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

Abstract

AbstractSoil 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 Sr2+ tracers (as SrBr2) under simulated rainfall, with the underlying soil initially dry or prewetted to a certain depth (about 7 cm). 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 Sr2+. After the rain, the column was sectioned in 1.15‐cm increments to determine soil water, Br−, and Sr2+ 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 Sr2+ movement much but increased Sr2+ content of runoff or macropore flow.