Surface Aggregates and Macropore Effects on Chemical Transport in Soil under Rainfall

Soil surface aggregates and macropores have the potential to greatly influence the transport of surface-applied agricultural chemicals in soil and to surface and groundwater. We studied the effects of a 1.0-cm layer of 4.5- to 12.5-mm surface aggregates, a 3-mm artificial macropore, and their combination on the transport of surface-applied Br and Sr 2+ tracers (as SrBr 2 ) under simulated rainfall, with the underlying soil initially dry or prewetted to a certain depth (about 7cm). Air-dried Kirkland soil (fine, mixed, thermic Udertic Paleustoll), <3.5-mm size, was packed in duplicate 15-cm-diam., 30-cm-long, sectionable Plexiglas columns (Cole-Parmer Instrument Co., Niles, IL) to a bulk density of 1.26 Mg m -3 . During the rainfall, surface runoff or macropore flow out of the column bottom was measured and analyzed for Br and Sr 2+ . After the rain, the column was sectioned in 1.15-cm increments to determine soil water, Br - , and Sr 2+ content distributions. Without a macropore, the surface aggregates reduced the overall movement of Br - in soil and increased Br in runoff, the effects being much greater in prewetted columns than in nonprewetted columns. A macropore, by itself, also reduced the movement of the main Br - pulse as it allowed surface solution (runoff) to move down and infiltrate laterally below the main wetting front. Surface aggregates greatly increased Br - content of the macropore flow and of the wetted soil around the macropore below the main wetting front. Aggregates or the initial condition did not change the Sr 2+ movement much but increased Sr 2+ content of runoff or macropore flow.