Rainfall Intensity Affects Transport of Water and Chemicals through Macropores in No‐Till Soil

Abstract

AbstractWater that infiltrates through cropland soils can carry agricultural chemicals through and below the root zone, possibly reaching groundwater. We investigated the effect of rainfall intensity on movement of water and chemicals through no‐till soil containing earthworm (Lumbricus terrestris L.) burrows and other macropores. Nine 30 by 30 by 30 cm blocks of soil from the surface of a no‐till corn (Zea mays L.) field received a surface application of 7.7 kg ha‐1 atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine) and 105 kg ha‐1 SrBr2·6H2O and were subjected to simulated rainfall treatments of 30 mm (2700 mL) of water applied in 15‐, 30‐, or 120‐min periods. Percolation through the blocks during and after the rainfall treatments was collected in 64 3.75 by 3.75 cm cells at the base of each block. Average time to start of percolation was 2.2, 4.4, and 51.5 min for the 15‐, 30‐, and 120‐min rainfall treatments, respectively. For the same treatments, percolation volumes averaged 468, 334, and 33 mL block‐1. Of the 64 possible percolation sampling cells, the average number of cells that collected percolate was 13.3 for the fast, 5.3 for the medium, and 3.3 for the slow treatments. The largest percolate sample for each block averaged 34, 60, and 51% of the total percolate for these intensity rates, emphasizing the lack of homogeneous flow through soils containing macropores. Average concentration of atrazine in the percolate ranged between 7 and 9 mg L‐1 and was not affected by rainfall intensity. A second 30‐min‐intensity storm was applied 1 wk later to all blocks, producing more percolate with lower chemical concentrations. Transport of each chemical was dominated by percolate volume rather than by rainfall intensity.