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

Numerous studies have reported on preferential solute transport, giving evidence that preferential flow is widespread. However there has been little field documentation of the relative importance of different preferential transport mechanisms. This study used a dye tracer to examine the extent and relative importance of different preferential transport mechanisms in a glacial outwash-derived soil that is used extensively for high input agriculture in central Minnesota, USA (Verndale sandy loam: coarse loamy over sandy, mixed, frigid, Udic Argiborolls). Experimental treatments included three initial soil water contents (WET, MEDIUM, and DRY) and three dye solution application rates (FLOOD, SPRINKLER-High, and SPRINKLER-Low). Thirteen cm of FD and C Blue no. 1 (also known as Brilliant Blue FCF) food dye solution (200 g l^-1) were applied to replicated 1 m x 1 m plots in a recently tilled 5 year old alfalfa stand and to two additional plots with no history of alfalfa. 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-ALFALFA plots. Our 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. Our initial goal of elucidating the relationship between rate of application and the relative number and depth of PFPs was aimed at evaluating water flow patterns. However, subsequent research found that dye retardation was increased at slower application rates, indicating that the dye patterns we observed were due to both the rate and the pattern of water movement. As a result, we caution that our findings of 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. It is possible that our observations may be indicative of patterns in movement of adsorbed solutes such as pesticides, however this contention would require further research.