Dye tracer infiltration in the plough layer after straw incorporation
Résumé
Straw residues may be accumulated in isolated zones in the plough layer after mouldboard ploughing. This may limit straw decomposition because of limited nitrogen availability depending on the prevalent water flow pathways induced by tillage. In this study, we used the food dye Brilliant Blue FCF (Color Index 42090) to make visible the flow pathways in the plough layer of a loamy soil after incorporation of 10 tonne of dry mass ha(-1), and to get a qualitative and quantitative interpretation of the small-scale flow behaviour. Approximately 1.3 L of a 4 g L-1 Brilliant Blue solution was applied at a matric head of -1 cm with an infiltrometer, equipped with a 25 cm-diameter disk. Horizontal cross-sections of 50 x 50 cm were photographed at 1.5 to 3.5 cm depth intervals. High-resolution spatial maps of Brilliant Blue concentration were derived from the scanned photographs, using separate calibration relationships between the measured Brilliant Blue concentrations and the color spectra and depth for the soil and the straw. Pronounced lateral dye movement was observed through the soil matrix owing to soil sorptivity. In cases of high sorptivity, the dye barely reached the depth of straw incorporation. Otherwise, enhanced preferential dye transport directed towards the incorporated straw was made visible and the stained water eventually ponded on the plough pan which induced lateral redistribution. Although the straw inclusions contributed to dye transport, they did not retain a substantial part of the applied dye mass, owing to the low density and surface area of straw. However, these inclusions may enhance the fast migration of potential pollutants such as nitrate or pesticides out of the tilled layer where much of the transformation occurs due to biological activity.