INITIAL STORM EFFECTS ON MACROPORE TRANSPORT OF SURFACE-APPLIED CHEMICALS IN NO-TILL SOIL

Previous research has established that macropores can rapidly transmit water through soil. This observation has raised concern that macropores may also promote rapid movement of agricultural chemicals to groundwater. This is a particular concern for no-till fields where lack of disruption by tillage can lead to the development of extensive macropore systems. In order to investigate the effect of initial rainfall on chemical transport, strontium bromide hexahydrate (SrBr2.6H2O) and atrazine (2-chloro-4-ethylamino-6-isopropylamino -s-triazine) were surface-applied to six 30 by 30 by 30 cm blocks of undisturbed soil obtained from a 25-yr-old, no-till corn (Zea mays L.) field with evidence of well-defined macropores attributable to earthworm activity. Half of the blocks then received a 1-h 5-mm simulated rain, which did not produce percolate. Two days later, the blocks received a 0.5-h 30-mm simulated rain, followed by another 0.5-h 30-mm rain 1 wk later. The remaining blocks received only the two 30-mm events. An average of 12% of the applied water passed through all the blocks during and shortly after the first 30-mm rain. Bromide, Sr, and atrazine losses in this percolate were 7, 10, and 2 times less, respectively, from blocks that received the 5-mm rain than from blocks not receiving this initial, light rain. The second 30-mm rain on the blocks not receiving the initial 5 mm produced 1.6X more percolate than the first 30-mm rain. Yet, transport and flow-weighted average concentrations of Br, Sr, and atrazine were all reduced. These results indicated that the first storm after application can move solutes into the soil matrix, thereby reducing the potential for transport in macropores during subsequent rainfall events.