Abstract

There is a concern that preferential flow (i.e., bypass of soil matrix) of water and chemicals through soil macropores may enhance leaching below the root zone of agricultural crops, which may impact groundwater quality. Our objective was to investigate the nature of preferential flow in a clay loam soil and to ascertain the potential impact of tillage practice and rainfall intensity. We utilized a long-term (since 1968) conventional tillage (heavy-duty cultivator) and no-till field located at Lethbridge in southern Alberta. Four large (46 x 46 x 51 cm) undisturbed soil blocks were excavated from each tillage field (unreplicated) during the fallow phase of a wheat-fallow rotation. The soil blocks were transported to the laboratory, stored under drying conditions (32 months), and rainfall-leaching experiments conducted under unsaturated, transient conditions. In the laboratory, we attached a funnel-lysimeter consisting of 64 square funnels or cells to the bottom of each block. Three rainfall experiments were conducted on each block and a solid chloride tracer was applied to the top of the soil block before each rainfall event. Blocks were at a soil water content of field capacity prior to each rainfall. A total of 22 mm of rainfall was applied in 60, 30 and 15 min, corresponding to rainfall intensities of 22, 44 and 88 mm h-1, respectively. The volume of water outflow and concentration of chloride in each cell was determined after each rainfall event. Water outflow through >98% of cells indicated that many soil pores contributed to leaching of water. Convex-shaped curves of cumulative water outflow from cells versus cumulative area indicated non-uniform water flow, but the degree of water flow preference was low. Semi-variograms of cell water outflow indicated random flow, and the pattern of cell water outflow was stable with time and among rainfall intensities. Chloride outflow in <14% of cells indicated that only a few soil pores contributed to leaching of chloride. Total mass loss of chloride from each soil block was <0.1% of the total amount applied, which suggested most chloride was retained within the soil block, and was more consistent with immobile water rather than anion exclusion. Tillage practice seemed to exert a stronger influence on water outflow parameters than rainfall intensity, with a trend towards greater preferential flow of water under CT than NT. Total block water outflow increased with greater rainfall intensity for CT, and to a lesser extent for NT. Mean values of maximum chloride concentration were higher for CT than NT at 88 mm h-1, but not at the two lower intensities. Rainfall intensity had no effect on mean values of maximum chloride concentration for either CT or NT. Our results indicated that although preferential leaching occurred in these wet clay loam soils under relatively high rainfall intensities, the amount of chloride leached past 50 cm was minimal. Caution is advised in extrapolating these results to scenarios where conditions are different from this study.

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