TILLAGE, RESIDUE, AND RAINFALL EFFECTS ON MOVEMENT OF AN ORGANIC TRACER IN EARTHWORM-AFFECTED SOILS

A field study was conducted to determine the effects of tillage, residue, and rainfall on the movement of rhodamine WT (C29H29N2O5Na2Cl), a strongly adsorbed organic dye, in earthworm-affected soils. Rhodamine WT was uniformly applied to the surface of two separate, 0.063-m(2) areas within either no-till or conventionally tilled treatments in the presence or absence of residues. One area was exposed to a high intensity rainfall of 12 mm, whereas the second area received 193 mm of rainfall in 13 events during a 28-d period. Following rainfall, soils were excavated in layers to a depth of 40 cm and dye concentrations were determined. For the single rain event, dye concentration at the 28- to 40-cm depth was 1.20 and 0.63 mg kg(-1) in the no-till and conventionally tilled treatments containing residues, respectively, compared with concentrations near the detection limit (0.10 mg kg(-1)) in treatments without residues. For multiple-rain plots, dye concentrations of 1.80 and 0.93 mg kg(-1) were detected at the 28- to 40-cm depth in the no till with surface residue and no till without residue treatments, respectively, compared with concentrations near the detection limit for the conventionally tilled treatments. Staining patterns indicated that earthworm channels were responsible for dye movement below 20 cm. The differences in dye movement for single- and multiple-rain events suggests that properties of the macropore system, such as continuity and surface position, control preferential flow under differing tillage, residue, and rainfall conditions.