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Transport of Nitrate in Soils as Affected by Earthworm Activities

Sciences International, Inc., Alexandria, VA 22314;

Masoud Ghodrati^*

Dep. of Environmental Science, Policy, and Management, Univ. of California, Berkeley, CA 94720.

^* Corresponding author (ghodrati{at}nature.berkeley.edu).

ABSTRACT

Earthworm (Lumbricus sp.) holes have long been recognized as an important conduit for water and solute transport in field soils. In this study we investigated preferential movement of NO₃ through artificially induced earthworm holes and compared three commonly used solute transport models with respect to their ability to describe NO₃ transport through the earthworm macropore system. Earthworms holes were created by introducing 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 leached at a range of fluxes and NO₃ breakthrough curves (BTCs) were determined. The columns also were traced with dyes to visualize the spatial distribution of the earthworm holes. The results showed that the earthworms completely altered the uniformity of the packed soil cores. The average saturated hydraulic conductivities of the earthworm hole columns (K_sm) increased 17.9- to 22.3-folds compared with the control columns. Significant preferential movement of NO₃ occurred in these columns even at relative fluxes as low as 0.014 K_sm. A nonlinear least squares program, CXTFIT, was used to fit three solute transport models to all BTCs. The physical nonequilibrium model (MIM) fitted the experimental data better than the convection-dispersion equation (CDE) and the stochastic model (SM), while none of them was adequate to describe the data well.

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