus, subsurface burrowers, as well as, L. terristris, a deeper burrowing species in the manure applied plot. Apporectodea tuberculata was the only species present in the inorganic fertilizer plot. Number of macropores and macroporosity varied with soil depth. The maximum macroporosity was <2.5% and it occurred at 2-cm depth. The predominant macropore sizes were between 1- and 2-mm radii for both treatments. during breakthrough experiments, Cl^- appeared earlier in soil columns taken from the manure plot thereby indicating a greater continuity of macropores in the manure compared with the inorganic fertilizer treatment. The early appearance of Cl^- in the manure treatment, however, was much slower than one would expect based on the number of macropores and their continuity estimated from the serial sectioning. This suggests that intrusive serial sectioning and image analysis techniques probably overestimate the continuity of macropores possibly due to vacuuming of the earthworm casts and other debris that plugs the macropore channels. Based on macropore size distribution with depth and related breakthrough curves, it is likely that most existing models of water and contaminant transport that simulate macropore flow, will not accurately predict the transport of water and contaminant because of their assumption that surface visible macropores are continuous to deeper soil depths. Data from this study showed that macropore size distribution could be described by a normal or log-normal distribution function. These functions in combination with information on continuity and tortuosity of macropores may be sufficient, when used in some current macropore models, to adequately describe the conducting efficiency of macropores in soils.