Surface Aggregates and Macropore Effects on Chemical Transport in Soil under Rainfall
1995
Heathman, G. C. | Timlin, D. J. | Ahuja, L. R. | Johnsen, K. E.
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 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.
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