Prevalence and initiation of preferential flow paths in a sandy loam with argillic horizon.

摘要

A dye tracer was used to examine the preferential transport mechanisms in a glacial outwash-derived soil widely used for intensive agriculture in central Minnesota, USA (coarse loamy over sandy, Udic Argiborolls). Experimental treatments included three initial soil water contents and three dye solution application rates (FLOOD, SPRINKLER-High, and SPRINKLER-Low). Thirteen cm of food dye solution (200 g/litre) were applied to replicated 1 m X 1 m plots in a recently tilled 5 year old lucerne [Medicago sativa] stand and to two additional plots with no history of lucerne. Vertical soil profile faces were exposed at 10-cm increments across each plot. Extensive and deeper preferential dye movement occurred under FLOOD conditions regardless of initial soil moisture or recent vegetation history. The two SPRINKLER rates generally resulted in relatively shorter preferential flow paths (PFPs). Within-plot variability of dye patterns - including depth and number of PFPs - was very high. Most PFPs observed were associated with roots and decayed roots, or with patterns in the abruptness and topography of the boundary between the Ap and Bt horizons. Open burrows were uncommon, but contributed to extensive preferential flow in the two no-lucerne plots. The findings indicate that preferential transport is prevalent under the variety of application rate and soil moisture conditions evaluated, and that observable soil features appear to be initiators of the majority of the PFPs. Only a few (10 of 126) of the profiles excavated had preferential flow paths that were not associated with visible soil features. The observed high variability gives support to the idea that observations of spatial variability in pesticide transport studies is due to preferential transport. Dye retardation was increased at slower application rates, indicating that the dye patterns observed were due to both the rate and the pattern of water movement. Thus the generally deeper and more extensive preferential dye transport under the higher velocity FLOOD application rate do not necessarily indicate more extensive preferential water (or non-adsorbing solute) transport at this rate compared to the intermittent SPRINKLER rate.