TRANSPORT OF NITRATE IN SOILS AS AFFECTED BY EARTHWORM ACTIVITIES

Earthworm (Lumbricus sp.) holes have long been recognized as an import ant conduit for water and solute transport in field soils. In this stu dy we investigated preferential movement of NO3 through artificially i nduced earthworm holes and compared three commonly used solute transpo rt models with respect to their ability to describe NO3 transport thro ugh the earthworm macropore system. Earthworms holes were created by i ntroducing earthworms into uniformly packed soil columns of 20-cm i.d. and 30- or 60-cm long. After 8 wk of incubation, the columns were lea ched at a range of flues and NO3 breakthrough curves (BTCs) were deter mined. The columns also were traced with dyes to visualize the spatial distribution of the earthworm holes. The results showed that the eart hworms completely altered the uniformity of the packed soil cores. The average saturated hydraulic conductivities of the earthworm hole colu mns (K-sm) increased 17.9- to 22.3-folds as compared with the control columns. Significant preferential movement of NO3 occurred in these co lumns even at relative hues as low as 0.014 K-sm. A nonlinear least sq uares program, CXTFIT, was used to fit three solute transport models t o all BTCs. The physical nonequilibrium model (MIM) fitted the experim ental data better than the convection-dispersion equation (CDE) and th e stochastic model (SM), while none of them was adequate to describe t he data well.