SOIL STRUCTURE FORMATION AND STABILIZATION BY EARTHWORMS AND TILLAGE EFFECTS ON SOIL POROSITY

Effects of tillage on soil porosity were investigated in plots planted to corn (Zea mays L.) for seven consecutive years. Micromorphometric analyses indicated that Ap horizons of no-till plots had less macroporosity than those of conventionally tilled plots and that loss of macroporosity was characterized by a decrease in mean pore size and a tendency for pores to become elongated, oriented parallel to the soil surface, and less tortuous. Obvious zoological activity, which consisted mainly of burrowing by earthworms, resulted in two to nine times more bioporosity in a no-till pedon than in a conventionally tilled pedon. Increased bioporosity may compensate for loss of macroporosity in the Ap of no-till plots by promoting rapid movement of water and gases from the soil surface to lower horizons. Specific effects of earthworms on soil structure were investigated by assessing the dispersibility of clay in worm casts. Freshly-deposited moist casts were 26 to 41% more dispersible than uningested moist soil due to disruption of some existing bonds by the peristalic action of the earthworm gut. Significant improvement in water stability of fresh, moist casts only occurred when food source derived organic matter was incorporated and casts were aged or dried, which facilitated close approach and bonding of plant and microbial polysaccharides and other organic materials to clay, predominantly via clay-polyvalent cation-organic matter (C-P-OM) linkages involving Ca. Casts produced by Lumbricus rubellus L. were less dispersible than casts produced by L. terrestris L. because they contained more incorporated organic debris. Mean reduction is dispersibility of dried casts when compared to uningested dried soil was up to 49% when worms were provided corn leaves; alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.) leaf diets reduced dispersibility to a lesser extent. Bromegrass (Bromus inermis L.) leaves were rejected by worms and did not reduce dispersibility. Surface casting by worms in areas that are subject to raindrop impact may contribute to soil degradation because casts are highly dispersible when first deposited. Casting activity should improve soil structure if casts are not subject to dispersion before becoming stabilized.